Alkenes imp notes

ALKENES
 It is an unsaturated hydrocarbons having at least a carbon-carbon double bond.
 Its general formula is CnH2n . It is also called olefins (Latin olefins = oil forming) becau
lower member of the series react with chlorine to form oily products.
 Each doubly bonded carbon atom is sp2 hybridized and forms three sigma bonds an
bond.
 Its bond angle is 120° and trigonal planar geometry.

, Preparation of alkenes
1. Dehydration of alcohol:
The removal of water molecule from alcohols by using dehydrating agent like conc. H
called dehydration of alcohols.
When alcohols are heated at 170°C with conc. H2SO4 alkene is formed. (By removing
atom and hydroxyl group from adjacent carbon atoms)(α,β-elimination)
conc. H2SO4 + H2 O
CH3-CH2-OH CH2=CH2
170°C
Ethanol Ethene


conc. H2SO4 H2 O
CH3-CH-CH3 CH2 CH-CH3 +
OH 170°C
Propan-2-ol Propene
The ease of dehydration of alcohol follows order: 3° ˃ 2° ˃ 1°
When vapours of alcohols are passed over heated alumina, alkenes are formed.
Al2O3 H2 O
CH3-CH-CH3 CH2 CH-CH3 +
OH 625K
Propan-2-ol Propene