The kinetics and mechanisms of the reactions of aluminium(III) with gallic acid, gallic acid methyl ester and adrenaline

Abstract

The kinetics and mechanisms of the reactions of gallic acid, gallic acid methyl ester and adrenaline with aluminium(III) have been investigated in aqueous solution at 25°C and an ionic strength of 0.5 M. A mechanism has been proposed which accounts satisfactorily for the kinetic data. This is consistent with a mechanism in which complex formation takes place almost exclusively by reaction of [Al(H 2O) 5OH] 2+ with the ligands. [Al(H 2O) 5OH] 2+ reacts with gallic acid, gallic acid methyl ester and adrenaline with rate constants of 1145, 1330 and 316 M −1 s −1 respectively. These data together with the equilibrium data enable the rate constants for reaction of [Al(H 2O) 6] 3+ with both gallic acid and gallic acid methyl ester to be calculated. In view of the dissociative nature of water exchange on [Al(H 2O) 6] 3+ and [Al(H 2O) 5(OH)] 2+ the complex formation rate constants are discussed in terms of the Eigen–Wilkins–Tamm mechanism. The overall mechanisms have been validated using global analysis. The results are compared with previously published data on the complex formation reactions of aluminium(III). In addition, the rate constants and mechanisms for replacement of maltol by gallic acid methyl ester and diethylenetriaminepentaacetic acid (dtpa) have been investigated.