Thursday, January 31, 2008

IIT JEE Revision Ch.10. CHEMICAL KINETICS Core Points

Jee Syllabus

Chemical kinetics:
Rates of chemical reactions;
Order of reactions;
Rate constant;
First order reactions;
Temperature dependence of rate constant (Arrhenius equation).
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The topic "Chemical kinetics" consists of reaction rate and reaction mechanism.

The branch of chemistry which deals with the rates of chemical reactions and the mechanism by which they occur, is called chemical kinetics.

Reaction rate is the speed with which a reaction takes place. This shows the rate or speed at which the reactants are consumed and products are formed.

Reaction mechanism is the path by which a reaction takes place.


Rate of reaction

The rate of reaction is a quantity that tells how the concentration of reactants or product changes with time.

So this can be expressed as Δ concentration/Δ time. That is change in concentration divided by time taken for the change.

Molar concentration i.e., moles per liter (M), is used in these equations.

The brackets, [ ] are always used to to indicate molar concentrations.

Rate law

The rate for a reaction is a mathematical expression that relates the rate of reaction to the concentrations of the reactants.

For the reaction aA + bB → products

The rate law is expressed as, rate of reaction is proportional to [A]^x[B]^y.
x and y are determined experimentally. These values can be whole or fractional numbers or zero.

Law of Mass Action

In 1867, Cato Guldberg, and Peter Waage, proposed this law. According to this law, for the rate determining step in a reaction, the rate of reaction is proportional to the product of the concentrations of the reactants, each raised to the power of its coefficient in the balanced equation.

For the reaction aA + bB → cC (when it is a rate determining step)

Rate of reaction is proportional to [A]^a[B]^b

The above proportionality can be written as an equation, by putting in a proportionality constant k.

Rate = k *[A]^a[B]^b

K is called the specific rate constant


Order of Reaction

From the rate law for a reaction order of reaction can be determined.

For a particular species or reactant, the order is equal to the exponent for that species in the rate law.

For example for Rate = k *[A][B]^2
for B the order of reaction is 2. For A it is 1.

The overall order of reaction is equal to the sum of all the individual orders of reactants.


Temperature

As temperature increases, the average kinetic energy increases. So there are more molecules with activation energy and hence reaction rate increases.

As a general approximation, the rate roughly doubles for each 10°C rise in temperature.

JEE Revision - Rates of chemical reactions

Reaction rate is the speed with which a reaction takes place. This shows the rate or speed at which the reactants are consumed and products are formed.

Reaction mechanism is the path by which a reaction takes place.


Rate of reaction

The rate of reaction is a quantity that tells how the concentration of reactants or product changes with time.

So this can be expressed as Δ concentration/Δ time. That is change in concenation divided by time taken for the change.

Molar concentration i.e., moles per liter (M), is used in these equations.

The brackets, [ ] are always used to to indicate molar concentrations.

Rate law

The rate for a reaction is a mathematical expression that relates the rate of reaction to the concentrations of the reactants.

For the reaction aA + bB → products

The rate law is expressed as, rate of reaction is proportional to [A]^x[B]^y.
x and y are determined experimentally. These values can be whole or fractional numbers or zero.

Rate = k[A]^x[B]^y

Law of Mass Action

In 1867, Cato Guldberg, and Peter Waage, proposed this law. According to this law, for the rate determining step in a reaction, the rate of reaction is proportional to the product of the concentrations of the reactants, each raised to the power of its coefficient in the balanced equation.

For the reaction aA + bB → cC (when it is a rate determining step)

Rate of reaction is proportional to [A]^a[B]^b

The above proportionality can be written as an equation, by putting in a proportionality constant k.

Rate = k *[A]^a[B]^b

K is called the specific rate constant

Note rate law and law of mass action are different expressions.

JEE Revision - Order of reactions

Rate law

The rate for a reaction is a mathematical expression that relates the rate of reaction to the concentrations of the reactants.

For the reaction aA + bB → products

The rate law is expressed as, rate of reaction is proportional to [A]^x[B]^y.
x and y are determined experimentally. These values can be whole or fractional numbers or zero.

Rate = k[A]^x[B]^y

Order of a reaction

The sum of the powers to which the concentration terms are raised in the rate law expression.

For the expression Rate = k[A]^x[B]^y, the order of the reaction is x+y. The order of the reaction is represented by n.

When n = 1, the reaction said to be first order reaction.
n = 2 second order reaction etc.

There are number of reactions where rate of reaction is independent of concentration of reactants. The order of reaction is zero.


IIT JEE Revision - Rate constant

Rate law

The rate for a reaction is a mathematical expression that relates the rate of reaction to the concentrations of the reactants.

For the reaction aA + bB → products

The rate law is expressed as, rate of reaction is proportional to [A]^x[B]^y.
x and y are determined experimentally. These values can be whole or fractional numbers or zero.

Rate = k[A]^x[B]^y

k = the rate constant.
[A] and [B] are molar concentrations of reactants mol/litre

Units of rate: Rate is the change in concentration with time.

If the concentrations are expressed in moles/litre and time in seconds, then the units for rate of reaction are mol litre-1 s-1 or mol L-1s-1

Units of rate constant

Units of rate constant are different for different orders of reaction.

For zero order reactions units of rate constant are mol L-1s-1

For first order reactions units of rate constant are s-1

For second order reactions units of rate constant are L mol-1s-1

Basically units of rate constant are changing to give the rate of reaction in required units mol L-1s-1 (change in concentration with time).

In case of gases, the concentrations are expressed in terms of pressure in the units of atmosphere. Therefore the rate of reaction has the units of atm per second.


Integrated Rate Expressions

For zero order reactions

k0 = {[A]0 - [A]}/t

Where k0 = rate constant in the case of zero order reactions
[A]0 = Initial concentration of reactant A

[A] = concentration of reactant A at time t.
t = time

This can be alternatively expressed.

a = Initial concentration of reactant A (in moles per litre)
x = moles reactants that changed into products in time t
a-x = concentration of reactant A after time t

k0 = x/t

Where k0 = rate constant in the case of zero order reactions
x = moles reactants that changed into products in time t

For first order reactions

k1 = (2.303/t)log{[A]0/[A]}

Where k1 = rate constant for first order equations

Alternative expression

a = Initial concentration of reactant A (in moles per litre)
x = moles reactants that changed into products in time t
a-x = concentration of reactant A after time t

k1 = (2.303/t)log{a/(a-x)}

IIT JEE Revision - Arrhenius equation

Reaction Rate Depends on Temperature

Temperature has influence on reaction rates. In general, an increase in temperature increases the rate of almost all reactions.

A general approximate rule is that the rate of a reaction becomes almost double for every 10° rise in temperature.

Activation Energy

For many reactions some extra energy is to be supplied to the reactants to initiate the reaction. This excess energy is required to bring the energy of reactants to the energy that is required to start the reaction. The energy of the reactants at which the reaction starts is called threshold energy.

Activation energy is the extra energy supplied to initiate the reaction. Thus activation energy is equal to the difference between the threshold energy and the average kinetic energy of the reacting molecules at the the given temperature (Note as activation energy is being given the temperature of the reactants increases)

Arrhenius Equation

Arrhenius proposed a quantitative relationship between rate constant and temperature

k = Ae(–Ea/RT)

where k = rate constant
A is a constant known as frequency factor. In a JEE problem it was termed as preexponential factor
–Ea is the activation energy
Both A and –Ea0 are characteristic of the equation
T is the absolute temperature and R is the gas constant

In log form the equation becomes

log k = log A - (Ea)/2.303 RT

As the activation energy –Ea0 increases, the value of k decreases and therefore, the reaction rate decreases.

Find the value of –Ea0

If log k is plotted against 1/T (both found through experiments), the intercept of the line will be equal to - (Ea)/2.303 R. Hence from the slope found from the graph - (Ea) can be found out as -2.303 R multiplied by slope.

Second Method

Measure rate constant at two temperatures k1 and k2 at T1 and T2

log (k2/k1) = (Ea/2.303 R)[ (1/T1) - (1/T2)]

JEE 2009 problem


For a first order reaction A→P, the temperature (T) dependent rate constant(k) was found to follow the equation logk = – (2000)(1/T) + 6.0
The pre-exponential factor A and the activation energy Ea, respectively, are -

Answer:
1.0 × 1066 s-1 and 38.3 kJ mol-1

IIT JEE Revision Ch. 11. SURFACE CHEMISTRY Core Points

JEE syllabus

Surface chemistry:
Elementary concepts of adsorption (excluding adsorption isotherms);
Colloids: types, methods of preparation and general properties;
Elementary ideas of emulsions, surfactants and micelles (only definitions and examples).
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The term adsorption implies the presence of excess concentration of any particular component in one of the three phases of matter (known as adsorbate) at the surface of liquid or solid phase (known as adsorbent) as compared to that present in the bulk of the material.

On the basis of the forces of attraction between adsorbent and adsorbate, two types of adsorption, namely, physisorption (i.e. physical adsorption) and chemisorption, may be identified.

Colloids or sols are the substances whose sizes lie in between the solutes present in a true solution (e.g., salt, sugar) and the solutes present in suspension (e.g., sand).

The diameters of colloidal particles may range from 1 to 100 nm. The particles in colloidal state do not settle down on standing, are not visible and they can pass through a filter paper. However, they do not pass through a perchment paper or animal membrane.

