Page 91 - Pharm.Org.Chem-I 02-06-05-101

1- Dehydrohalogenation of alkyl halides (-HX)

• Elimination of X from α C of the alkyl halide and elimination of H from β carbon
(β-elimination mechanism).

• Heating in strong base: alcoholic KOH or sodium ethoxide / ethanol

R-

alc. KOH /  CH3CH=CH2
CH3CH2CH2Br propene

Bromopropane

Zaitsev’s Rule: Formation of the More
Substituted Alkene (using small base)

β αβ alc. KOH / 

H3C H2C CH CH3

Br

2-Bromobutane

When using a bulky base as K tert.butoxide, the Hoffmann alkene is the major
product.

89