For some theory questions on alkenes
http://iit-jee-chemistry-ps.blogspot.com/2007/10/iit-jee-chemistry-questions-22-alkenes.html
For some application questions
http://iit-jee-chemistry-ps.blogspot.com/2007/12/application-questions-22-alkenes.html
For pass JEE questions
http://iit-jee-chemistry-ps.blogspot.com/2007/12/past-jee-questions-ch22.html
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Syllabus
Preparation, properties and reactions of alkenes:
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;
Preparation of alkenes by elimination reactions;
Electrophilic addition reactions of alkenes with X2, HX, HOX and H2O (X=halogen);
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Main Topics Covered in the TMH Book
METHODS OF PREPARATION
CHEMICAL PROPERTIES
The material covered is brief. I am trying to include more details.
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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
The C=C bond is made of a σ (sigma) bond and a π (pi) bond.
The carbon-carbon bond length in ethene is 134 pm and it is shorter than the carbon-carbon bond length in ethane which is 154 pm.
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Nomenclature of alkenes
Isomerism of alkenes
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.
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Methods of Preparation
From
alkyl halides
alcohols
dihalogen derivatives (dihalides)
alkynes
potassium salts of dicarboxylic acids
1. Dehydrohalogenation of alkyl halides. (What are alkyl halides see: )
2. Dehydration of alcohols
3. Dehalogenation of vicinal dihalides
4. partial reduction of alkynes (Specially mentioned in syllabus)
5. Kolbe's electrolytic method
Physical properties
State
M.P.
B.P.
Dipole moments
Solubility
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.
Chemical reactions
Addition reactions
-- of halgens
-- halogen acids
-- water
-- hypohalous acids
-- sulphuric acid
Addition of water (Specially mentioned in JEE syllabus)
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.
Addition of hypohalous acid (HOX) - specially mentioned in JEE syllabus
The reaction gives halohydrings. The addition occurs according to Markownikov's rule. It is important to note that halogen becomes the positive part and OH the negative part.
Ethene + HOCL ---> Ethylene chlorohydrin
Ethene + HOBr ---> Ethylene bromohydrin
Addition of sulphuric acid
Coldi 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.
Oxidation reactions
-- combustion
-- hydroboration oxidation
-- oxidation with potassium permanganate
-- oxidation with hot potassium permanganate
-- catalytic oxidation
-- oxidation with ozone
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.
Ethene gives ethanediol (glycol).
Propene gives propane-1,2 diol (propylene glycol)
The alkaline potassium permanganate is known as Baeyer's reagent. It has bright pink colour. Glycols have no colour. So 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
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Acid catalysed hydration of alkenes
Weak Brønsted acids such as water (pKa = 15.7) and acetic acid (pKa = 4.75) do not normally add to alkenes. However, the addition of a strong acid serves to catalyze the addition of water, and in this way alcohols may be prepared from alkenes. For example, if sulfuric acid is dissolved in water it is completely ionized to the hydronium ion, H3O(+), and this strongly acidic (pKa = -1.74) species effects hydration of ethene and other alkenes.
CH2=CH2 + H3O(+) ——> HCH2–CH2OH + H(+)
The importance of choosing an appropriate solvent for addition reactions should be clear from the above equation. If the addition of HCl, HBr or HI is desired, water and alcohols should not be used. These strong acids will ionize in such solvents to give ROH2(+) and the nucleophilic oxygen of the solvent will compete with the halide anions in the final step, giving alcohol and ether products. By using inert solvents such as hexane, benzene and methylene chloride, these competing solvent aditions are avoided. Because these additions proceed by way of polar or ionic intermediates, the rate of reaction is greater in polar solvents, such as nitromethane and acetonitrile, than in non-polar solvents, such as cyclohexane and carbon tetrachloride.
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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).
The chief difference in these conditions is that 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. The decomposition of the final ozonide results in carbonyl products by either a reductive or oxidative workup.
In the reaction with ozone, carbon-carbon double bonds breaks and carbon-oxygen double bonds from in the two separate compounds.
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Reduction
-- 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. Regioselectivity is not an issue, since the same group (a hydrogen atom) is bonded to each of the double bond carbons. The simplest source of two hydrogen atoms is molecular hydrogen (H2), but mixing alkenes with hydrogen does not result in any discernable reaction. Although the overall hydrogenation reaction is exothermic, a high activation energy prevents it from taking place under normal conditions. This restriction may be circumvented by the use of a catalyst.
Catalysts are substances that changes the rate (velocity) of a chemical reaction without being consumed or appearing as part of the product. Catalysts act by lowering the activation energy of reactions, but they do not change the relative potential energy of the reactants and products. Finely divided metals, such as platinum, palladium and nickel, are among the most widely used hydrogenation catalysts. Catalytic hydrogenation takes place in at least two stages, as depicted in the diagram. First, the alkene must be adsorbed on the surface of the catalyst along with some of the hydrogen. Next, two hydrogens shift from the metal surface to the carbons of the double bond, and the resulting saturated hydrocarbon, which is more weakly adsorbed, leaves the catalyst surface. The exact nature and timing of the last events is not well understood.
Three C5H10 alkenes which give the same alkane product (2-methylbutane).
Alkene Isomers
(CH3)2CHCH=CH2 3-methyl-1-butene
CH2=C(CH3)CH2CH3 2-methyl-1-butene
(CH3)2C=CHCH3 2-methyl-2-butene
Polymerisation of alkenes
-- polymerisation of ethene
-- polymerisation of vinyl chloride
-- polymerisation of styrene
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Note
The group CH2=CH- is known as vinyl group.
The group CH2-CH-CH2- is known as allyl group
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Cause - Effect relations
Anti-Markownikoff addition does not occur with HCl because H-Cl bond is quite strong and it doesnot form chloride free radical.
In case of HI, Anti-Markownikoff addition does not occur. In this case, iodide free radicals are formed but they are too less reactive that, they do not attack alkenes,
but combine with each other to form a iodine molecule.
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Electrophilic addition reactions of alkenes with X2, HX, HOX and H2O (X=halogen);
http://www.cem.msu.edu/%7Ereusch/VirtTxtJml/addene1.htm#add1
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Web-sites for reference:
The structure and naming-nomenclature of ALKENES
http://www.docbrown.info/page06/AlkeneStructure.htm
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7
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.
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JEE Question 2007 paper II
Cyclohexene on ozonolysis followed by reaction with zinc dust and water gives compound E. Compound E on further treatment with aqueous KOH yields compound F. Compound F is
Options given as pictures
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