Emulsion is a liquid dispersed in a liquid.

Any substance which can decrease the surface tension of water to a large extent is known as surfactant. Examples of soap and detergents. Such substances have larger concentrations at the surface of water as compared to the bulk of the solution.


Surfactants in solution are often association colloids, that is, they tend to form aggregates of colloidal dimensions, which exist in equilibrium with the molecules or ions from which they are formed. Such aggregates are termed micelles.

IIT JEE Revision Elementary concepts of Adsorption

Elementary concepts of adsorption (excluding adsorption isotherms);

The term adsorption implies the presence of excess concentration of any particular component in one of the three phases of matter (known as adsorbate) at the surface of liquid or solid phase (known as adsorbent) as compared to that present in the bulk of the material.

On the basis of the forces of attraction between adsorbent and adsorbate, two types of adsorption, namely, physisorption (i.e. physical adsorption) and chemisorption, may be identified.

In physisorption only van der waals forces attration are present between molecules of absorbent and absorbate.

In chemisorption covalent bonding takes place between molecules of absorbent and absorbate. Ex: Iron nitride.


The characteristics of physisorption are;

1. the forces of attraction are of van der waals type (weak forces).
2. Predominates at low temperature.
3. All gases show this adsorption at low temperatures.
4. Heat of adsorption is low, about 40 kJ/mol.
5. Reversible in nature.
6. Low activation energy (appx. 5 kJ)
7. Adsorption is multilayer.

The characteristics of chemisorption are:

1. the forces of attraction are of a chemical nature (strong forces)
2. Predominates at high temperature.
3. This is highly specific in nature.
4. Heat of adsorption is large 9appx. 80 to 420 kJ/mol)
5. Usually irreversible.
6. Large activation energy
7. Adsorption is monolayer.

The extent of adsorption of gases increases with increase in the pressure of the gases and it decreases with increase in temperature of the gas.

IIt JEE Revision - Emulsions, Surfactants and Micelles

JEE syllabus

Elementary ideas of emulsions, surfactants and micelles (only definitions and examples).


Emulsions:

Emulsion is a liquid dispersed in a liquid.

Any two immiscible liquids form an emulsion.

For example, milk is a naturally occuring emulsion in which particles of liquid fats are dispersed in water.

Since two immiscible liquids do not mix well, the emulsion is generally unstable and separation of liquids may take place on standing for some time.

Emulsifiers are substances which are added to make emulsions more stable.

Emulsifiers reduce the interfacial tension between the two liquids.

Two types of emulsions: Oil in water and water in oil.

Identifications of two types of emulsions:
Dilution test: If on addition of water, the emulsion becomes dilute, it means it is oil in water emulsion.

Dye test: An oil soluble dye is used and if the whole solution becomes coloured it is water in oil emulsion. If only drops become coloured, it is oil in water emulsion.

Any substance which can decrease the surface tension of water to a large extent is known as surfactant. Examples of soap and detergents. Such substances have larger concentrations at the surface of water as compared to the bulk of the solution.


Surfactants in solution are often association colloids, that is, they tend to form aggregates of colloidal dimensions, which exist in equilibrium with the molecules or ions from which they are formed. Such aggregates are termed micelles.

Tuesday, January 29, 2008

IIT JEE Revision Ch.21 Alkanes - Core Chapter Points

Syllabus

Preparation, properties and reactions of alkanes:

Homologous series,
Preparation of alkanes by Wurtz reaction
Preparation of alkanes decarboxylation reactions.
physical properties of alkanes (melting points, boiling points and density); Combustion and halogenation of alkanes;
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Alkanes: Introduction

Alkanes are saturated hydrocarbons containing only carbon-carbon single bonds in their molecules.

Thye are also called paraffins (meaning little affinity or reactivity, we will see later why it is so).

Alkanes are divided into 1. Open chain or acyclic Alkanes and 2. CycloAlkanes or cyclic alkanes.

The general formula of alkanes is CnH2n+2



Preparation of alkanes



1. From unsaturated hydrocarbons (alkenes and alkynes)
2. From alkyl halides
3. From carboxylic acids and their salts

1. From unsaturated hydrocarbons (alkenes and alkynes)
By catalytic hydrogenation alkenes and alkynes are converted into alkanes (Note that this point will come in alkenes and alkynes chapter as reactions of them).
Ni, Pt or Pd in the form of fine powder are used as catalysts. A temperature of 523-573 K needs to be employed.

Methane cannot be prepared by this method because alkenes or alkynes will have two carbons at their lowest level.





2. Wurtz reaction (From alkyl halides)
When an alkyl halide (usually bromide or iodide) is treated with sodium in dry ether, a symmetrical alkane containing both twice the number of carbon atoms of alkyl halide is obtained.

3. Decarboxylation reaction
When sodium salt of a monocarboxylic acid is heated with soda lime (amixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.

Physical properties of alkanes

1. State: CH4 to C4H10 are gases, C5H12 to C17H36 are liquids and higher ones are solids
2. Boiling point: Boiling point increases with molecular mass. Branched isomers have a lower boiling point than normal alkanes.
3. Melting point
4. Solubility: Being nonpolar, these are insoluble in water.
5. Density: Liquid alkanes lighter than water


Combustion
Large quantity of heat generated in the combustion of alkanes

Halogenation of alkanes

This involves substitution o fhydrogen atom by halogen atom. The order of reactivity is F2>Cl2>Br2(>I2). The mechanism of chlorination and bromination involves free radicals.

IIT JEE Revision Alkanes - Introduction

Alkanes are also called paraffins

Gen formula CnH2n+2

Open alkanes
Cyclo alkanes

They are saturated.

Under normal conditions of temperature and pressure they do not react with reagents like acids, bases, oxidizing agents and reducing agents

IIT JEE Revision Alkanes Nomenclature

1. The unbranched alkanes are named according to the number of alkanes of carbons. Examples: Methane, Ethane, Propane etc.

2. For alkanes containing branched carbon chains, the principal chain of the compound needs to be determined. This is the longest continuous carbon chain in the molecule.

3. In determining the principal chain sometimes, two or more chains in the molecule may have same number of carbons in their chains. In such a case, the chain having the greater number of branches is chosen as the principal chain of the molecule.

4. Numbers have to be given for the carbon atoms, to which the branches are attached. To give numbers, principal chain of the carbon atoms is numbered from the direction that gives the lower number to the first branching point.

5. Branching groups are in general termed as substituents. Each group is to be given an IUPAC name (prefix name) and is prefixed to the name of the principal chain of the molecule.

6. Compound is named according to the pattern “number- alkyl group prefix name principal chain name”. Name of the group (prefix) and name of the principal chain are written together as one word.

7. Where there are more than one substituent groups, each substituent group must be given its own number depending on the carbon to which it was attached.

8. In case of identical substituents, prefixes di, tri, tetra etc., are used before the group prefix name to indicate number of identical groups.

9. Substituent groups are written in alphabetical order regardless of their location in the principal chain but the prefixes di, tri… as well as the prefixes tert- and sec- are ignored in deciding the alphabetical order, and the prefixes iso, neo, and cyclo are considered.

10. If the number of carbon atoms of the principal chain from either gives identical numbers to the substituent attachment carbons, the direction which gives the lower number to the first written group (according to the alphabetical order) is chosen.







Examples

2,2,4,4-Tetramethylhexane

3-Ethyl-2,2-dimethylpentane

Revision - Conformations in Alkanes

Isomerism

Conformations

The different arrangements of atoms in a molecule which can be obtained due to rotation about carbon-carbon single bond are called conformations

To present ocnformations, the diagrams used by chemists a. Saw horse representation and (b) Newman projection can be used.

Conformations of Ethane

Of all the conformations for ethane, only two extreme conformations are important and these are

Staggered conformation
Eclipsed conformation

The staggered conformation is more stable than the eclipsed conformation.

Conformations of Butane

In butane (CH3CH2CH2CH3) the rotation about the single bond between the two inner carbon atoms (C2 and C3) is examined.
The lowest energy conformation will be theo one, in which the two methyl groups are as far apart as possible i.e., 180° away from each other. This conformation will be maximum staggered and is called anti conformation.

Order of stability

Anti>Skew or Gauche>Eclipsed>Fully eclipsed

IIT JEE Revision - Preparation of alkanes

General methods

1. From unsaturated hydrocarbons (alkenes and alkynes)
2. From alkyl halides
3. From carboxylic acids and their salts

From unsaturated hydrocarbons (alkenes and alkynes)

By catalytic hydrogenation alkenes and alkynes are converted into alkanes.
Ni, Pt or Pd in the form of fine powder are used as catalysts. A temperature of 523-573 K needs to be employed.

Methane cannot be prepared by this method because alkenes or alkynes will have two carbons at their lowest level.

2. From alkyl halides

Wurtz reaction: When an alkyl halide (usually bromide or iodide) is treated with sodium in dry ether, a symmetrical alkane containing twice the number of carbon atoms of alkyl halide is obtained.



Reducing agents can be used to add hydrogen to the halide and remove the halogen atom.
i) Zinc + HCl is one reducing agent.
ii) Catalytic hydrogenation using Pd or Pt as catalyst
iii) Hydrogen iodide (halogen acid) in the presence of red phosphorous also acts as reducing agent. In this reaction phosphorous combines with iodine to form phosphorous triiodide.
iv) zinc copper couple and alcohol


By the use of Grignard reagent

Alkyl halides react with magnesium metal in diethyl ether to form alkyl magnesium halides which are called as Grignard reagents.

