Mechanism of the complexation reaction of aluminum chloride with o-hydroxy-benzophenone

Abstract

A theoretical and experimental study on the formation reaction of the complex between AlCl 3 and 2-hydroxy-benzophenone in primary alcohols was performed. The influence of the temperature, the ionic strength and the solvent on the reaction rate were investigated. Under the adopted experimental conditions the stoichiometric composition of the formed complex is 1:1. In the theoretical study, thermodynamic magnitudes, torsional angles, length bonds, energy and reactivity indices were used. The calculations were performed at the RHF/3-21G and RHF/6-31G(d) levels of theory, using the Onsager's and COSMO methods. To explain the kinetic results obtained we proposed a reaction mechanism that includes the acid–base dissociation of 2-hydroxy-benzophenone, the reaction between AlCl 3 and a solvated proton to form the AlCl 2 + cation, and the formation of the metal complex by an ionic reaction between the 2-hydroxy-benzophenonate anion and the metal cation. This reaction between two ions of opposite charges is the rate-determining step of the complexation reaction analyzed. On the other hand, the theoretical conclusion on the greater planarity of the molecules of the complex with respect to the free molecules of 2-hydroxy-benzophenone is in agreement with the batochromic shift experimentally observed. Also, the proposed mechanism satisfactorily explains the global rate of the second order reaction, and the dependence of the rate constant with the temperature, ionic strength and permittivity of the reaction medium.