- Synthesis of an alkene by dehydrohalogenation is almost always better achieved
by an E2 reaction:
CC
X
B − H
β α E2 CC + B H + X−
- A secondary or tertiary alkyl halide is used if possible in order to bring about an
E2 reaction.
- A high concentration of a strong, relatively nonpolarizable base, such alkoxide ion,
is used to avoid E1 reaction.
- A relatively polar solvent such as an alcohol is employed.
- To favor elimination generally, a relatively high temperature is used.
- Sodium ethoxide in ethanol and potassium tert-butoxide in tert-butyl alcohol are
typical reagents.
- Potassium hydroxide in ethanol is used sometimes:
OH– + C 2 H 5 OH H 2 O + C 2 H 5 O–
7.6A E2 REACTIONS: THE ORIENTATION OF THE DOUBLE BOND IN THE
PRODUCT ZAITSEV’S RULE
- For some dehydrohalogenation reactions, a single elimination product is possible:.
CH 3 CHCH 3 CH 2
Br
CHCH 3
C 2 H 5 O−Νa+
C 2 H 5 OH
55 oC 79%
CH 3 CCH 3
Br
CH 2 C
C 2 H 5 O−Νa+
C 2 H 5 OH
55 oC 100%
CH 3
CH 3
CH 3
