dasdas

Master Organic Chemistry Reaction Guide

Halogenation Of Ketones via Enols

Description: Treatment of ketones with bromine (Br2) in the presence of acid will result in formation of a new C-Br bond at the “alpha” position.

description of bromination of enols under acidic conditions

Notes: The purpose of the acid is to catalyze formation of the enol from the ketone, which is the active nucleophile in the reaction. Instead of Br₂, one can use a similar source of electrophilic bromine like N-bromosuccinimide (NBS). Various acids can be used (e.g. HBr, acetic acid (AcOH).

Examples:

examples of alpha bromination of ketones with br2

Notes: Example 1 shows formation of a new C-Br bond using HBr and Br2. Example 2 shows the use of acetic acid (AcOH). Example 3 shows how this reaction can occur on acid halides (this is a key step in the Hell-Vollhard-Zelinsky reaction).

Mechanism:

The reaction begins with protonation of the carbonyl oxygen with acid (Step 1, arrows A and B) which accelerates keto-enol tautomerism. Deprotonation of the C-H bond on the alpha-carbon (Step 2, arrows C , D, and E) results in the formation of the enol, which is the active nucleophile in this reaction. The nucleophilic enol carbon then reacts with the excellent electrophile, bromine (Step 3, arrows F, G, H) resulting in a new C-Br bond. The carbonyl oxygen is then deprotonated with weak base (Step 4, arrows I and J) to give the alpha-bromo ketone.