CH 3 CH 2 CH 2 O CH 3
CH 3 CH 2 CH 2 Br
Methyl propyl ether
HBr
Heat
Propyl bromide
+ CH 3 OH
Mechanism.
CH 3 CH 2 CH 2 O CH 3
CH 3 CH 2 CH 2 O CH 3
H
H Br
Heat
Propyl bromide
CH 3 CH 2 CH 2 Br
..
..
+
..
+
+ CH 3 OH
S N 2
Br: −
Preparation of alcohols Ethylene oxide can be easily cleaved by HBr to
give bromoethanol. The oxygen is protonated to form a protonated ethylene
oxide, which, being attacked by the halide, gives bromoethanol.
CH 2
H 2 C
O
+ HBr
Ethylene oxide
Bromoethanol
BrCH 2 CH 2 OH
Mechanism.
H Br
CH 2
H 2 C
O
CH 2
H 2 C
OH
Bromoethanol
BrCH 2 CH 2 OH
+
: :
..
+
Br: −
Acid-catalysed cleavage of epoxides
In the case of acid-catalysed unsymmetrical epoxide, the weak nucleophiles
(H 2 O and ROH) attack the most substituted carbon of the ring, and produce
1-substituted alcohol. This reaction follows S N 1 reaction.
Preparation of diols Acid-catalysed epoxides are easily cleaved by water.
Water reacts as the nucleophile, and this is referred to as a hydrolysis. For
example, hydrolysis of ethylene oxide in the presence of acid-catalyst
produces 1,2-ethanediol (ethylene glycol).
CH 2
H 2 C
O
CH 2 CH 2
OH OH
Ethylene oxide
1,2-Ethanediol
H +
H 2 O
+ H 3 O +
Mechanism.
CH 2 CH 2 OH
O
H
H
H 2 C
OH
CH 2
H 2 C
O
CH 2 CH 2
OH OH
+
H 2 O
..
..
+
H +
:
CH 2
:
..
:
H 2 O
..
..
+ H 3 O +
..
246
CH5 ORGANIC REACTIONS