Esters, acid hydrolysis acyl-oxygen fission

Transesterification occurs by mechanisms that are identical with those of ester hydrolysis—except that ROH replaces HOH—that is, by the acyl-oxygen fission mechanisms. When alkyl fission takes place, the products are the acid and the ether. 

Isotopes can also be used to solve mechanistic problems that are non-kinetic. Thus the aqueous hydrolysis of esters to yield an acid and an alcohol could, in theory, proceed by cleavage at (a) alkyl/ oxygen fission, or (b) acyl/oxygen fission ... 

If the reaction is carried out in water enriched in the heavier oxygen isotope 180, (a) will lead to an alcohol which is 180 enriched and an acid which is not, while (b) will lead to an l80 enriched acid but a normal alcohol. Most simple esters are in fact found to yield an lsO enriched acid indicating that hydrolysis, under these conditions, proceeds via (b) acyl/oxygen fission (p. 238). It should of course be emphasised that these results are only valid provided that neither acid nor alcohol, once formed, can itself exchange its oxygen with water enriched in 180, as has indeed been shown to be the case. 

Thiols are more acidic than the corresponding alcohols and are converted into their salts using alkali metal hydroxides or alkoxides. The thiolate salts are powerful nucleophiles, a property which has been used in the cleavage of methyl esters and methyl ethers. The combination of a hard acid and a soft base such as lithium propanethiolate (Li SPr) may favour attack on the methyl group of an ester and lead to alkyl-oxygen fission of the ester, rather than the more common addition of a nucleophile to the carbonyl group and consequently acyl-oxygen fission. This is particularly useful in the hydrolysis of hindered esters. 

II. Unimolecular Acid-Catalyzed Reactions Involving Acyl-Oxygen Fission. At least one instance is known in which decomposition of the protonated complex does not seem to depend upon the attack of water or an alcohol molecule. It is the formation or hydrolysis of esters of 2,4,6-trimethylbenzoic acid in sulfuric acid solution. Since cryoscopic studies have shown that the acid gives a molar freezing point depression of four (p. 39), and the ester five (p. 225), we must conclude that decomposition of the protonated complex to the acyl carbonium ion 0... 

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