IIT JEE Revision Ch. 22/15 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);
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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

15.9 Preparation and Properties of Alkenes

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.

Chemical Reactions of Alkenes

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.

15.7 Isomerism in Alkenes

Alkenes show the following types of strunctural isomerism

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.

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



Jauhar Chapter 15 contents of Alkenes

Chemistry of Alkenes
15.6 Nomenclature of Alkenes
15.7 Isomerism in Alkenes
15.8 Stability of of Alkenes
15.9 Preparation and Properties of Alkenes


Updated om 11 January 2020
28 January 2008