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

Abstract

Second-order rate constants (<TEX>$k_N$</TEX>) have been measured spectrophotometrically for reactions of 3,4-dinitrophenyl benzoates (5b) with a series of alicyclic secondary amines in 80 mol % <TEX>$H_2O$</TEX>/20 mol % DMSO at 25.0 <TEX>${\pm}$</TEX> 0.1 <TEX>${^{\circ}C}$</TEX>. 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 (<TEX>$T^{\pm}$</TEX>) with a change in the rate-determining step (RDS). Substrate 5a is more reactive than 5b when breakdown of <TEX>$T^{\pm}$</TEX> is the RDS but less reactive when formation of <TEX>$T^{\pm}$</TEX> is the RDS. Dissection of kN values into the microscopic rate constants (e.g., <TEX>$k_1$</TEX> and <TEX>$k_2/k_{-1 }$</TEX> ratio) has revealed that 5a results in larger <TEX>$k_2/k_{-1}$</TEX> 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 <TEX>$k_2/k_{-1}$</TEX> 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.