KINETICS AND MECHANISM OF COVALENT ADDITION OF S(IV) SPECIES TO THE ACRIDINIUM CATION

The reaction of acridine with S(IV) species (SO2 . H2O, HSO3-, and SO3 2-) to form the adduct acridine-S (IV) has been studied spectrophotome trically throughout the pH range 2.6-8 in aqueous solutions. The obser ved pseudo-first-order rate constants, k(obs), were determined at 25 d egrees C and ionic strength I = 0.11 M, and the pH profile of the rate reached a maximum at pH approximate to 6.1. At constant pH the k(obs) values were a linear function of the total S(IV) concentration with s lopes that increased significantly with pH. These data are consistent with the rate-determining attack of SO3H- and SO32- upon the C-9 of th e acridinium cation. A nonlinear least-squares fitting of the experime ntal values to the model equation, within the overall pH region studie d, yields the pH-independent rate constants k(1) = 3.7 +/- 0.1 and k(2 ) = (6.24 +/- 0.01) x 10(4) M(-1) s(-1) for the attack of these two sp ecies, respectively. The experimental results agree very well with the kinetic model. Due to the experimental conditions used we did not det ect any possible pseudobase formation in the pH range studied. The rea ctivity of the S(IV) species with acridine follows the order: SO32- mu ch greater than HSO3- much greater than SO2 . H2O. The value obtained for the ratio k(1)/k(2) is similar to the results given for other addi tion reactions of S(IV) species to the double bond of carbonyl compoun ds such as benzaldehyde and formaldehyde.