7.7 DEHYDRATION OF ALCOHOLS
- Dehydration of alcohols:
- Heating most alcohols with a strong acid causes them to lose a molecule of water
and form an alkene:
HA
heat CC + H^2 O
HOH
CC
- The reaction is an elimination and is favored at higher temperatures.
- The most commonly used acids in the laboratory are Brønsted acids ––– proton
donors such as sulfuric acid and phosphoric acid.
- Lewis acids such as alumina (Al 2 O 3 ) are often used in industrial, fas phase
dehydrations.
- Characteristics of dehydration reactions:
- The experimental conditions –– temperature and acid concentration –– that
are required to bring about dehydration are closely related to the structure
of the individual alcohol.
i) Primary alcohols are the most difficult to dehydrate:
concd
Ethanol (a 1o alcohol)
CC +
H
H
H
H
H 2 O
H O H
H 2 SO 4
180 oC
CC
H H
H H
ii) Secondary alcohols usually dehydrate under milder conditions:
OH 85% H
3 PO 4
165-170oC
Cyclohexanol Cyclohexene (80%)
+ H 2 O
