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
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