Grignard reagents are highly reactive and are easily decomposed by water or alcohol to form alkanes

RMgX + HOH (H2O) ---> RH + Mg(OH)X

3. From carboxylic acids

i) Reduction of carboxylic acid: Carboxylic acids are reduced to alkanes by hydroiodic acid (HI). In this reaction COOH group in the carboxylic acid is reduced to CH3 group.


ii) When sodium salt of a monocarboxylic acid is heated with soda lime (a mixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.

RCOONa + NaOH -->RH + Na2CO3

In this reaction a CO2 group is removed from carboxylic acid and therefore the reaction is called decarboxylation.



iii) Kolbe's reaction

When an acqueous solution of sodium or potassium salt of carboxylic acid is eletrolysed alkane is evolved at the anode.

Kolbe's reaction can also be used like wurtz reaction for preparing alkanes with even number of carbon atoms.



The methods in this section can be summarised as

R-COONa ---> RH
R-COOK ---> R-R - Kolbe's reaction
R-COOH---> R-CH3

Industrial method: Petroleum provides the natural source of alkanes.

Revision - Preparation of alkanes by Wurtz reaction

When an alkyl halide (usually bromide or iodide) is treated with sodium in dry ether, a symmetrical alkane containing both twice the number of carbon atoms of alkyl halide is obtained.

RX + 2Na + XR ---> R-R + 2NaX (catalyst in dry ether)

There is a possibility, in the reaction to use different alkyl halides instead of a single halide. If two different halides are taken with the aim of preparing an alkane with odd number of carbon atoms, a mixture of products is obtained in stead of a single alkane. This is because in this case three reactions takes place and three different products are obtained.

Monday, January 28, 2008

IIT JEE Revision Preparation of Alkanes by Decarboxilation

Decarboxylation reaction

When sodium salt of a monocarboxylic acid is heated with soda lime (a mixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.

RCOONa + NaOH -->RH + Na2CO3

In this reaction a CO2 group is removed from carboxylic acid and therefore the reaction is called decarboxylation.

IIT JEE Revision Alkanes Physical Properties

1. State: CH4 to C4H10 are gases, C5H12 to C17H36 are liquids and higher ones are solids
2. Boiling point: Boiling point increases with molecular mass(carbon atoms).
Branched isomers have a lower boiling point than normal alkanes.
3. Melting point: Melting points increase with molecular mass. But the increase is not a regular variation. In general, the alkanes with even number of carbon atoms have a higher melting points as compared to the immediately next lower alkanes with odd number of carbon atoms.
4. Solubility: Alkanes are almost non-polar molecules. Being nonpolar, these are insoluble in water. They dissolve in non-polar solvents such as ether, benzene, carbon tetrachloride etc. The solubility generally decreases with increase in molecular mass.
5. Density: Liquid alkanes lighter than water. the density increases with the increase in the number of carbon atoms.

IIT JEE Revision Alkanes Chemical Reactions

Chemical properties or reactions

1. Substitution reactions of alkanes
a)Halogenation
Involves the replacement of one or more atoms of hydrogen by the halogen atoms.

b) Nitration
Involves the replacement of a hydrogen atom of alkane with -NO2 group.
Nitration of higher alkanes (hexane or higher) is carried out by boiling alkane with nitric acid.

Nitration of lower alkanes can be carried out in vapour phase by heating alkane and nitric acid to very high temperatures in the range of 723-773 K. At such higher temperature C-C bonds of alkane breakes, and hence a mixture of nitroalkanes may obtained.

For example in the reaction between ethane and nitric acid at very high temperature, both nitroethane and nitromethane(due to breaking of one C-C bond in ethane) are obtained as mixture.

c) sulphonation
This involves the replacement of a hydrogen atom by -SO3H group.
Fuming sulphuric acid reacts with alkane at higher temperature.

Higher alkanes (hexane and above) only give this reaction.

Lower alkanes particularly methane, ethane do not give this reaction.

2. Oxidation
a) Complete combustion
b) Incomplete combustion
c) Controlled oxidation

3. Action of steam
This reaction is used for the production of hydrogen from natural gas.

On passing a mixture of steam and methane over heated nickel(over alumina Al2O3) catalyst at 1273 K, methane gets oxidized to carbon monoxide and all hydrogen atoms get released.

4. Isomerisation

branched isomers of alkanes are obtained by heating alkanes with anhydrous aluminium chloride (AlCl3) and hydrogen chloride at 573 K under a pressure of about 30-35 atmosphere.

5. Aromatization

Alkanes containing 6 or more carbon atoms get converted to aromatic compounds, when heated at about 773 K under higher pressures of the order of 10-20 atm in the presence of catalysts - like oxides of chromium, molybdenum or vanadium supported on alumina gel.

6. Thermal decomposition or fragmentation
When higher alkanes are heated to high temperatures (700-800 K) in the presence of alumna or silica catalysts, they break down to lower alkanes and alkenes.

If methane is heated to high temperature up to 1500 K, it breaks down to its elements (carbon and hydrogen)

IIT JEE Revision - Combustion of Alkanes

Complete combustion

Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide and water.


C3H8 + 5O2 --> 3CO2 + 4H2O

2C4H10 + 13O2 --> 8CO2 + 10H2O

Incomplete combustion (where there isn't enough oxygen present) can lead to the formation of carbon or carbon monoxide.

A simple explanation is that, hydrogen in the hydrocarbon gets the first chance at the oxygen, and the carbon gets whatever is left over.

The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke.

Carbon monoxide is produced as a colourless poisonous gas.

Controlled combustion

a) When a mixture of methan and oxygen in the molar ratio of 9:1 is compresed to abut 1100 atmospheres and passed through copper tubes at 575 K, methane is oxidized to methanol - Output is methanol

b) When methane and oxygen are passed through heated molybdenum oxide (Mo2O3), it is oxidized to methanal (formaldehyde).

Alkanes having teriary hydrogen atom can be oxidized to alcohols in the presence of potassium permangante.

Alkanes are oxidized to carboxylic acids by silver oxide (Ag2O)

IIT JEE Revision Alkanes Halogenation

Halogenation of alkanes

This involves substitution o fhydrogen atom by halogen atom. The order of reactivity is F2>Cl2>Br2(>I2).

The mechanism of chlorination and bromination involves free radicals.

Step 1 (Initiation)

Heat or uv light cause the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and starting the chain process.

Step 2 (Propagation)

(a) A bromine radical abstracts a hydrogen to form HBr and a methyl radical, then
(b) The methyl radical abstracts a bromine atom from another molecule of Br2 to form the methyl bromide product and another bromine radical, which can then itself undergo reaction 2(a) creating a cycle that can repeat.

Step 3 (Termination)
Various reactions between the possible pairs of radicals allow for the formation of ethane, Br2 or the product, methyl bromide. These reactions remove radicals and do not perpetuate the cycle. There will be equilibrium and reaction terminates.

IIT JEE Revision Ch. 22 Alkenes - Core Chapter Points

Syllabus

Preparation, properties and reactions of alkenes:

Preparation of alkenes by elimination reactions;
Physical properties: boiling points, density and dipole moments
Acidity;
Acid catalysed hydration of alkenes(excluding the stereochemistry of addition and elimination);
Reactions of alkenes with KMnO4 and
Reactions of alkenes with ozone;
Reduction of alkenes;
Electrophilic addition reactions of alkenes with X2, HX, HOX and H2O (X=halogen);
---------------

Introduction

Alkenes are unsaturated hydrocarbons having carbon-carbon double bond(C=C) in their molecules.

Their general formula is C-nH-2n.
The simplest alkene is ethene, C-2H-4

Methods of Preparation

1. Dehydrohalogenation of alkyl halides.

2. Dehydration of alcohols

3. Dehalogenation of vicinal dihalides

Physical properties

State: Ethene, propene and butene are gases at room temperature. From pentene onwards till alkenes having 18 carbon atoms, they are liquids. Still higher members of the family are solids.



Addition of water

water adds to alkenes in the presence of mineral acids. Hence it is termed catalytic hydration of alkenes. Addition occurs in accordance with Markownikov's rule. We get alcohols from this addition.

Oxidation with potassium permanganate (specially mentioned in syllabus)
Alkenes react with cold dilute potassium permanganate solution(alkaline) to form 1,2-diols called glycols. The glycols contain two -OH groups on adjacent carbon atoms.

Reaction with ozone

Ozone, O3, is an allotrope of oxygen that adds rapidly to carbon-carbon double bonds. Since the overall change in ozonolysis is more complex than a simple addition reaction, its mechanism has been extensively studied. Reactive intermediates called ozonides have been isolated from the interaction of ozone with alkenes, and these unstable compounds may be converted to stable products by either a reductive workup (Zn dust in water or alcohol) or an oxidative workup (hydrogen peroxide).


Reduction
Addition of hydrogen to a carbon-carbon double bond is called hydrogenation. The overall effect of such an addition is the reductive removal of the double bond functional group.

Polymerisation
-- polymerisation of ethene
-- polymerisation of vinyl chloride
-- polymerisation of styrene

Addition of hydrogen to a carbon-carbon double bond is called hydrogenation. The overall effect of such an addition is the reductive removal of the double bond functional group.

