Aminolysis of 2,4-Dinitrophenyl and 3,4-Dinitrophenyl Benzoates: Effect of ortho-Nitro Group on Reactivity and Mechanism

Second-order rate constants (k(N)) have been measured spectrophotometrically for reactions of 3,4-dinitrophenyl benzoates (5b) with a series of alicyclic secondary amines in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The kinetic data have been compared with the data reported previously for the corresponding reactions of 2,4dinitrophenyl benzoates (5a) to investigate the effect of changing the nucleofuge from 2,4-dinitrophenoxide to 3,4-dinitrophenoxide on reactivity and mechanism. The kinetic results show that aminolyses of 5a and 5b proceed through the same mechanism, i.e., a zwitterionic tetrahedral intermediate (T-+/-) with a change in the rate-determining step (RDS). Substrate 5a is more reactive than 5b when breakdown of T-+/- is the RDS but less reactive when formation of T-+/- is the RDS. Dissection of k(N) values into the microscopic rate constants (e.g., k(1) and k(2)/k(-1) ratio) has revealed that 5a results in larger k(2)/k(-1) ratios but smaller k(1) values than 5b for all the amines studied. Since 2,4-dinitrophenoxide is less basic and a better nucleofuge than 3,4-dinitrophenoxide, the larger k(2)/k(-1) ratios determined for the reactions of 5a than for those of 5b are as expected. The steric hindrance exerted by the ortho-nitro group on 5a contributes to the smaller k(1) values found for the reactions of 5a than for those of 5b.