AQUEOUS CHLORINATION KINETICS AND MECHANISM OF SUBSTITUTED DIHYDROXYBENZENES

The initial chlorination kinetics of several substituted dihydroxybenz enes, including chlorinated resorcinol compounds, was studied over the pH range of 2-12 at 22 degrees C. For each of the resorcinol substrat es, the apparent chlorination rates are a minimum in the pH range of 3 -6 and a maximum at pH values between 8-11. A mechanism that involves the reaction of HOCl with ArX(OH)(2), ArX(OHO-, and ArX(O-)(2), and an acid-catalyzed pathway at pH < 4 was proposed to explain this pH depe ndence. Over natural water pH conditions, the reactions of HOCl with t he anion and dianion forms of resorcinol groups are the most important . Although the intrinsic reactivity of HOCl with resorcinol substrates decreases with the extent of chlorine substitution on aromatic ring, the apparent reactivity of HOCl increases for more chlorinated resorci nols. In the presence of excess HOCl, monochloro- and dichloro resorci nol intermediates, therefore, should not accumulate when resorcinol gr oups undergo chlorine substitution. Linear free energy relationships f or the reactivity of HOCl with resorcinols and phenols were developed. The sequential chlorination kinetics of resorcinol up to trichlorores orcinol can now be modeled.