SA-nucleophilic catalysis

The realization of the occurrence of intermolecular GA catalysis comes from the experimental fact of the presence of a kinetic term kgJBH+][NuH][Sub] (where BH+ is the GA, NuH is the nucleophile, and Sub is the substrate) in the rate law for the nucleophilic reaction between NuH and Sub in the presence of BH+. But this kinetic term is kinetically indistinguishable from kinetic term k[H+][B][NuH][Sub], which represents SA-GB catalysis or SA-nucleophilic catalysis. However, these kinetically indistinguishable catalytic mechanisms can be resolved, at least qualitatively, as described in the following text. 

An interesting example of the resolntion between SA-nucleophilic catalysis and SA-GB catalysis is found in the kinetic study on the rate of hydrolysis of N-benzyl-P-snltam in the presence of carboxylate buffers. " Pseudo-first-order rate constants (kg J, obtained at a constant pH, ionic strength, temperature, and different total carboxylic acid buffer concentrations ([BufJx), follow Equation 2.68... 

The observation on GA-catalyzed hydrolysis is in contrast to the GB catalysis seen in the buffer-catalyzed hydrolysis of P-lactams of penicillins. " Thus, the probable mechanism of buffer catalysis in this case involves SA-nucleophilic catalysis (Scheme 2.44). The evidence for this catalytic proposal comes from the following observations (1) the mixed acid anhydride intermediate (Intn) has been trapped with aniline to give acetanilide as identified by HPLC (2) the kinetic runs for hydrolysis of A-benzyl-P-snltam in acetate buffers revealed biphasic plots of observed absorbance vs. reaction time an initial exponential bnrst of UV... 

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