Chapter 10, Problem 10.13P

Interpretation Introduction

Interpretation:

The complete mechanism for this reaction is to be shown and the major product is to be predicted.

Concept introduction:

In acidic conditions, the carbonyl oxygen of an aldehyde or a ketone is protonated. This leads to keto-enol tautomerization. Any acidic $\text{α}$ hydrogen present can then be extracted by a strong base. This generates a strong nucleophile that can be used for nucleophilic substitution on a halogen molecule in an ${\text{S}}_{\text{N}}\text{2}$ reaction. The result is $\text{α}$ halogenation of the aldehyde/ketone.

Unlike the $\text{α}$ halogenation under basic conditions, the reaction stops at a single halogenation as the electronegativity of the halogen prevents the protonation of the carbonyl group necessary for the second keto-enol tautomerization.