Kinetics and mechanism of covalent addition of S(IV) species to the acridinium cation

Abstract

The reaction of acridine with S(IV) species (SO2•H2O, HSO3−, and SO32−) to form the adduct acridine–S(IV) has been studied spectrophotometrically throughout the pH range 2.6–8 in aqueous solutions. The observed pseudo-first-order rate constants, kobs, were determined at 25 °C and ionic strength I = 0.11 M, and the pH profile of the rate reached a maximum at pH ≈ 6.1. At constant pH the kobs values were a linear function of the total S(IV) concentration with slopes that increased significantly with pH. These data are consistent with the rate-determining attack of SO3H− and SO32− upon the C-9 of the acridinium cation. A nonlinear least-squares fitting of the experimental values to the model equation, within the overall pH region studied, yields the pH-independent rate constants k1 = 3.7 ± 0.1 and k2 = (6.24 ± 0.04) × 104 M−1 s−1 for the attack of these two species, respectively. The experimental results agree very well with the kinetic model. Due to the experimental conditions used we did not detect any possible pseudobase formation in the pH range studied. The reactivity of the S(IV) species with acridine follows the order: [Formula: see text] The value obtained for the ratio k1/k2 is similar to the results given for other addition reactions of S(IV) species to the double bond of carbonyl compounds such as benzaldehyde and formaldehyde. Key words: covalent addition, acridine, acridine – S(IV) adducts, kinetics and mechanism.