Hydroxylation of chlorotoluenes and cresols: a pulse radiolysis, laser flash photolysis, and product analysis study

Abstract

The reactions of ^•OH, O^•- and SO₄^•- with 2-, 3-, and 4-cresols were studied by pulse radiolysis, laser flash photolysis, and product analysis techniques. The rates of OH reaction with cresols are very high (k ≈ 1 × 10¹⁰ M^-1 s^-1), whereas O^•- was found to be less reactive (k ≈ 2.4 × 10⁹ M^-1 s^-1). The second-order rate constants for SO₄^•- reaction with cresols are in the range (3-6) × 10⁹ M^-1 s^-1. The transient absorption spectra measured in OH reaction exhibited peaks in the range 295-325 nm with a red shift for the meta isomer. The absorption spectra obtained for O^•- reaction with 2-cresol has a peak at 360 nm, which is different from those measured with the m and p isomers (λ _max = 310 and 380 nm). The absorption spectra of the transient species in SO₄^•- reaction obtained by pulse radiolysis and flash photolysis techniques are similar, with absorption maxima centered around 290 and 390 nm in all three isomers. The intermediates formed in ^•OH, O^•-, and SO₄^•- reactions are assigned to OH adducts, substituted benzyl radicals, and radical cations, respectively. The rates for oxygen addition to OH adducts of 2-, 3-, and 4-chlorotoluenes and cresols are high, with k_f values lying in the range (1-2.7) × 10⁸ M^-1 s^-1. The relatively higher stability constants of peroxyl radicals formed with cresols (K = k_f/k_r = (2-5) × 10⁴ M^-1) than with chlorotoluenes suggest that the product-forming reaction competes effectively with the reverse reaction in cresols. Dihydroxytoluenes with OH groups ortho to each other were only formed in oxygenated solutions of cresols, and the mechanism involves the addition of oxygen to 1,3-type OH adducts at the carbon carrying the hydroxyl function.