Pulse radiolysis studies of aminobenzenesulfonates: Formation of cation radicals

Sulfanilic acid and anilinedisulfonic acids (ADS) react with OH radicals (k = 8.2 × 10⁹ and 5.9 × 10⁹ M^-1 s^-1) to form the corresponding OH adducts. In acid solutions the adducts react with protons to yield cation radicals (k = 5.3 × 10⁸ and 8.7 × 10⁸ M^-1 s^-1). N₃ oxidizes sulfanilic acid directly to the cation radical by an electron-transfer reaction at a diffusion-controlled rate constant, k = 6.5 × 10⁹ M^-1 s^-1, while the rate of oxidation of ADS by N₃ is only 7.6 × 10⁷ M^-1 s^-1. SO₄^- on the other hand oxidizes ADS to the cation radical at a rate of 1.8 × 10⁹ M^-1 s^-1. Both cation radicals deprotonate to the anilino-type radicals in acid-base equilibria. The pK_a of deprotonation of the sulfanilic cation radical is 5.8 ± 0.05 and that of the ADS cation radical is 4.3 ± 0.05.