Chapter 18: Enols and Enolates
Basic
a
-Halogenation of Aldehydes and Ketones
Reaction type :
Nucleophilic substitution
Summary
Under basic conditions,
a
-halogenation occurs.
Typically the reaction can not be limited to
mono
halogenation and
poly
halogenation occurs.
This is because each electronegative halogen further enhances the stability of the enolate.
Chlorination, bromination and iodination all occur at the same rate.
The reaction proceeds via the very reactive enolate.
Polyhalogenation of methyl ketones (
RCO-CH
3
) is known as the
Haloform reaction
.
Related Reactions
a
-halogenation under acidic conditions
The Haloform reaction
MECHANISM OF
a
-HALOGENATION OF ALDEHYDES AND KETONES UNDER BASIC CONDITIONS
Step 1:
First, an acid-base reaction. Hydroxide functions as a base and removes the acidic
a
-hydrogen giving the enolate.
Step 2:
The nucleophilic enolate reacts with the halide giving the halogenated ketone and a bromide ion.
