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 % H2O/20 mol % DMSO at 25.0+/-0.1 degrees 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 I. The Bronsted-type plot for the reactions of 2 exhibits a downward curvature (i.e., beta(2) = 0.84 and beta(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 Bronsted curvature, defined as pK(a)(o), has been found to be 8.5 and 9.5 for the reactions of 2 and 1, respectively. Dissection of k(N) 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 ratio has been found to be independent of the electronic nature of Ph and PhO.