Base-catalysed hydrolysis of esters by a Kumar

Base-catalysed hydrolysis of esters is irreversible: You can’t make esters from carboxylic acids and alcohols under basic conditions because the base deprotonates the carboxylic acid. However, you can reverse that reaction and hydrolyse an ester to a carboxylic acid (more accurately, a carboxylate salt) and an alcohol. This time the ester is, of course, not protonated first as it would be in acid, but the unprotonated ester is a good enough electrophile because OH–, and not water, is the nucleophile. The tetrahedral intermediate can collapse either way, giving back ester, or going forward to acid plus alcohol Without an acid catalyst, the alcohol cannot react with the carboxylic acid; in fact, the backward reaction is doubly impossible because basic conditions straight away deprotonate the acid to make a carboxylate salt (which, incidentally, consumes the base, making at least one equivalent of base necessary in the reaction).