Kinetic Study on Aminolysis of 4-Chloro-2-Nitrophenyl X-Substituted-Benzoates in Acetonitrile and in 80 mol % H2O/20 mol % DMSO: Effect of Medium on Reactivity and Reaction Mechanism

Abstract

A kinetic study on aminolysis of 4-chloro-2-nitrophenyl X-substituted-benzoates (6a-i) in MeCN is reported. The Hammett plot for the reactions of 6a-i with piperidine consists of two intersecting straight lines, while the Yukawa-Tsuno plot exhibits an excellent linear correlation with <TEX>${\rho}_X$</TEX> = 1.03 and r = 0.78. The nonlinear Hammett plot is not due to a change in rate-determining step (RDS) but is caused by the resonance stabilization of substrates possessing an electron-donating group in the benzoyl moiety. The Br<TEX>${\phi}$</TEX>nsted-type plot for the reactions of 4-chloro-2-nitrophenyl benzoate (6e) with a series of cyclic secondary amines is linear with <TEX>${\beta}_{nuc}$</TEX> = 0.69, an upper limit for reactions reported to proceed through a concerted mechanism. The aminolysis of 6e in aqueous medium has previously been reported to proceed through a stepwise mechanism with a change in RDS on the basis of a curved Br<TEX>${\phi}$</TEX>nsted-type plot. It has been concluded that instability of the zwitterionic tetrahedral intermediate (<TEX>$T^{\pm}$</TEX>) in MeCN forces the reaction to proceed through a concerted mechanism. This is further supported by the kinetic result that the amines used in this study are less reactive in MeCN than in <TEX>$H_2O$</TEX>, although they are more basic in MeCN over 7 <TEX>$pK_a$</TEX> units.