OH RADICAL-INDUCED OXIDATION OF CHLOROBENZENE IN AQUEOUS-SOLUTION IN THE ABSENCE AND PRESENCE OF OXYGEN

The reaction of hydroxyl radicals, generated radiolytically in N,O-con taining aqueous solutions [G((OH)-O-.) approximate to 5.8 x 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-chl orophenol (1.75), Pulse radiolysis with conductometric detection (in t he absence of the oxidant) shows that prompt HCl formation (which occu rs upon (OH)-O-. radical addition at the ipso-position) is only a very minor process (0.03), and it is concluded that (OH)-O-. radical addit ion 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 th e pava and ortho positions and a considerable steric, hindrance of the addition to the ipso position, In the absence of the oxidant, the yie ld of the chlorophenols is low [G(total chlorophenols) = 0.4 x 10(-7) mol J(-1)] and dimeric products [bis(hydroxychlorocyclohexadienyls)] p redominate, Most of these primary dimers are unstable and rearomatize under dehydration, which can be promoted by heating at pH 1, to variou s dichlorobiphenyl, chlorohydroxybiphenyl and dihydroxybiphenyl isomer s. In the presence of O-2, the chlrohydroxycyclohexadienyl radicals ar e (reversibly) converted into the corresponding peroxyl radicals (k(fo rward) = 2.6 x 10(8) dm(3) mol(-1) s(-1); k(reverse)= 5.5 x 10(4) s(-1 )). Out of this equilibrium two reactions occur (k(product formation) = 1.1 x 10(3) s(-1)): HO2-elimination leading to the three isomers of chlorophenol [G(chlorophenol total) = 2.1 x 10(-7) mol J(-1)] and form ation of endo-peroxidic structures which in turn are converted into pe roxyl radicals and subsequently undergo fragmentation. Most of these p roducts, which number in excess of 30 by GC analysis, are free of chlo rine [G(chloride ion) = 3.3 x 10(-7) mol J(-1)], apart from a few whic h contain the vinyl chloride function.