IITJEE Revision Alkenes Introduction

Hydrocarbons having at least one double bond

Their general formula is CnH2n.

The carbon carbon double bond is made of a σ bond and a π bond.

The bond dissociation enthalpy of double bond is 610 kJ mol-1.

The carbon-carbon bond length in ethene is 134 pm.

Revision Alkenes Nomenclature

Ethene
Propene
Butene
Pentene

The rules used for naming alkenes under IUPAC system are similar to alkanes with some special ones.

i)The suffix to be used is -ene.

ii)The longest continuous chain should include both the carbon atoms of the double bond.

iii) The chain is to be numbered from the end that gives the lower number to the first carbon atom of the double bond.

iv) If there are two or more double bonds, the suffix used is -adiene or -atriene.


Some more examples

Buta-1,3-diene

Revision Alkenes Isomerism

Isomerism in alkenes

1. Structural isomerism: Alkenes show chain isomerism and position isomerism.

a. Chain isomerism: C-4H-8 exists as two chain isomers, n-Butene and Isobutene.
b. Position isomerism: The isomers differ in the position of the double bonds. C-4H-8 can have the double bond as the terminal bond or it can be in the middle. The one with the terminal bond is n-Butene and the one with the double bond in the middle is but-2-ene. Thus butane with the structural formula C-4H-8 has three structural isomers.

2. Geometrical isomerism: Alkenes exhibit geometrical isomerism.

Molecules of the type C2A2B2 are available as geometrical isomers. The two atoms attached to the same carbon atom are different. In this symbols, A and B are different.

The molecule in which similar atons or groups lie on the same side of the double bond is called cis-isomer. Both As are attached to the carbon on one side of the double bond. Simiarly two Bs.


If they are attached on the opposite sides, it is a trans-isomer

Examples
CH3HC=CHCH3 exhibits geometrical isomerism.

As CH3 and H are different groups

Maleic acid and Fumaric acid are geometrical isomers.
HOOCHC=CHCOOH

Cis isomer is maleic acid and trans isomer is fumaric acid.

If they are attached on the opposite sides, it is a trans-isomer

Revision Alkenes - Methods of Preparation

From
alkynes
alkyl halides
dihalogen derivatives (dihalides)
alcohols
potassium salts of dicarboxylic acids

1. Partial reduction of alkynes
Alkynes can be reduced to alkenes using palladium-charcoal catalyst in catalytic hydrogenation (addition of hydrogen). The reduction (addition of hydrogen) can also be done using sodium in liquid ammonia.

2. Dehydrohalogenation of alkyl halides.
From alkyl halides, an atom of hydrogen and an atom of halogen are removed by treating it with alcoholic KOH.

As a hydrogen atom and a halogen atom are removed from the molecule, the reaction is called dehydrohalogenation.

3. Dehalogenation of vicinal dihalides (dihalogen compounds)
Vicinal dihalides are converted to alkenes by heating with zinc dust in ethyl alcohol.


4. Dehydration of alcohols
Alcohols are heated with sulphuric acid or phosphoric acid at about 443 K. A H2O molecule gets removed from the alcohol giving alkene.

The reaction occurs in multiple steps.

5. Kolbe's electrolytic method (alkenes from salts dicarboxylic acids)
The electrolysis of potassium salts of dicarboxylic acids gives alkenes.

Revision Preparation of alkenes by elimination reactions

One of the principal methods for alkene synthesis in the laboratory is the elimination of alkyl halides, alcohols and similar compounds.

When an alkyl halide is used, the reaction is called a dehydrohalogenation.

Alkenes can be synthesized from alcohols via dehydration, in which case water is lost. For example, the dehydration of ethanol produces ethene:
C2H5OH --> C2H4 + H2O

IIT JEE Revision - Alkenes - Physical properties

1. State: Ethene, propene and butene are gases at room temperature.
From pentene onwards till alkenes having 18 carbon atoms, they are liquids.
Still higher members of the family are solids.

2. Melting points: Alkenes have higher melting points than the corresponindg alkanes. Intermolecular forces of attraction in double bond are stronger.

The melting points increase with molecular mass of alkenes.

Among isomers, trans-alkenes have higher melting points than their corresponding cis alkenes.

3. Boiling points: The boiling points increase with increase in carbon atoms. The branched chain alkenes have lower boiling points than the corresponding straight chain alkenes.

Among geometric isomers, cis-alkenes have higher boiling points than the corresponding trans-isomers.

4. Dipole moments: Alkenes are weakly polar. Their dipole moments are higher than those of alkanes.

In case of geometricl isomers, symmetrical trans alkenes are nonpolar and have zero dipole moment due to symmetry. But unsymmetrical trans alkenes are polar. Cis isomers are polar and have dipole moments.

Unsymmetrical terminal alkenes such as propene and but-1-ene have some dipole moment.

5. Solubility: Alkenes are lighter than water. These are insoluble in water. They readily dissolve in organgic solvents like alcohol, benzenes, ether, carbon tetrachloride etc.

Alekenes Chemical Properties

Alkenes undergo electrophilic addition reactions.
They also participate in oxidation, polymerization and some replacement reactions.

Addition

Halogen acids
Water
Hypohalous acid
Sulphuric acid

Oxidation reactions

Combustion
Alkens burn in oxygen or air to give carbon dioxide , water and large amount of heat.

Hydroboration oxidation
Potassium permanganate - cold and hot
Catalytic oxidation
Reaction with Ozone

Reduction

Polymerisation
of ethene
of vinyl chloride
of styrene
of tetrafluoroethylene


Addition of sulphuric acid

Cold and concentrated sulphuric acid adds to alkenes forming alkyl hydrogen sulphate.

This adds to water to give alcohol. Actually the final reaction is same as acid catalysied hydration of alkenes.

IIT JEE Revision - Acidity of alkenes

Alkynes are weakly acidic.

Compared to alkynes, alkenes are still weak in acidic behaviour. Practically they do not shown any acidic behaviour.

The acidic property is because of s character.

Alkynes are sp hybridized. (50% s character)
Alkenes are sp² hybridized (33% s character)
Because of lesser s character, these carbon atoms are much less electronegative than the carbon atoms taking part in the triple bond.

Therefore, the release of H+ ion from an alkene molecule is difficult and they do not show acidic character.

In term of Ka

Alkynes have 10^-26 and

alkenes have 10^-36.

Revision Acid catalysed hydration of alkenes

water adds to alkenes in the presence of mineral acids. Hence it is termed catalytic hydration of alkenes. Addition occurs in accordance with Markownikov's rule. We get alcohols from this addition.

Ethene gives ethanol
Propene gives Propan-2-ol as the major product.

IIT JEE Revision - Reactions of alkenes with KMnO4

Oxidation of Alkenes with potassium permanganate

Alkenes react with cold dilute potassium permanganate solution(alkaline) to form 1,2-diols called glycols. The glycols contain two -OH groups on adjacent carbon atoms.

Ethene gives ethanediol (glycol).
Propene gives propane-1,2 diol (propylene glycol)

The alkaline potassium permanganate (known as Baeyer's reagent) has bright pink colour. Glycols have no colour and the pink colour will disappear after reaction. So Baeyer's test is used to find the presence of double bond.

Reaction with hot potassium permanganate: In this case, the alkene gets split up at the double bond forming acids or ketones.

=C(R-R') carbon attached to two alkyl groups gets oxidized to ketone
=C(R-H) carbon attached to one alkyl group and one hydrogen gets oxidized to carboxylic acid
=C(H-H) carbon attached to two hydrogen atoms gets oxidized to CO2

IIT JEE Revisin - Reactions of Alkenes with Ozone

Ozone, O3, is an allotrope of oxygen that adds rapidly to carbon-carbon double bonds.

Reactive intermediates called ozonides form from the interaction of ozone with alkenes.

These unstable compounds may be converted to stable products by either a reductive workup (Zn dust in water or alcohol) or an oxidative workup (hydrogen peroxide).

Reductive workup gives an aldehyde product when hydrogen is present on a double bond carbon atom, whereas oxidative workup gives a carboxylic acid or carbon dioxide in such cases.

Ozonide formation: a process that is believed to involve initial syn-addition of ozone, followed by rearrangement of the extremely unstable molozonide addition product.

In the reaction with ozone, carbon-carbon double bonds breaks and carbon-oxygen double bonds form in the two separate compounds.

The double bond of the original molecule is obtained by joining the carbon atoms of the two carbonyl compounds which are the final products of this reaction.

Hence ozonolysis helps in locating the double bond in the alkene.

IIT JEE Revision - Reduction of Alkenes

Alkenes react readily with hydrogen in the presence of finely divided nickel, platinum or palladium to give alkanes.

This reaction is used in the manufacture of vanaspati ghee from vegetable oils.

Ethene - Ethane
Propene - Propane

Sunday, January 27, 2008

IIT JEE Revision Ch.23 Alkynes - Core Chapter Points

syllabus

Preparation, properties and reactions of alkynes:

Physical properties of alkynes (boiling points, density and dipole moments);
Acidity of alkynes;
Acid catalysed hydration of alkynes (excluding the stereochemistry of addition and elimination);

Reduction of alkynes;
Preparation of alkynes by elimination reactions;
Addition reactions of alkynes;
Metal acetylides.
-----------------

1. Alkynes are hydrocarbons with triple bonds. General formula CnH2n

2. Methods of Preparation of Alkynes;

1. Dehydrohalogenation of vicinal dihalides
2. Reaction of metal acetalides with primary alkyl halides. This method can be used to generate large alkyne from the smaller one.

