Question

Propose a mechanism for the acid-catalyzed bromination of pentan-3-one.

Short answer

The halogenation of ketones is catalyzed by acid. The ketone is dissolved in acetic acid, which serves as both the solvent and the acid catalyst.

Acidic halogenation can selectively replace just one hydrogen or more than one, depending on the amount of the halogen added.

Step-by-step solution

Acid-Catalyzed Alpha Halogenation

The halogenation of ketones is catalyzed by acid. The ketone is dissolved in acetic acid, which serves as both the solvent and the acid catalyst.

Acidic halogenation can selectively replace just one hydrogen or more than one, depending on the amount of the halogen added.

Mechanism of Acid-catalyzed bromination of pentan-3-one

Under acid catalysis, the ketone( pentan-3-one) is in equilibrium with its enol form. The enol acts as a weak nucleophile, attacking bromine to give a resonance-stabilized intermediate. Loss of a proton (deprotonation) results in the formation of the product 2-bromopentan-3-one.

Formation of 2-bromopentan-3-one