Ch.22 Alkenes - Core Points for Revision

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);
---------------

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
-- 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.