Transesterification requires what?

• A. Water and alcohol under acid
• B. A ketone and an alcohol with base
• C. An ester and an alcohol in acid or base to exchange the alkoxy group
• D. An ester and an alcohol under acid or base to exchange the alkoxy group

Answer: (C)

Transesterification is the alkoxy exchange at an ester, driven by nucleophilic substitution at the carbonyl carbon. The reaction needs an ester and another alcohol, with acid or base catalysis to speed things up. The alcohol acts as the nucleophile and attacks the carbonyl, forming a tetrahedral intermediate. Through proton transfers and collapse of that intermediate, the original alkoxy group leaves as the corresponding alcohol, giving a new ester where the alkoxy group has been swapped. Water isn’t required, and a ketone isn’t the right substrate for this process. The presence of an ester and an alcohol under acid or base conditions is precisely what enables the alkoxy exchange to occur.