Reactions involving alkyl-oxygen cleavage

Many of the esters which are hydrolyzed by the AalI mechanism in acid are also hydrolyzed with alkyl-oxygen fission under neutral condi-tions60,67 74 75 84 85 88 89. These reactions have the high enthalpies and entropies of activation characteristic of unimolecular reactions, and involve the ionization of (usually) tertiary alkyl esters, to the carbonium ion and a carboxylate anion in the rate-determining step, viz. 

These are best regarded as SN1 reactions, in which the leaving group is the carboxylate anion. The point is brought out very well by Moffat and Hunt s comparison79 of the solvolyses of /-alkyl trifluoroacetates in 70% aqueous acetone with the reactions of the corresponding /-alkyl halides (Table 26). The activation parameters, and the fate of the carbonium ion, as measured by the percentage of olefin formed by the parallel El reaction, are closely similar for the two types of substrate. 

COMPARISON OF THE HYDROLYSIS OF /-ALKYL FLUOROACETATES WITH THE SOLVOLYSIS OF /-ALKYL HALIDES AT 25°C 

Compound Relative rate Elimination (%) A H (kcal-mole ) AS (eu) Solvent 

Another well-established class of ester reactions which involve alkyl-oxygen cleavage belongs to the large category of SN2 reactions. 

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The results of analysis of the samples prepared via reactive extrusion of PMMA with dimethylamine are shown in Tables 1 and 2, and Figure 1. It is apparent from these results that the main product of the reaction was methacrylic anhydride, confirming the hypothesis that alkyl-oxygen cleavage was involved in the reaction of PMMA with amines (at least with dimethylamine). However, additional subtleties of the reaction are apparent from a more detailed analysis of the reaction products vs. degree of conversion and reaction conditions. 

The AaiI (acid-catalyzed, unimolecular, alkyl-oxygen cleavage) mechanism involves formation of a carbocation in a process that is analogous to the ionization step in an SnI or El reaction (Figure 7.18). ... 

The detailed mechanism of the formation of acyl cations is not yet clear. It seems certain that the leaving group is a neutral water or alcohol molecule, and thus that a proton transfer is involved. It is not possible at this time, however. to distinguish between a pre-equilibrium formation of the tautomeric form protonated on the alkyl-oxygen, which would then cleave in the ratedetermining step (route A)14 28 and the similar mechanism in which the proton transfer and cleavage reactions are concerted (route B)14. 

The proposed reaction mechanisms can be distinguished by two factors (i) whether ester cleavage occurs between the oxygen atom and the acyl carbon atom or between the oxygen atom and the alkyl carbon atom, and (ii) whether the reaction involves only a single step or involves an intermediate. ... 

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