Chapter 10, Problem 10.9YT

Interpretation Introduction

Interpretation:

The complete, detailed mechanism for the given reaction is to be drawn, which is identical to the one in Equation 10-30, except that ${\text{Br}}_2$ takes the place of ${\text{Cl}}_2$.

Concept introduction:

Alpha halogenation is the halogenation at the alpha carbon atom of an aldehyde or a ketone under acidic conditions. Under these conditions, a negatively charged enolate ion is highly basic and cannot exist at substantial concentrations. However, the alpha carbon atom of a ketone can still become electron-rich via acid-catalyzed keto-enol tautomerism.

Under the acidic conditions, the first step is the protonation of carbonyl oxygen. In the second step, the acetate ions serve as a base and abstract an alpha proton from ketone; this yields an enol form. In the enol form, the alpha carbon is electron rich, due to the small contribution by the resonance structure in which a negative charge and a lone pair of electrons are located on the alpha carbon. Alpha halogenation occurs only once as the electron withdrawing effect of bromine destabilizes the positive charge, if the carbonyl oxygen gets protonated further. This reduces the chances of the second halogenation.