Chapter 18: Enols and Enolates

Summary

• Under acidic reaction conditions, selective monohalogenation at the a-postion is observed.
• Chlorination, bromination and iodination all occur at the same rate.
• The reaction can be catalysed by addition of acid, but it is actually autocatalytic as the by-product is the hydrogen halide.
• The reaction proceeds via the enol tautomer.

• a-halogenation under basic conditions
• The Haloform reaction

MECHANISM OF a-HALOGENATION OF ALDEHYDES AND KETONES
Step 1: First, tautomerise the carbonyl to its enol tautomer (mechanism not shown here : review ?)
Step 2: Here the electrons from the oxygen are used as they enhance the nucleophilicity of the alkene, as a result we end up with an oxonium ion and a bromide ion.
Step 3: An acid / base reaction. Here the bromide ion is used to deprotonate the oxonium ion.