# Mechanism of Acid Hydrolysis

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

$$\ce{A_{AC}1}$$
$$\ce{A_{AC}2}$$
$$\ce{A_{AL}1}$$
$$\ce{A_{AL}2}$$

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:

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?

• Perhaps you could explain in a few words what those mechanisms mean, or provide a reference. I think that's not too widely known. – Karl Jun 9 at 21:23
• @Karl Edit made. Reference added. – vik1245 Jun 9 at 21:26
• +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. ;-) – Karl Jun 9 at 21:36
• @Karl edited it further so now it appears in the introduction rather than the end! – vik1245 Jun 9 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.