Kinetics and mechanism of the solid-state reaction between p-substituted anilines and p-benzoquinone

Abstract

Reactions of the p-substituted anilines p-anisidine (I), p-bromoaniline (II) and p-chloroaniline (III) with p-benzoquinone have been studied in the solid state. The reaction products are 1:1 molecular complexes which may be polymeric or agglomeric in nature. DSC studies revealed that the products obtained from solution and by solid-state reactions are not identical in the systems of I and III, whereas in system II they are identical. However, when the reaction products of system I and III obtained from solution are heated at 150°C, the entrapped solvent molecules become liberated and the products obtained from solution and solid-state reactions become identical. Spectroscopic studies show that the reaction products are charge-transfer complexes stabilized by hydrogen bonding. ESR spectroscopic studies confirm the presence of unpaired electrons in the complexes. Electrical conductivity measurements of solid pellets proved that the reaction products have semiconducting properties.

In the course of the reactions it was found that the p-benzoquinone molecules diffuse towards the p-substituted anilines, forming brown-coloured reaction products. Diffusion occurs through surface migration, which is related to the geometry of the molecules. The higher the dipole moment of the p-substituted anilines, the lower the rate of the reaction. Also, the higher the dissociation constants of the p-substituted anilines, the higher the rate of the reaction. The rate of the reactions have confirmed the validity of the Hammett equation. This equation has been verified for the first time for organic solid-state reactions.