Esters, acid hydrolysis alkyl-oxygen fission

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 ... 

The acid-catalysed hydrolysis of phosphinic esters (158) is relatively insensitive to the substituent (R = Me, Et, Pr1, or Bu1) attached to phosphorus this contrasts with the base-catalysed process.126 The acid hydrolysis of the phosphinic esters (159) (see Organophosphorus Chemistry , Vol. 7, p. 129 for the base hydrolysis) takes place with alkyl-oxygen fission in an SnI process.127... 

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. 

It is possible to shift ester hydrolyses away from the normal Aac2 or Bac2 mechanisms by structural changes in the substrate. When the ester is derived from a tertiary alcohol, acid-catalyzed hydrolysis often shifts to a mechanism involving alkyl-oxygen fission. This shift is due to the stability of the carbonium ion that can be formed by C-O heterolysis, and probably also to a decrease in the rate of nucleophilic attack at the carbonyl group because of steric factors. Alkenes, as well as alcohols, may be produced from the carbonium ion, since water can function either as a nucleophile or as a Br0nsted base. 

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. 

In protic solvents, the allylic carbonium ion formed by acid-catalyzed alkyl carbon-oxygen bond fission can recombine either with the carboxylic acid molecule or with a solvent molecule. The electrostatic attraction between the carbonium and carboxylate ions, which is a major factor in isomerization of allylic esters by ion-pair internal return during solvolysis, is absent in the acid-catalyzed reaction. The more numerous, usually more nucleophilic, solvent molecules in the solvation shell of the carbonium ion should compete effectively with the departed carboxylic acid molecule and solvolysis rather than isomerization should be the predominant reaction. For example, in the presence of 0.05 M perchloric acid, solvolyses of cis- and //- //7.s-5-methyl-2-cyclohexenyl p-nitrobenzoates are not only very much faster than in the absence of the acid, but polarimetric and titrimetric rates of solvolysis of optically-active esters were identical within experimental error. For these esters, the acid-catalyzed solvolysis was not accompanied by a detectable amount of isomerization. Braude reported, on the basis of indirect evidence, that isomerization accompanies acid-catalyzed hydrolysis of a-ethynyl-y-methylallyl acetate in aqueous dioxane. It was shown that, under some experimental conditions, the spectrophotometrically determined rate of appearance of the rearranged 1 -yne-3-ene chromophore exceeds the titrimetrically determined rate of hydrolysis,... 

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