6.16 ELIMINATION REACTIONS OF ALKYL HALIDES
CC
Y Z
elimination
(−YZ) CC
6.16A DEHYDROHALOGENATION
- Heating an alkyl halide with a strong base causes elimination to happen:
CH 3 CHCH 3
Br
CC^2 H^5 ONa + NaBr
2 H 5 OH, 55oC
H 2 CCHCH 3
(79%)
+ C 2 H 5 OH
C
CH 3
H 3 CBr
CH 3
C CH 2
CH 3
H 3 C +NaBr+ C 2 H 5 OH
(91%)
C 2 H 5 ONa
C 2 H 5 OH, 55oC
- Dehydrohalogenation:
β α
+ B− ++H B X−
Base
C Dehydrohalogenation
H
C
X
(−HX) CC
X = Cl, Br, I
1) alpha (α) carbon atom:
2) beta (β) hydrogen atom:
3) β-elimination (1,2-elimination):
6.16B BASES USED IN DEHYDROHALOGENATION
- Potassium hydroxide dissolved in ethanol and the sodium salts of alcohols (such
as sodium ethoxide) are often used as the base for dehydrohalogenation.
- The sodium salt of an alcohol (a sodium alkoxide) can be prepared by treating an
alcohol with sodium metal:
