Which of the following best describes the first step in the acid-catalyzed transesterification mechanism of an ester with an alcohol?

• A. Nucleophilic attack of the alcohol on the carbonyl carbon before any protonation
• B. Protonation of the carbonyl oxygen of the ester by the acid catalyst
• C. Direct elimination of the leaving group from the ester
• D. Deprotonation of the alcohol to form an alkoxide ion (${\mathrm{RO}}^{-}$)

Answer: (B)

Verified step by step guidance

1. Understand that acid-catalyzed transesterification involves converting one ester into another by reaction with an alcohol in the presence of an acid catalyst.
2. Recognize that the acid catalyst first protonates the carbonyl oxygen of the ester, increasing the electrophilicity of the carbonyl carbon and making it more susceptible to nucleophilic attack.
3. Write the protonation step as: \(\text{Ester} + \text{H}^+ \rightarrow \text{Protonated ester}\), where the carbonyl oxygen gains a proton.
4. After protonation, the alcohol acts as a nucleophile and attacks the now more electrophilic carbonyl carbon, leading to the formation of a tetrahedral intermediate.
5. Note that nucleophilic attack before protonation is less favorable because the carbonyl carbon is less electrophilic without protonation, and direct elimination or deprotonation of the alcohol to form an alkoxide ion are not the initial steps in this acid-catalyzed mechanism.