I wanted to ask a question about acid catalysed hydrolysis.

I learnt in an introductory course to Physical Organic Chemistry that there are different ways that an ester can be hydrolysed acidically:


There are different ways that esters can be hydrolysed: – catalysis by acids (A) or bases (B) – cleavage of acyl-oxy (AC) or alkyl-oxy (AL) bonds – the molecularity of the key step (1 or 2).

Using Acids (A) there are 4 different ways to hydrolyse an ester using acid catalysis:

$\ce{A_{AC}1}$ Cleavage of Acyl-Oxy Bond Unimolecular
$\ce{A_{AC}2}$ Cleavage of Acyl-Oxy Bond Dimolecular
$\ce{A_{AL}1}$ Cleavage of Alkyl-Oxy Bond Unimolecular
$\ce{A_{AL}2}$ Cleavage of Alkyl-Oxy Bond Dimolecular

An explanation can be found here.

But consider this question, with the marking scheme given below: enter image description here

Our professor did not mention anything regarding how to choose which type of hydrolysis takes place. From this question, he states and goes further onto mention that either $\ce{A_{AC}1}$ or $\ce{A_{AC}2}$ can take place, but no other.

I'm looking at these structures and thinking

Why can't $\ce{A_{AL}1}$ or $\ce{A_{AL}2}$ take place for these two? Why must it be the two given in them marking scheme?

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    $\begingroup$ Perhaps you could explain in a few words what those mechanisms mean, or provide a reference. I think that's not too widely known. $\endgroup$ – Karl Jun 9 '19 at 21:23
  • $\begingroup$ @Karl Edit made. Reference added. $\endgroup$ – vik1245 Jun 9 '19 at 21:26
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    $\begingroup$ +1 , but I wouldn't add this as a P.S. People have a short attention span, they read five lines, find they don't get it, and move on. ;-) $\endgroup$ – Karl Jun 9 '19 at 21:36
  • $\begingroup$ @Karl edited it further so now it appears in the introduction rather than the end! $\endgroup$ – vik1245 Jun 9 '19 at 21:42

You should have read your explanation material more carefully! It clearly states that $A_{AL}2$ has not even been observed in acid hydrolysis due to water being a poor nucleophile in an $S_N2$ process. As for $A_{AL}1$, that only happens when the esterified radical can leave as a stable carbocation which, in this case methyl is a very very unstable carbocation.

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    $\begingroup$ Well if I am such a fool then I will learn today! Thanks very much this isn't actually my material but I read it so fast I couldn't actually make the point clear to myself! My apologies! $\endgroup$ – vik1245 Jun 9 '19 at 22:14

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