Ester hydrolysis Bac2 pathway

The p value for base-catalysed hydrolysis (+2-51) is +ve and quite large, reflecting the development of not inconsiderable -ve charge at the reaction centre in the rate-limiting step—attack on this centre by OH (step CD in the BAC2 pathway). By contrast, the p value for acid-catalysed hydrolysis (+0-03) is very nearly zero which means, of course, that the rate of this hydrolysis does not vary significantly from one ester to another, no matter what the m- or... 

Tertiary benzoyloxymethylsulfonamides (17) undergo hydrolysis via pH-independent and acid- and base-catalysed processes. Reactions are also buffer catalysed for buffer species with pXa values > 10.5. For the pH-independent pathway, hydrolysis takes place via formation of an Al-sulfonyliminium ion (Scheme 2). The mechanism of the acid-catalysed process involves pre-equilibrium protonation of the substrate followed by iminium ion formation. The base-catalysed pathway involves the normal Bac2 mechanism of ester hydrolysis. The buffer-catalysed reaction gives rise to a curved Brpnsted plot, with values of 1.6 and 0.25 for nucleophiles with pXa values <12.5 and >13, respectively. This is indicative of nucleophilic catalysis associated with a change in rate-limiting step from formation of the tetrahedral intermediate for buffer species with pXa > 13 to decomposition of the tetrahedral intermediate for buffer species with pXa < 12.5. ... 

If we now extend our consideration of base-catalysed (BAC2), and acid-catalysed (Aac2), hydrolysis to esters in general, including aliphatic ones (RC02Et), we see that there is a close similarity between the transition states (42b or 42a) for the rate-limiting step in each of the two pathways they are both tetrahedral and differ... 

That a single solvent molecule clustered to a nucleophile can drastically change the reaction pathway has been demonstrated by studying the reaction of phenyl acetate with methoxide ion in the gas phase [671, 672]. Alkaline hydrolysis of esters in solution is known to proceed by attack of the nucleophile at the carbonyl carbon atom to form a tetrahedral intermediate, followed by cleavage of the acyl-oxygen bond (Bac2 mechanism) cf. Eq. (5-138a). 

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