3. Physical properties

State; first three members are gases at room temperature. thenext eight are liquids while the higher ones are solids.

Solubulity: are mostly insoluble in water nbut are soluble in organic solvents such as petroleum, ether, carbon tetrachlorde. benzene etc.

4. Chemical Properties

The alkynes have at least one triple bond in them, therefore, they are quite reactive chemically.

They readily take part in addition reactions and can also be easily oxidized.

5. Acidic property of acetylene

Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.


6. Addition of water (hydration of alkynes) (
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.

Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).

7. Reduction of Alkynes

Reaction Type: Addition


Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
This reaction is stereospecific giving only the trans-alkene via an anti addition.

8. Preparation of alkynes by elimination reactions; To be posted

9. Formation of metal acetylides

Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.

Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.

IIT JEE Revision - Alkynes - Introduction

Alkynes are hydrocarbons having one or more triple bonds.

They are unsaturated.

General formula CnH2n-2

Simplest member of the class

Ethyne C2H2

Other members
Propyne
Butyne
Pentyne

IIT JEE Revision - Alkynes - Nomenclature

Trivial names
Common system

IUPAC System

Acetylene - Ethyne

IIT JEE Revision Alkynes Isomerism

Alkynes exhibit and chain isomerism and position isomerism

Chain isomerism
The arrangement of the chain is different in these isomers

Ex: Pent-1-yne Linear chain pentyne with triple bond at the terminal carbon.
3-Methylbut-1-yne - branched isomer having a methyl group at the 3rd carbon and triple at the terminal carbon.

Position isomerism - position of triple bond changes. Alkynes having more than four carbon atoms exhibit this isomerism.

1-Alkyne 2 Alkyne

Alkynes do not have geometrical isomerism as the bond angle is 180°

IIt JEE Revision - Alkynes - Preparation

1. Synthesis of carbon and hydrogen
A stream of hydrogen is passed through electric arc struck between carbon electrodes at 3270 K. Ethyne (acytylene) is obtained.

2. From acetylene (ethyne)
Higher alkynes are prepared from ethyne (acytylene) by treating its sodium salt with alkyl halide. Sodium salt of ethyne is prepared from the reaction of ethyne and sodamide (NaNH2).
Sodium salt obtained from ethyne and sodamide is Sodium acetylide (HC≡CNa).
If methyl bromide is added to it, propyne is obtained.
If ethyl bromde is added to it, But-1-yne is obtained.

As higher and higher alkyl bromide are added higher alkynes are obtained in this process.

3. Action of zinc on tetrahalogen derivatives of alkanes
On treatment with zinc, tetrahalides get dehalogated (eliminated from the molecule) and triple bonds forms and alkyne is obtained
1,1,2,2-tetrabromoethane + zinc --> ethyne +zinc bromide

4. Dehydrohalogenation of vicinal dihalides
Vicinal dihalides (having halogen atoms on the adjacent carbon atoms) get dehalogenated on treatment with alcoholic solution of potassium hydroxide.

5. By electrolysis of aqueous solution of potassium salt of fumaric acid
Fumaric acid is a dicarboxylic acid.
Electrolysis of acqueous solution Potassium fumerate gives ethyne

6. Action of water on calcium carbide

CaC2 + 2H2O gives ethyne and Calcium hydroxide

Calcium carbide required is obtained by heating calcium oxide (from limestone) and coke in an electric furnace at 2275 K.


--------------------







1. Dehydrohalogenation of vicinal dihalides


The process takes place in two stages.

In stage I KOH(alc) reacts with vicinal dihalide and vinylic halide is formed.


second stage: Vinyl halide is unreactive and hence vigorous conditions are required and a strong base NaNH2 (sodamide) is used in the reaction to produce the corresponding alkyne.

2. Reaction of metal acetalides with primary alkyl halides. This method can be used to generate large alkyne from the smaller one.

Secondary and tertiary halides cannot be used because eliminatin is the predominant reaction which results in the formation of alkenes.

3. Hydrolysis of CaC two and Mg two C three.

Calcium carbide in reaction with water gives ethyne and calcium hydroxide.(came in X class)
Magnesium carbide in reaction water gives propyne and magnesium hydroxide.

4. Kolbe's Electrolytic Method:
The electrolysis of an acqueous solution of potassium salt of an unsaturated dicarboxylic acid forms alkyne





5. Dehalogenation of vic-tetrahalogen compounds

Tetrahalogenated alkane in reaction with zinc with ethyl alcohol as catalyst gives alkyne and zinc halide.

Preparation of Acetylene or Ethyne

1. By the action of Alcoholic Potash on Ethylene Bromide

BrH two C - CH two Br + 2KOH = C Two H Two + @KBr + 2 H two O

At the end of first state vinyl bromide is formed.

2. By heating tetra-bromoethane with Zinc

C-two H-two Br-four +2Zn = C-two H-two + 2ZnBr-two

3. By the electrolysis of Acqueous solution of Potassium salts of maleic acid (Kolbe's method)

4. By the action of water on calcium carbide (already covered earlier)

5. By heating Iodoform with silver powder

2HCI-three + 6Ag = C-twoH-two + 6AgI

6. By partial oxidation of Methane

2CH-four + 3[O] = C-twoHtwo + 3H-two O

7. By direct synthesis of carbon and hydrogen (Berthelot's synthesis)

2C + H-two = C-twoH-two

------------------------------------------------------------------



1. Action of water on calcium carbide
2. Dehydrohalogenation of vicinal dihalides
3. Action of zinc on tetrahalogen derivatives of alkanes
4. From acetylene
5. By electrolysis of aqueous solution of potassium salt of fumaric acid
6. Synthesis of carbon and hydrogen

Industrial preparation
Ethyne is prepared on an industrial scale by treating calcium carbide with water.

IIT JEE REvision - Preparation of Alkynes by Elimination Reactions

Partial oxidation of Methane

2CH-4 + 3[O] = C2H2 + 3H2O

Methane on thermal decomposition by pyrolysis gives Ethyne + Hydrogen at around 1500 degree Celsius in an electric arc.

IIT JEE Revision - Alkynes - Physical Properties

State:
First three members are gases at room temperature.
The next eight are liquids while the higher ones are solids.

Solubulity: are mostly insoluble in water but are soluble in organic solvents such as petroleum, ether, carbon tetrachlorde. benzene etc.

Melting point
Slightly higher compared to alkanes and alkenes. This is because of linear structure which allows more close packing. The magnitude of attractive forces are higher among them.

Melting points increase with increase in molecular mass among alkynes.

B.P.
The behaviour is similar to melting point.

IIT JEE Revision - Alkynes - Chemical Properties and Reactions

Reactions specifically mentioned in the syllabus are posted in more detail as separate topics.

Oxidation

a) Combustion

b) Oxidation with alkaline potassium permanganate

c) Ozonolysis

Alkynes react with ozone to form ozonides.

These ozonides on decomposition with water in the presence of zinc give diketones(two carbonyl groups).

Ethyne gives glyoxal on reacting with ozone. Glyoxal also has two carbonyl groups.

III. Polymerization

a) Acytylene dimerizes (two molecules come together as one molecule) in the presence of cuprous chloride and ammonium chloride to give vinyl acetylene which on reacting with HCl gives chloroprene.

The later polymerizes to give neoprene - a synthetic rubber.

b) Cyclic polymerization

This takes place when alkyne is passed through red hot iron tube at 400 degress celsius.

Occurs in two stages. Acetylene becomes mesitylene.

IV. Isomerization

1-alkyne gets converted to 2-alkyne and vice versa.

IIT JEE Revision - Acidity of Alkynes

Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.

They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.

Acetylides react with alkyl halides to give higher alkynes.

Explanation for the acidic character of alkynes

The acidic character of 1-alkynes can be explained on the basis of sp hybridisation state of the carbon atoms in alkynes.

In sp hybridisation, s-character is 50% and due to this large s-character, the electons in sp hybrid orbitals are held more tightly by the nucleus and are quite electronegative.

Consequently the eletron pair of H-C≡C bond gets displaced more towards the carbon atom and helps in release of H+ ion.

In the case of nonterminal alkynes, no hydrogen atom is attached directly to the triple bonded carbon atom (triple bond to carbon and one single bond to another carbon) and hence hydrogen atoms are not released easily.

Alkynes are weakly acidic but alkenes and alkanes do not show acidic behaviour.

acidic charater
HC≡HC > CH2=CH2 > CH3-CH3

The relative acidity of acetylene is more than that of ammonia but less than that of water.

IIT JEE Revision - Acid Catalysed Hydration of Alkynes

In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K.

The catalyst in this reaction is HgSO4 (Mercuric sulphate).

The final products of this reaction are carbonyl compounds aldehydes and ketones.

Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess.

Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).

Example:
Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.

In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.

The conversion of enol form into keto form is termed tautomerism

IIT JEE REvision - Reduction of Alkynes

Reaction Type: Addition

Summary

Alkynes can be reduced to trans-alkenes using Na in NH3 (l)

This reaction is stereospecific giving only the trans-alkene via an anti addition.

