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

摘要

Second-order rate constants (An) 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 °C. The Bransted-type plot for the reactions of 4 exhibits downward curvature (i.e., β1 = 0.21 and β2 = 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°, 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 k1 and k2/k_(-1) ratio has revealed that the reactions of 4 result in smaller k1 values but larger A2/A-1 ratios than the corresponding reactions of 3. The larger k2/k_(-1) ratios have been concluded to be responsible for the smaller pK_a° found for the reactions of 4.