Addition and elimination kinetics in OH radical induced oxidation of phenol and cresols in acidic and alkaline solutions

The rates of the two consecutive reactions, OH radical addition and H2O/OH- elimination, were studied by pulse radiolysis in highly acidic (pH = 1.3-1 .9) and alkaline (pH approximate to 11) solutions, respectively, for phenol and for the three cresol isomers. The rate coefficient of the addition as measured by the build-up of phenoxyl radical absorbance and by a competitiv e method is the same (1.4 +/- 0.1) x 10(10) mol(-1) dm(3) s(-1) both in aci dic and alkaline solution. The rate coefficient of the H2O elimination in a cidic solution is (1.6 +/- 0.2) x 10(6) s(-1), whereas the coefficient of t he OH- elimination in alkaline solutions is 6-8 times higher. The kinetics of the phenoxyl radical formation was described by the two-exponential equa tion of the consecutive reactions: the first exponential is related to the pseudo-first-order addition, while the second to the elimination reaction. No considerable structure dependence was found in the rate coefficients, in dicating that the methyl substitutent in these highly acidic or alkaline so lutions influences neither the addition nor the elimination rate. (C) 1999 Elsevier Science Ltd. All rights reserved.