Formation and use of carboxylate nucleophile in SN2 formation of an ester.

There is a long MCAT tradition of questions about proposed mechanisms for formation of an ester with ¹⁸O labeled carboxylic acid reagent. If the ester has been arrived at via nucleophilic acyl substitution, some ¹⁸O will be in the solvent, having left during the collapse of the tetrahedral intermediate. (Remember that no matter how they draw it, the two oxygens in a carboxylic acid are equivalent because the proton exchanges 100,000 times a second.)

On the other hand, if the the ester were arrived at via SN2 reaction with carboxylate nucleophile, all of the ¹⁸O will still be present in the ester product. There are a million variations of this question. The heart of the question is always about seeing that you understand the mechanism of nucleophilic acyl substitution.