Kinetics and Mechanism of Uncatalysed Substitution of Cyanide in Hexacyanoferrate(Ii) by Isoniazid

Abstract

The kinetics and mechanism of the uncatalysed ligand substitution reaction between potassium ferrocyanide, K4[Fe(CN)6] and isoniazid (INH) in aqueous medium has been investigated spectrophotometrically at 435 nm, i.e. the Λmax of the golden yellow complex, [Fe(CN)5(INH)]3-, as a function of [Fe(CN)64-] and [INH] under the conditions: pH = 3.5 ± 0.02, temperature = 30 ± 0.1°C and ionic strength, I = 0.05 mol/l (KNO3). The reaction is first order each in [Fe(CN)64-] and isoniazid (INH) at low [INH]. The initial rate of reaction is determined from the slopes of absorbance versus time plots. As [INH] increases, the rate of reaction increases, passes through a maximum and then falls, suggesting that the course of the reaction is different at low and high [INH]. A tentative mechanism has been proposed, which is further supported by the results of studies, such as variation in [Fe(CN)64-], effect of temperature and pH of the reaction medium. The composition of the complex formed in the present reaction has been established to be 1:1 by the mole ratio and slope ratio methods. A repetitive spectral scan is also provided in support of the exchange of coordinated cyanide ions by isoniazid in hexacyanoferrate(II).