 Solution

One equivalent supplies carbon; the other acts as a base for the product. The newly formed 1,3-diketone is appreciably more acidic than the ketone from which D was prepared. proton transfer from its central CH2 to a second enolate equivalent is therefore favourable. Product-enolate stabilization also helps drive the acyl-substitution sequence towards completion.

The second enolate is consequently consumed as base, yielding pinacolone, rather than supplying a second carbon fragment to E. acid workup reverses the product deprotonation. Ignoring this acid–base step would predict that a single equivalent of D is sufficient to consume every equivalent of acyl chloride, despite the product immediately quenching part of the available reagent.