Unusual solvent effects in the reactions of 1-halo-2,4-dinitrobenzenes andaniline in aprotic and dipolar-aprotic solvents. Effects of aggregates

Abstract

The kinetics of reactions between 1-halogeno-2,4-dinitrobenzene (halogen = F, Cl) and aniline were studied in toluene, acetone, dimethylsulfoxide and in toluene-acetone mixtures. Unexpectedly, when chlorine is the nucleofugue, the second rate coefficients, kA, in toluene shows a parabolic dependence on the amine concentration which indicates a third order in amine kinetic law, while in the dipolar aprotic solvent the dependence is linear. Small additions of acetone to the solvent produce an important increase in rate, much greater then the increase in the permittivity, which is surprising on the basis of the classical effect of a dipolar medium on a dipolar transition state. The reactions with fluorine as the nucleofugue show similar behaviours. All these results are well explained in the frame of the "dimer nucleophile" mechanism.