Ketone halogenation

In organic chemistry ketone halogenation is a special type of halogenation.

The position alpha (next) to the carbonyl group in a ketone is easily halogenated, due to the ability to form an enolate in basic solution, or an enol in acidic solution. An example is the bromination of acetone in basic solution:

CH₃-CO-CH₃ + OH⁻ → CH₃-CO-CH₂⁻ + H₂O

CH₃-CO-CH₂⁻ + Br₂ → CH₃-CO-CH₂Br + Br⁻

In acidic solution, usually only one alpha hydrogen is replaced by a halogen, because each successive halogenation is slower that the first. The halogen decreases the basicity of the carbonyl oxygen, thus making protonation less likely. However, in basic solution successive halogenations are more rapid, because the halogen withdraws electrons by induction and makes remaining hydrogens more acidic. In the case of methyl ketones, this results in what is called the haloform reaction.