= Solution
\b[One equivalent supplies <carbon>; the other acts as a <base (chemistry)> 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 (chemistry)>, yielding <pinacolone>, rather than supplying a second <carbon> fragment to E. <acid (chemistry)> workup reverses the product <deprotonation>. Ignoring this <acid (chemistry)>–<base (chemistry)> 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>.
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