Chapter 18: Enols and Enolates

Basic α-Halogenation of Aldehydes and Ketones

Summary

• Under basic conditions, α-halogenation occurs.
• 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 reaction.

• α-halogenation under acidic conditions
• The Haloform reaction

MECHANISM OF α-HALOGENATION OF ALDEHYDES AND KETONES UNDER BASIC CONDITIONS
Step 1: First, an acid-base reaction. Hydroxide functions as a base and removes the acidic α-hydrogen giving the enolate.
Step 2: The nucleophilic enolate reacts with the halide giving the halogenated ketone and a bromide ion.

© Dr. Ian Hunt, Department of Chemistry