The thermodynamics and kinetics of the proton‐transfer reactions between bromophenol blue and some aromatic amines in aprotic solution

Abstract

AbstractThe equilibria and the stopped‐flow kinetics of the reaction between the acid bromophenol blue (BPB) and several aromatic amines in toluene to form ion pairs have been investigated. Contrary to expectations, the rate of the proton‐transfer increases with increasing activation enthalpy. This is explained in terms of the entropy of the transition state.The role of the electronic structure of the amine and of the solvent are discussed. Solvent effects were further investigated by comparing the activation parameters for the reaction between BPB and pyridine in chlorobenzene, toluene, and benzene. The solvent clearly plays a vital role in determining the rate of proton‐transfer via the entropy of activation. The mechanism of the reaction (that proton‐transfer is almost complete in the transition state, and is followed by amine migration and ring opening) is confirmed. © 1993 John Wiley & Sons, Inc.