Catalytic Efficiency vs. Ester Reactivity

Substrate Pre-steady state (initial rate) Steady-state Slow step 

It is apparent that an increase in the substrate-to-catalyst ratio dramatically decreases the catalytic efficiency for the pNPOAc reactions, but affects to a much lower extent the POAc reaction. This is easily understood with reference to Table 5.3. Since in the reaction of pN POAc the rate-determining step is mainly deacetylation, an increase in ester concentration causes a proportional increase in the rate of background methanolysis, but hardly affects the rate of deacetylation, with the result that catalytic efficiency varies inversely to ester concentration. Conversely, the reaction of POAc approaches a situation in which acetylation of the catalyst is rate determining, which implies that both acetylation and background reactions increase on increasing ester concentration. 

Extrapolation of the above considerations to acetate esters less reactive than POAc, i.e., with leaving groups more basic than PO , leads to the prediction of lower catalytic efficiencies because the rate of acetylation of the catalyst is expected to decrease more rapidly than the rate of background methanolysis. In contrast, with esters of phenols more acidic than pNPOH, the catalytic reaction is predicted to be obscured by increased rates of background methanolysis. Thus, for one reason or 

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