The stereochemistry of this reaction complements that of catalytic hydrogenation (syn)

The reaction proceeds via single electron transfer from the Na with H coming from the NH3

These reaction conditions do not reduce alkenes, hence the product is the alkene.

MECHANISM FOR THE REDUCTION OF ALKYNES WITH Na / NH3

Step 1:
Sodium transfers an electron to the alkyne giving a radical anion.

Step 2:
The radical anion removes a proton from the ammonia in an acid / base reaction.

Step 3:
A second atom of sodium transfers another electron to the alkyne giving an anion.

Step 4:
The anion removes a proton from the ammonia in an acid / base reaction.

IIT JEE Revision Addition reactions of alkynes

The alkynes have at least one triple bond in them, therefore, they are quite reactive chemically.

They readily take part in addition reactions and can also be easily oxidized.

I. Addition reactions (specially given in jee syllabus)

1. Addition of Hydrogen

If the triple bond is not present at the end of the chain of the molecule (it is not a terminal alkyne), its reduction (addition of hydrogen) produces either a cis alkene or a trans alkene depending upon the choice of reducing agent.

2. Addition of halogens

chlorine and bromine add on alkali

3. Addition of hydrogen halides

This addition takes place in accordance with Markonikov's rule(do you remember the rule?).
Peroxides have the same effect on addition of the HBr to acetylenes (alkynes) as they have on alkenes (do you remember the effect?).

4. Addition of water (hydration of alkynes) (specially given in jee syllabus)
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.

Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).

Examples:
Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.

In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.

The conversion of enol form into keto form is termed tautomerism



5. Addition of hypohalous acid (HOX)

Alkynes react with two molecules of hypohalous acids in two stages.
For example take ethyne or acytelene and HOCl.
In the first stage HO gets added to one carbon and Cl adds to the other carbon.
In the second stage one more HOCl gets added to the intermediate product which has a double bond. The addition now follows markonikov's rule. OH gets added to HC-OH and Cl gets added to CH-Cl. Two OHs create instability and H2O molecule gets removed.
An aldehyde 2,2-Dichloroethanal (Dichloroacetaldehyde) is formed.

6. Addition of H2SO4
Acetylene adds two molecules of concentrated H2SO4 in two stages and forms ethylidene hydrogen sulphate as the final product.

In the first stage Vinylhydrogen sulphate H2C=CH-OSO3H is formed. (H gets added to one CH and OSO3H gets added to the other CH).
The addition of second molecule follows markownikov's rule. H gets added to CH2 and OSO3 gets added to the other carbon. Thus two functional groups OSO3H gets added to one carbon.
The final product is Ethylidine hydrogen sulphate.

IIT JEE Revision - Metal Acetylides

Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.

They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.

Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.

Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.

Saturday, January 26, 2008

IIT JEE Revision - Ch. 24 Benzene - Main chapter points

syllabus

Structure

Aromaticity

Electrophile Substitution Reactions
---Halogenation
---Nitration
--- Sulphonation
--- Friedel-Crafts Alkylation
--- Friedel-Crafts Acylation

Effect of --, m- and p- directing groups in mono-substituted benzenes
---------

1. Benzene has the molecular formula C6H6. It has hexagonal ring of six carbon atoms with three double bonds in alternate positions.

Arenes are the aromatic hydrocarbons which contain one or more hexagonal rings of carbon atoms with double bonds in alternate positions.


2. Preparation of benzene and its homologues

1. From alkynes: acetylene and other alkynes polymerise at high temperatures to give benzene and other arenes.

3C2H2 gives C6H6

Benzene was first synthesized by Berthelot by passing acetylene through red hot iron tube.

2 Decarboxylation of aromatic acids: by heating sodium benzoate with soda lime

Decarboxylation: Removal carboxyl group

3. From phenol: by distillation of phenol with zinc.


3. Physical properties

i) colour less liquids up to eight carbon atoms
ii) aromatic hydrocarbons are insoluble in water ut soluble in organic solvents.
iii) They are inflammable and burn with sooty flame

4. Chemical properties

Even though double bonds are present, benzene is quite stable and does not undergo common addition reactions undergone by alkenes.

Benzene and other arenes undergo following types of reactions.

1. substitution
2. addition
3. oxidation

5. Halogenation

benzene will react with a mixture of Cl-2 and FeCl-3.


The output is a combination of benzene with Cl, Cl diplacing one hydrogen atom from benzene(Chlorobenzene).

6. Nitration
A mixture of nitric acid and sulphuric acid is the nitrating agent.

7. Sulphonation

The product is a combination Benzene and SO-3H that displaced one hydrogen atom from benzene.

For sulphonation we require excess of H-2SO-4 along with SO-3.


8. Friedel-Crafts Alkylation

Benzene reacts with a combination of alkyl halide and AlCl-3. AlCl-3 acts as a Lewis acid.

The alkyl group replaces one hydrogen atom in benzene.

9. Friedel-Crafts Acylation

Acylation is the term given to substituting an acyl group such as CH-3CO- into another molecule. An acyl group is a hydrocarbon group attached to a carbon-oxygen double bond.

The most commonly used example of an acyl group is the ethanoyl group, CH3CO-.

10. Effect of o-, m- and p- directing groups in mono-substituted benzenes

In planning syntheses based on substitution reactions of mono-substituted benzenes, you must be able to predict in advance which of the available positions of the ring are most likely to be substituted.

Basically, three problems are involved in the substitution reactions of aromatic compounds: (a) proof of the structures of the possible isomers, o, m, p, that are formed; (b) the percentage of each isomer formed, if the product is a mixture; and (c) the reactivity of the compound being substituted relative to some standard substance, usually benzene.

IIT JEE Revision Benzene Introduction

Molecular formula C6H6

Gen formula CnH2n-6

Bicyclic arenes CnH2n-12

If m rings are present CnH2n-6m

IIT JEE Revision Benzene Nomenclature

Toluene

Ethyl benzene

0-xylene

m-xylene

p-xylene

Mesitylene

IIt JEE Revision Structure of Benzene

Resonance


Resonance energy

Orbital Struncture

IIT JEE Revision Arenes Isomerism

Arenes exhibit position isomerism o, m, and p

IIt JEE Revision Aromaticity Huckel Rule

Aromatic compounds are those which resemble benzene in chemical behaviour.
They contain alternate single and double bonds in a cyclic structure.
They undergo substitution reactions rather than addition reactions

Criteria for Aromaticity- Contribution by Huckel

1. Delocalisation: The molecule should contain a cyclic cloud of delocalized Pi electrons above and below the plane of the molecule.

2. Planarity:For a molecule to be aromatic, the ring must be planar.

3. Huckel Rule: the pi electron cloud must contain a total of 4n+2 pi electrons whre n is an integer equal to 0,1,2,3.

Benzene has 6 pi electrons, napthalene has 10.

IIT JEE Revision Benzene Preparation

Preparation of benzene and other aromatic compounds or arenes


1. From alkynes: acetylene and other alkynes polymerise at high temperatures to give benzene and other arenes.

3C2H2 gives C6H6

Benzene was first synthesized by Berthelot by passing acetylene through red hot iron tube.

2. From aryl halides:
Benzene is obtained from chlorobenzene by reducing it with Ni-Al alloy in the presence of sodium hydroxide

Arenes are obtained by reaction fo aryl halide, sodium metal and alkyl halide in dry ether.

Bromobenzene + sodium + Ethylbromide give Ethylbenzene and Sodiumbromide

3. Arenes from Benzene and alkyl halides
Arenes can also be obtained from benzene and alkyl halides in the presence of anhydrous aluminium chloride. This is called Friedel Craft's reaction.

4.From Grignard reagent (Phenyl magnesium halide)
Arenes are also prepared by reacting aromatic Grignard reagent and alkyl halide.

5. From phenol: by distillation of phenol with zinc.

6 Decarboxylation of aromatic acids: by heating sodium benzoate with soda lime. Decarboxylation: Removal carboxyl group

7. From diazonium salts: Benzene diazonium is reduced by hypophosphorus acid.

IIT JEE Revision Benzene Physical Properties

is colourless, volatile liquid with burning state and characteristic smell.

Melting point is 278.5 K and boiling point is 353 K (80 C)

Insoluble in water but soluble in organic solvents.
Highly Inflammable and burns with sooty flame

It is lighter than water and its specific gravity is 0.878

Benzene itself is a good solvent for fats, oil, resin etc.

Benzene is toxic in nature

IIT JEE Revision - Benzene Chemical Properties and Reactions

Even though double bonds are present, benzene is quite stable and does not undergo common addition reactions undergone by alkenes.

Benzene and other arenes undergo following types of reactions.

1. substitution
2. addition
3. oxidation

Substitution reactions are covered in different posts

Addition reactions

1. hydrogen
2. halogens
3. ozone

Oxidation

combustion
Oxidation of benzene
Oxidation of alkyl side chain

IIT JEE Revision Benzene - Halogenation

Halogenation

Benzene will react with a mixture of Cl-2 and FeCl-3 (catalyst).


The output is a combination of benzene with Cl, Cl displacing one hydrogen atom from benzene(Chlorobenzene).

AlCl3 can also be used as catalyst.

Bromine also combines with benzene with AlCl3 as catalyst and forms bromobenzene.
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At a slightly elevated temperature, chlorination of benzene in presence of a catalyst gives rise to a mixture of orthochlorobenzene and para chlorobenzene.

