Volume profiles for reactions of Malachite Green with anionic nucleophiles

Abstract

Volumes of activation and volumes of reaction for the reaction Malachite Green⁺+ Y^–→ Malachite Green-Y have been measured for six different Y^– in eight different solvents. The volumes of activation are negative (–4 to –20 cm³ mol^–1) whereas volumes of reaction are positive (+10 to +30 cm³ mol^–1) which is expected for a reaction involving charge neutralisation. In these solvents, contributions to the volumes of activation from reaction of neutral solvent are absent and the explanation of the negative values is sought in two possible models. Either, the reaction requires an initial anion separation from its counterion with which it exists as a relatively unreactive ion-pair or, the activation process requires rehybridisation and charge concentration on the Malachite Green central carbon leading to an increase in electrostriction during activation.