How does adding a Brønsted acid help the reaction? The water produced by the reaction is incapable of being a nucleophile for the reverse reaction if it is protonated. The carbonyl group of the carboxylic acid becomes more susceptible to attack by nucleophiles if the carbonyl oxygen is protonated. Once protonated, the hydroxyl group of the alcohol becomes more susceptible to attack by the carboxylic acid. The acid catalyzes only the forward reaction.