Aminolysis of 2,4-dinitrophenyl and 3,4-dinitrophenyl benzoates: Effect of ortho-nitro group on reactivity and mechanism

Abstract

Second-order rate constants (kN) 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°C. The kinetic data have been compared with the data reported previously for the corresponding reactions of 2,4-dinitrophenyl 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 kN values into the microscopic rate constants (e.g., k1 and k 2/k-1 ratio) has revealed that 5a results in larger k 2/k-1 ratios but smaller k1 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 k2/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 k1 values found for the reactions of 5a than for those of 5b.