On prolonged chlorination, benzene forms hexachlorobenzene.

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Reaction mechanism

Reaction is carried out in the presence of ferric salts i.e, FeCl3 in case of chlorine and FeBr3 in case of bromine.

The metal catalyst is a Lewis acid (Fe is electron deficient) and hence polarises the halogen molecule.

Cl2 + FeCl3 --> FeCl4ˉ + Cl+

Cl+ attacks the benzene molecule.

It has been suggested by some that free chloronium ion (Cl+) may not have actual existence. The electrophile is supposed to be made available by a complex between FeCl3 and Cl2

The electrophile attack creates a carbocation.

The carbocation loses a proton(H+) to FeCl4ˉ and the formation of aryl halide takes place.

IIT JEE Revision - Benzene Nitration

6. Nitration
A mixture of nitric acid and sulphuric acid is the nitrating agent.

Heating benzene with the nitrating mixture consisting of concentrated nitric acid and sulphuric acid to about 330 K.

Product is nitrobenzene

Benzene + HNO3---> Nitrobenzene + H2O (H2SO4 and 330 K)

IIT JEE Revision Benzene Sulphonation

The product is a combination Benzene and SO-3H that displaced one hydrogen atom from benzene.

For sulphonation we require excess of conc. H-2SO-4 along with SO-3(Fuming sulphuric acid or oleum)

Sulphonation can also be carried by treating benzene with chlorosulphonic acid(ClSO3H). The product is benzene sulphonic acid

IIT JEE Revision Friedel-Crafts Alkylation

Benzene reacts with a combination of alkyl halide and AlCl-3. AlCl-3 acts as a Lewis acid.

The alkyl group replaces one hydrogen atom in benzene.


Benzene + Methyl chloride --> Toluene + HCl

Toluene = Benzene with CH3 substitution

IIT JEE Revision Friedel-Crafts Acylation

Acylation is the term given to substituting an acyl group such as CH-3CO- into another molecule. An acyl group is a hydrocarbon group attached to a carbon-oxygen double bond.

The most commonly used example of an acyl group is the ethanoyl group, CH3CO-.

On treatment with an acid chloride (Acyl chloride) in the presence of anyhydrous aluminium chloride, acylation occurs.

Benzene + Acetyl chloride --> Acetophenone + HCl


Mechanism

The attacking electrophile an acyl carbocation RCO is supplied by acid chloride RCOCl in the presence of anhydrous AlCl3

IIT JEE Revision Benzene

Effect of o-, m- and p- directing groups in mono-substituted benzenes

In planning syntheses based on substitution reactions of mono-substituted benzenes, you must be able to predict in advance which of the available positions of the ring are most likely to be substituted.

Basically, three problems are involved in the substitution reactions of aromatic compounds: (a) proof of the structures of the possible isomers, o, m, p, that are formed; (b) the percentage of each isomer formed, if the product is a mixture; and (c) the reactivity of the compound being substituted relative to some standard substance, usually benzene.

the Pattern of Orientation in Aromatic Substitution

The reaction most studied in connection with the orientation problem is nitration, but the principles established also apply for the msot part ot the related reactions of halogenation, sulfonation, alkylation and acylation.

The group present on the benzene ring affect the incoming attacking groups.

Two effects: orientation, reactivity

Orientation effect: The three possible disubstituted products -ortho, meta and para are not formed in equal amounts.

some substituents groups are ortho directors and some others or meta directors or para directors

Reactivity:some activate the ring and make it more reactive. Some deactivate and make it (benzene) less reactive.

Three groups are classified

1. Ortho and para directing activators: Groups release electrons and activate the benzene ring

-OH, -OCH3, NH2, -NHCOCH3, -CH3

2. Meta directing deactivating groups: They withdraw electrons from the benzene ring and deactivate it

-NO2, -CN, -CHO

3. Ortho and para directing deactivating groups

Halogens -F, Cl, Br, I

IIT JEE Revision - Ch 30 Amines - Core Points

JEE Syllabus

Amines:
Preparation, Properties, Reactions
Characteristic reactions
Basicity of substituted anilines and aliphatic amines,
Preparation from nitro compounds,
Reaction with nitrous acid,
Azo coupling reaction of diazonium salts of aromatic amines,
Sandmeyer and related reactions of diazonium salts;
Carbylamine reaction;


Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by alkyl or aryl group.

Preparation from nitro compounds,

Reduction of nitro compound to obtain amine can be done by using either molecular hydrogen and a catalyst (Ni or Pt) or a metal (usually granulated tin) and an acid (HCl)

Reaction with nitrous acid,
Aliphatic primary amine in reactin with nitrous acid forms unstable diazonium salt which on decomposing liberates nitrogen and mixture of alcohols and alkenes.

Basicity of substituted anilines and aliphatic amines,
Nitrogen of amines contains lone pair of electrons, which can be shared with other species and thus these act as Lewis bases.

Azo coupling reaction of diazonium salts of aromatic amines,

Sandmeyer and related reactions of diazonium salts;
The diazonium salt is treated with cuprous chloride or cuprous bromide.

Carbylamine reaction;
The treatment of a primary amine with chloroform and alcoholic potash produces carbylamine (isocyanide) which has most offensive smell. This reaction is not exhibited by secondary and tertiary amines.

IIT JEE Revision Amines - Introduction - Classification

Amines are derivatives of ammonia

Primary

Secondary

Tertiary

Aliphatic

Aromatic

Simple and Mixed

IIT JEE Revision - Amines Nomenclature

Nomenclature of aliphatic amines
Common system:
In the common system, amines are called alkylamines. The suffix amines is added to the name of the corresponding alkyl group.

Ex: Methylamine, Ethylamine, Propylamine
Second common system
In the second system of the common system, primary amines are named as the amino derivatives of the corresponding hydrocarbons and are names as aminoalkanes. The position of the amino group is indicated by Arabic numeral. The numbering of the primary chain is to be done in such a way that the carbon atom containing the amino group gets the lowest possible number.

Secondary and tertiary amines are named as nitrogen substituted primary amine.
Example: N-Methylaminoethane
Primary amine is aminoethane and N-Methyl is added to it. N- indicates that the methyl group is attached to the nitrogen atom.

IUPAC system

The aliphatic amines are called alkamines. The letter ‘e’ in the alkane is replaced by suffix amine.
Ex: methanamine, ethanamine

The position of the amino group is indicated by Arabic numeral. The numbering of the primary chain is to be done in such a way that the carbon atom containing the amino group gets the lowest possible number.
Ex: 1-propanamine, 2-propanamine

Aromatic Amines

Common system
Aromatic amines are called aryl amines. Suffix amine is added to the aryl group.

IUPAC System
The simplest aromatic amine C6H5NH2 is called benzanamine.
Other aromatic amines are named as derivatives of benzenamine and positions of other groups are indicated by numbers

Ex: Benzenamine
N-Methylbenzenamine, N,N-Dimethylbenzenamine, 2-Methylbenzenamine, 3-Methylbenzenamine

In IUPAC system, benzenamine may also be written as amino benzene.

IIT JEE Revision - Amines - Isomerism

1. Chain
Aliphatic amines containing four or more carbon atoms show chain isomerism as branched and straight chain alkyl groups can be attached to the nitrogen atom.

Example: Butan-1-amine, 2-Methylpropan-1-amine, and 2 Methylpropan-2-amine are chain isomers.

2. Position Isomerism
The amino group can be bonded to different carbon atoms of the alkyl group.

Example: Propan-1-amine and Propan-2-amine

3. Functional

Primary, secondary and tertiary amines having the same molecular formula are isomers. This isomerism is functional isomerism because different alkyl groups (functional groups) are present in the molecules and give rise to isomerism
4. Metamerism
In amines metamerism, a type of isomerism in which different alkyl groups are attached to the Nitrogen atom of the amino group exists(molecular formula for isomers is same).
Example: Diethylamine, Methyl-n-propylamine, and Isopropylmethylamine are metamers.

5. Chiral amines

Nitrogen atom undergoes sp3 hybridisation. Hence secondary and tertiary amines have a chiral nitrogen atom. But chiral amines cannot be resolved. This is because the two enantiomeric forms rapidly interconvert and it is very difficult to isolate a pure sample of either enantiomer.

IIT JEE Revision Preparation of Amines

1. Ammonolysis
Hoffmann’s method: When an aqueous or alcoholic solution of ammonia is heated with an alkyl halide at 373 K in a sealed tube, a mixture of three amines (primary, secondary and tertiary) is obtained. It is very difficult to separate the mixture.

2. Gabriel phthalimide synthesis
In this synthesis, phthalimide is treated with alcoholic KOH to give potassium phthalimide.
Potassium phthalimide is treated with alky halide to form N-alkyl phthalimide.
The hydrolysis of N-alkyl phthalimide with 20% HCl under pressure or refluxing with NaOH gives primary amine.
This method can be used for preparing only primary amines.

3. Reduction of nitriles (cyanides)
Nitriles can be reduced to corresponding amines using H2/Raney Li or Pt, LiAlH4 or Na, C2H5OH

When sodium and alcohol are used, the reaction is called Mendius reaction

4. Reduction of isonitriles (isocyanides)
Isonitriles can be reduced to secondary amines using H2/Raney Li or Pt, LiAlH4 or Na, C2H5OH

5. Reduction of amides (Hoffman degradation method)
Amides on treatment with Br2 and KOH give primary amines. The amine formed contains one carbon atom less than the parent amide.

