Wednesday, August 15, 2007

TMH-JEE-Chemistry- book-chapters Ch.21 Alkanes Study Guide/Note

JEE Syllabus
Preparation, properties and reactions of alkanes:
Homologous series,
physical properties of alkanes (melting points, boiling points and density); Combustion and halogenation of alkanes;
Preparation of alkanes by Wurtz reaction and decarboxylation reactions.
Main Topics Covered in the TMH Book

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.

Nomenclature of alkanes


4-(1-methyl ethyl) heptane or 4-Isopropylheptane

Straight chain alkanes or normal alkanes (n-alkanes): All the carbon atoms are attached by covalent bonds in a continuous chain.

Branched alkanes:

Iso-alkanes: In these, one carbon chain is attached to the second carbon atom of the long chain (parent chain).

Neo-alkanes: In these two single carbon branches are attached to the second carbon atom of the long chain.

Conformations in alkanes

The C-C bond in alkanes is a sigma bond and has cylindrical symmetry. Due to this, atoms involved can rotate about the axis of the bond without affecting the overlap of orbitals that form the bond. So rotation about C-C bond is quite free. Therefore hydrogen atoms can have different spatial arrangement while maintaing the bond and the bond angle between H-C-H. Such different spatial arrangments are called conformations, conformational isomers (conformers) or rotational isomers (rotamers).

the different arrangements of atoms in a molecule which can be obtained due to rotation about carbon-carbon single bond are called conformations.
To represent conformations Saw horse diagrams and Newman projection diagrams are used.

Conformations of ethane, propane, butanecyclopropane, cyclopentane, cyclohexane are discussed in Jauhar's book.

Preparation of alkanes

General methods

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. From alkyl halides

a) Wurtz reaction (specially in syllabus)
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. The equation of the reaction will make the statement more clear.

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

In the reaction different alkyl halides can also be used in stead 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.

b) Reduction of alkyl halides

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

c) 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. (This reaction will come in alkyl halides chapter also)

Grignard reagetns 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 and their salts
a) Decarboxylation reaction
b) Kolbe's reaction
c) Reduction of carboxylic acid

a) 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.

RCOONa + NaOH -->RH + Na2CO3

In this reaction a CO2 group is removed from carboxylic acid and therefore the reaction is called decarboxylation.
(Note this reaction will be discussed in carboxylic acids chapter)

b) 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.

c) 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.

The methods in this section can be summarised as

R-COONa ---> RH
R-COONa ---> R-R
R-COOH---> R-CH3

Industrial method: Petroleum provides the natural source of alkanes.

Physical properties of alkanes

1. State
2. Boiling point
3. Melting point
4. Solubility
5. Density

Chemical properties or reactions

1. Substitution reactions of alkanes
2. Oxidation
3. Action of steam
4. Isomerisation
5. Aromatization
6. Thermal decomposition or fragmentation

Alkanes – Simple Questions

1. What happens to boiling point of alkanes as molecular mass increases?
2. What happens to boiling point of branched isomers of alkanes?
3. Under what conditions does monochlorination of methane takes place?
4. What is the `reaction in Kolbe Electrolytic method of preparing alkanes?
5. What are the reactants and products in hydrolysis of Grignard reagent?
6. In the case of halogenation of alkanes, what is order of reactivity among chlorine, fluorine, and bromine?
7. At ordinary temperatures, why is ultraviolet radiation required to initiate a reaction between halogens and alkanes?
8. What do you understand by 1 degree H, 2 degree H and 3 degree H?
9. What is the difference in reactions between an alkane and fluorine and an alkane and bromine?
10. What is the major product in reaction between bromine and n-butane?
11. What are the products of reaction between n-butane and chlorine at room temperature and sunlight?
12. How do you get iso-butane from n-butane?
13. When do you get aromatic compounds from alkanes?
14. What is the more common name of Methane?
15. What is % composition of Methane in coal gas?
16. How do you produce methane from aluminium chloride?
17. What are the reactants and products of Wurtz reaction?

Answer these questions. This will help you to answer other complex ways of asking questions like MCQs, multiple matching, passage based questions and fact & cause questions.
web sites
The homologous aliphatic series of ALKANES

No comments: