˙OH radical-induced oxidation of chlorobenzene in aqueous solution in the absence and presence of oxygen

Abstract

The reaction of hydroxyl radicals, generated radiolytically in N₂O-containing aqueous solutions [G(˙OH)≈ 5.8 × 10^–7 mol J^–1], with chlorobenzene has been studied. In the presence of Fe(CN)₆^{3 –} the major products (G values in units of 10^–7 mol J^–1 in parentheses) are 2-chlorophenol (2.5), 3-chlorophenol (1.25) and 4-chlorophenol (1.75). Pulse radiolysis with conductometric detection (in the absence of the oxidant) shows that prompt HCl formation (which occurs upon ˙OH radical addition at the ipso-position) is only a very minor process (0.03), and it is concluded that ˙OH radical addition to the ortho : meta : para : ipso positions occurs in a proportion of 1: 0.5 :0.7 : < 0.01, i.e. there is a noticeable preference of attack at the para and ortho positions and a considerable steric hindrance of the addition to the ipso position. In the absence of the oxidant, the yield of the chlorophenols is low [G(total chlorophenols)= 0.4 × 10^–7 mol J^–1] and dimeric products [bis(hydroxychlorocyclohexadienyls)] predominate. Most of these primary dimers are unstable and rearomatize under dehydration, which can be promoted by heating at pH 1, to various dichlorobiphenyl, chlorohydroxybiphenyl and dihydroxybiphenyl isomers.

In the presence of O₂, the chlorohydroxycyclohexadienyl radicals are (reversibly) converted into the corresponding peroxyl radicals (k_forward= 2.6 × 10⁸ dm³ mol^–1 s^–1; k_reverse= 5.5 × 10⁴ s^–1). Out of this equilibrium two reactions occur (k_{product formation}= 1.1 × 10³ s^–1): HO₂-elimination leading to the three isomers of chlorophenol [G(chlorophenol total)= 2.1 × 10^–7 mol J^–1] and formation of endo-peroxidic structures which in turn are converted into peroxyl radicals and subsequently undergo fragmentation. Most of these products, which number in excess of 30 by GC analysis, are free of chlorine [G(chloride ion)= 3.3 × 10^–7 mol J^–1], apart from a few which contain the vinyl chloride function.