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 (kN) 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 oC. The Brønsted-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 pKa at the center of the Brønsted curvature, defined as pKa o, has been analyzed to be 6.6 for the reactions of 4 and 8.5 for those of 3. Dissection of kN 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 k2/k–1 ratios than the corresponding reactions of 3. The larger k2/k–1 ratios have been concluded to be responsible for the smaller pKa o found for the reactions of 4.