iii) Tertiary alcohols are usually dehydrated under extremely mild conditions:
20% aq. H 2 SO 4
85 oC
tert-Butyl alcohol 2-Methylpropene (84%)
H 3 C
C
CH 2
CH 3
C + H 2 O
CH 3
H 3 COH
CH 3
iv) Relative ease of order of dehydration of alcohols:
3 o Alcohol 2 oAlcohol 1 oAlcohol
C >>
R
R OH
R
C
R
R OH
H
C
H
R OH
H
2) Some primary and secondary alcohols also undergo rearrangements of their
carbon skeleton during dehydration.
i) Dehydration of 3,3-dimethyl-2-butanol:
H 3 C
OH
C
CCH 3
CH 3
H 3 C H^3 C
CH 3
C
CCH 3
CH 3
H 2 C
CH 3
C
CCH 3
CH 3
85% H 3 PO 4
80 oC
3,3-Dimethyl-2-butanol 2,3-Dimethyl-2-butene
(80%)
2,3-Dimethyl-1-butene
(20%)
+
ii) The carbon skeleton of the reactant is
CC while that of the products is
C
C
CCC
CC
C CC
7.7A MECHANISM OF ALCOHOL DEHYDRATION: AN E1 REACTION
- The mechanism is an E1 reaction in which the substrate is a protonated alcohol
(or an alkyloxonium ion).
