Chapter 18: Enols
of Aldehydes and Ketones
Reaction type : Nucleophilic
- Under basic conditions, α-halogenation
- Typically the reaction can not
be limited to monohalogenation and polyhalogenation 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-CH3) is known as the Haloform
MECHANISM OF α-HALOGENATION
OF ALDEHYDES AND KETONES UNDER BASIC CONDITIONS
First, an acid-base reaction. Hydroxide functions as a base and removes
the acidic α-hydrogen giving the enolate.
The nucleophilic enolate reacts with the halide giving the halogenated
ketone and a bromide ion.