6. Reduction of oximes
Oximes are obtained from aldehydes and ketones by reaction with hydroxylamine. The oximes of aldehydes or ketones can be reduced to primary amines with either Na/CH2H5OH or LiAlH4.

7. Reductive amination of aldehydes and ketones
Reaction between aldehydes or ketones and ammonia results in the formation of imines. Imines are reduced to primary amines with H2, Ni.

IIT JEE Revision Amines - Preparation from nitro compounds

By reduction of nitro compounds

a. by hydrogen in the presence of Raney Ni, Pt or Pd as catalyst at room temp.

b. with active metals such as Fe, Sn, Zn, and conc. HCl acid

c. with lithium aluminium hydride - LiAlH4

IIT JEE Revision Amines - Physical Properties

1. Lower members of the family such as methylamine, dimethylamine, ethylamine are gases at ordinary temperatures and smell like ammonia.

The higher members are mostly liquids haivng fishy smell.

Amines are polar compounds and with the exception of tertiary amines can form intermolecular hydrogen bonds.

Amines hve higher boiling points than non-polar compounds of the same molecular mass.

Amines have lower boiling points than those of alcohols and carboxylic acids.

Primary amines have the highest boiling points and tertiary the lowest.

The lower amines are soluble in water.

Higher amines containing six or more carbon atoms are insoluble.

The amines are also soluble in less polar solvents like ether, alcohol or benzene etc.

Friday, January 25, 2008

IIT JEE Revision Amines-Chemical Reactions

Amines Chapter

Characteristic Reactions or Chemical Properties

1. Reaction with water (Basic character of amines) (specially mentioned in JEE syllabus - Covered in more detail in a separate post)


2. Reaction with acids
Amines reacts with acids to form salts. The salts are ionic compounds and are soluble in water.

3. Reaction with metal ions
Amines combine with metal ions such as Ag+ and Cu2+ to form complex ions. The lone pair of electrons in -NH2 group is used to form a coordinate bond with metal ions.

For example, silver chloride dissolves in methylamine to form a soluble silver amine complex.

4. Alkylation
primary and secondary amines react with alkyl halides to form tertiary amines

5. Acylation (reaction with acid chlorides and acid anhydrides)
Primary and secondary amines react with acid chloride or acid anhydride to form substituted amides.

6. Benzoylation
Aliphatic and aromatic amines react with benzoyl chloride in the presence of a base such as pyridine or acqueous NaOH to form benzoyl derivatives in which C6H5CO- group is introduced.

7. schiff's base formation
Both aliphatic and aromatic primary amines react with aldehydes to form Schiff's bases or anils.

8. Oxidation
Primary aliphatic amines on oxidation with potassium permanganate followed by hydrolysis give aldehydes and ketones.

Secondry amines react with Caro's acid to give corresponding N-hydroxy amine.

Tertiary amines are oxidized by Caro's acid, ozone or H2O2 to corresponding N-oxides.

Aninline can be readily oxidized in the presence of K2Cr2O7 and H2SO4, and gives p-benzoquinone.

9. Carbalamine reaction (specially mentioned in JEE syllabus - covered separately)

10. Reaction with nitrous acid (specially mentioned in JEE syllabus - covered separately)

11. Reaction with Grignard reagent
Primary and secondary amines react with Grignard reagents to form alkanes.

Tertiary aliphatic amines do not react with Grignard reagent because they do not have hydrogen atom attached to the nitrogen atom.

12. Carbon disulphide
Primary amines react with carbon disulphide to form dithio-alkyl carbamic acids which decompose on heating with mercuric chloride (HgCl2) to give alkyl isothio cyanates. (Hoffman must oil reaction - a test for primary amines).

13. Carbonyl chloride
Primary and secondary aliphatic amines react with carbonyl chloride to form substituted ureas.

14. Ring substitution in aromatic amines
Aromatic amines give the aromatic substitution reactions as given by benzene.

15. coupling of diazonium salts (specially mentioned in JEE syllabus - covered separately)

16. Sandmeyer and related reactions (specially mentioned in JEE syllabus - covered separately)

IIT JEE Revision Basicity of Amines

Chapter Amines

Basicity of substituted anilines and aliphatic amines

Nitrogen has a lone pair in Ammonia are sharing an electron with each of the hydrogen atoms.

Due to the presence of lone pair on nitrogen atom of the -NH2 group, the aliphatic amines are generally basic in nature.


They have a tendency to donate this lone pair of electrons to different electron deficient compounds and, therefore, behave as bases.

The amines react with water to from hydroxides which inonise to furnish hydroxyl (OH-) ions.

The basic strength is expressed in terms of dissociation constant Kb for the dissociation reaction which is a reversible reaction.

Greater the Kb value, which means greater the dissociation at equilibrium, the stronger the base.

Aliphatic amines are stronger bases than ammonia.

It is observed that secondary amines are the strongest bases, next come the primary amines and the weakest among the three are tertiary amines.

The reasons for the strength order of basicity is explained in terms of steric hindrance and solvation of ions.

Aromatic amines are also bases.

Among aniline (NH2 substituted in benzene) and ethylamine, aniline is a weaker base. This is due to the fact tath among the resonating structures of aniline three of them have positive charge on nitrogen. In these cases the lone pair is not available for protonation (for donating the lone pair to the H+ ion). Hence aniline is less basic than ethylamine.

In substituted amines, electron releasing groups like -OCH3, -CH3 increase the basic strength. Electron withdrawing groups like -X (halogen), -NO2, -CN decrease basic strength.

The effects of substituents is more marked at p-position than a m-position.

Every o-substituted aniline (electron releasing or electron withdrawing) is less basic than aniline due to ortho effect.

Base weakening effect of electron withdrawing group is very large at o-position.

When hydrogen atoms of the amino group of arylamines are replaced by electron donating alkyl groups, the basic character of the resulting arylamine increases.

For example, N-methylaniline is a stronger base than aniline and N,N-dimethylaniline is even stronger.

In contrast, if hydrogen atoms of the amino group of arylamines are replaced by electron withdrawing groups (such as phenyl group-aryl group), the basic character of the resulting arylamine increases.

Aniline is stronger than diphenylamines and this is a stronger base than triphenylamine.

In case of aralkylamines where amine group is attached to an alkyl group attached to the carbon of the benzene ring, the lone pair of nitrogen atom is not delocalized and hence readily available for protonation (accepting the H+ ion). Therefore aralkylamines are more basic than arylamines.

Hence benzylamines is a stronger base than aniline.

Thursday, January 24, 2008

IIT JEE Revision Amines - Reaction with Nitrous Acid

Amines - Reaction with nitrous acid



Nitrous acid (HNO2) is an unstable acid.

It must be freshly made.

Primary amines react rapidly with nitrus acid to form alcohol and evolved nitrogen.

Primary aromatic amines react with nitrous acid in the cold (below 273 K) to form diazonium salts. The process is called diazotisation.

If the temperature is more than 278 K, aromatic amines form phenol with the evolution of N2 gas.

Secondary aliphatic and aromatic amines react with nitrous acid slowly in the cold to form yellow oily nitroso amines.

The yellow only nitrosoamine gives a green solution when warmed with ohenol and conc. H2SO4. On dilution with water and addition sodium hydroxide, the colour changes to greensih blue to violet (Libermann's nitroso reaction. a test for secondary amines)

Tertiary amines dissolve in cold nitrous acid to form salts which decompose on warming to nitrosoamine and alcohol.

Aromatic tertiary amines react with nitrous acid to give coloured sustituted nitroso compound.

IIT JEE Revision - Azo coupling reaction of Diazonium salts

Amines chapter

Azo coupling reaction of diazonium salts of aromatic amines

Revision points

Benzene diazonium chloride couples with electron rich aromatic compounds like phenols and anilines to give azo compounds.

The azo compounds contain -N=N- bond and the reaction is called coupling reaction.

Benzene diazonium chloride + phenol gives p-Hydroxyazobenzene (colour yellow).

Benzene diazonium chloride + aniline gives p-Aminoazobenzene (orange).

Coupling occurs para to hydroxy or amino group.

All azo compounds are strongly coloured and are used as dyes.

Methyl orange is an important dye obtained by coupling the diazonium salt of sulphanilic acid with N,N-dimethylaniline.

IIT Revision Sandmeyer and Related Reactions of Diazonium Salts

The diazonium salts have the general formula ArN2+X-, where X- may be an anion like Cl-, Br- etc.

The group N2+ (-N≡N+) is called diazonium ion group.

Sandmeyer reaction is a substitution reaction.

In the substistution reactions, nitrogen of the salts is lost as N2 and different groups are introduced in its place.

Various substitution reactions of diazonium salts

1. Replacement by -OH group. By boiling or by steam boiling. Phenol is formed

2. Replacement by hydrogen. By treatment with hypophophorous acid. Benzene is obtained.

3. Replacement by Cl and Br group.When the salt is warmed with cuprous chloride or cuprous bromide the corresponding halide is formed. This reaction is called Sandmeyer reaction

IIT Revision Carbylamine Reaction

Amines Chapter

The treatment of a primary amine with chloroform and alcoholic potash produces carbylamine (isocyanide) which has most offensive smell.

This reaction is not exhibited by secondary and tertiary amines.

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