Pyridinolyses of 2,4-dinitrophenyl phenyl carbonate and 2,4-dinitrophenyl benzoate: Effect of nonleaving group on reactivity and mechanism

Abstract

Second-order rate constants (k N) have been measured for reactions of 2,4-dinitrophenyl phenyl carbonate (2) with a series of pyridines in 80 mol % H 2O/20 mol % DMSO at 25.0 ± 0.1 °C, and compared with the k N values reported for the corresponding reactions of 2,4-dinitrophenyl benzoate (1) to investigate the effect of nonleaving group on reactivity and mechanism. The reactions of 2 result in larger k N values than those of 1. The Brønsted-type plot for the reactions of 2 exhibits a downward curvature (i.e., β 2 = 0.84 and β 1 = 0.16), which is typical for reactions reported to proceed through a stepwise mechanism with a change in rate-determining step. The pK a at the center of the Brønsted curvature, defined as pK q o, has been found to be 8.5 and 9.5 for the reactions of 2 and 1, respectively. Dissection of kN into the microscopic rate constants (e.g., k 1 and k 2/k -1 ratio) has revealed that the reactions of 2 result in larger k 1 values than those of 1, indicating that PhO behaves as a stronger electron-withdrawing group than Ph. However, the k 2/k -1 ratio has been found to be independent of the electronic nature of Ph and PhO.