Dinitrophenyl benzoates, hydrolysis

Hubbard and Kirsch (1972) have recently proposed that histidine may act as a nucleophile in a-chymotrypsin acylation reactions of esters having a good leaving group (jO-nitrophenol). This suggestion was based on a similarity in p-value for acylation by p-substituted nitrophenyl and dinitrophenyl benzoates and nucleophilic attack on these compounds by imidazole, in contrast with less positive p-values for hydroxide ion catalysis. Hammett p-values for hydrolysis of substituted phenyl esters are given in Table 6 and show little apparent trend. The values for hydroxide ion and alcoholate ions are... 

As expected, carboxylate and other oxyanions catalyze the hydrolysis of esters with good leaving groups. And evidence specifically implicating the nucleophilic mechanism is available from work using labelled substrates. Bender and Neveu279 hydrolyzed 2,4-dinitrophenyl benzoate in the presence... 

Product analysis Product identity or distribution attributable only to reaction via intermediate formed by nucleophilic attack I "0]Acetate-catalyzed hydrolysis of 2,4-dinitrophenyl benzoate gives C 0]-benzoic acid 86... 

The reaction of oarboxylate with various substrates is an example of a system which necessitates a careful search for products. Early examples of these reactions demonstrated that direct nucleophilic attack takes place. For example, using a dilatometric method, formate was found to catalyze strongly the hydrolysis of acetic anhydride whereas propionate and butyrate slow down the reaction (Kilpatrick, 1928). These results are attributable to mixed anhydride formation with the total rate being determined by the reactivity of the mixed anhydride. The reaction of acetate with 2,4-dinitrophenyl benzoate results in the formation of an unstable mixed anhydride, as was shown in an experiment in which 0 labeled acetate was used the benzoic acid product contains 75% of the O label (Bender and Neveu, 1958). The other 25 % of the O label is presumed to be lost to acetate by solvent attack at the benzoyl carbon if direct nucleophilic interaction is the sole path. 

Experimental evidence for such nucleophilic catalysis is sununarized in Figure 7.27. Hydrolysis of 2,4-dinitrophenyl benzoate catalyzed by acetate ion labeled with on both oxygen atoms led to product in which both benzoic acid and acetic acid were labeled with 0, but about 75% of the label derived from one 0 in the reactant was found in the benzoate ion. The data were interpreted as evidence for the intermediacy of the mixed anhydride CH3CO2COC6H5. 

Fig. 10. Effect of inhibiting salts upon the hydrolysis of 2,6-dinitrophenyl phosphate in 3 X lO- MCTABat 25-0° and pH 9 0 A, sodium chloride .sodium methane sulfonate I. sodium P toluene sulfonate O. sodium benzoate . dipotassium phthalate o. disodium terephthalate disodium phenyl phosphate (Bunton et al., 1968).

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