The stabilities of Meisenheimer complexes. Part 41. Kinetic studies of adduct formation and nucleophilic substitution in the reactions of hydroxide ions with 1-X-3,5-dinitrobenzenes and with 1-X-4-chloro-3,5-dinitrobenzenes (X = H, CF3, CO2–) in water–dimethyl sulphoxide mixtures. The effect of a chloro-substituent on the rate of nucleophilic attack

Abstract

Kinetic and equilibrium measurements are compared for the reactions of (1) and (4)(X = CF₃, CO₂^–, H) with hydroxide ions in dimethyl sulphoxide–water mixtures. The adducts (3), formed from (1), have higher thermodynamic stabilities than their isomers (2) although there is little kinetic discrimination in the rates of hydroxide attack at the three unsubstituted positions. The presence in (4) of the chlorine atom results in a general reduction in the rate of nucleophilic attack, the effect being particularly large for ipso-reaction (attack at the 4-position). Hence the reaction of (4) with base results in the initial formation of adducts (5). Contrary to a recent report the intermediates (6) on the reaction pathway to the phenols (7) are not sufficiently stable to allow spectrophotometric detection.