The reaction referred to is the nucleophilic acyl substitution reaction, where the nucleophile is a carboxylate ion and the electrophile is the ester. I am having a difficult time visualizing why this cannot occur.

  • $\begingroup$ It may be somewhat plausible in the E conformation of the ester, but certainly not in the Z form $\endgroup$ – Yusuf Hasan Dec 31 '18 at 8:17
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    $\begingroup$ Carboxylate is a poor nucleophile, alkoxide a much better one. If the reaction occured, the alkoxide would then go on to react with the anhydride - a very much better electrophile - so the outcome would be ester plus carboxylate again. $\endgroup$ – Waylander Dec 31 '18 at 11:20
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    $\begingroup$ @YUSUFHASAN I don't see how ester conformations are relevant. Even if it made a difference to the fundamental reactivity of esters vs anhydrides as Waylander explained (and it doesn't), it's not as if you could isolate two different conformations of the ester. $\endgroup$ – orthocresol Dec 31 '18 at 19:44

The reaction you are asking about is a carboxylate plus an ester reacting to form an anhydride plus an alcohol. The reverse reaction, making an ester from an alcohol and an anhydride, is one commonly performed in teaching labs.

So the equilibrium lies on the side of the ester. The principle of micro-reversibility would suggest that this reaction can go in either direction, but you would have trouble detecting formation of the anhydride and alcohol from the carboxylate and ester because of where the equilibrium lies.


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