Pyridinolysis of 2,4-Dinitrophenyl Phenyl Thionocarbonate: Effect of Changing Electrophilic Center from C=O to C=S on Reactivity and Mechanism

Second-order rate constants (k(N)) have been measured spectrophotometrically for nucleophilic substitution reactions of 2,4-dinitrophenyl phenyl thionocarbonate 4 with a series of Z-substituted pyridines in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Bronsted-type plot for the reactions of 4 exhibits downward curvature (i.e., beta(1) = 0.21 and beta(2) = 1.04), indicating that the reactions proceed through a stepwise mechanism with a change in rate-determining step. It has been found that 4 is less reactive than its oxygen analogue, 2,4-dinitrophenyl phenyl carbonate 3, although the thionocarbonate is expected to be more electrophilic than, its oxygen analogue. The pK(a) at the center of the Bronsted curvature, defined as pK(a)(o), has been analyzed to be 6.6 for the reactions of 4 and 8.5 for those of 3. Dissection of k(N) into the microscopic rate constants k(1) and k(2)/k(-1) ratio has revealed that the reactions of 4 result in smaller k(1) values but larger k(2)/k(-1) ratios than the corresponding reactions of 3. The larger k(2)/k(-1) ratios have been concluded to be responsible for the smaller pK(a)(o) found for the reactions of 4.