Pulse radiolysis study of the reactions of SO₄^•- with some substituted benzenes in aqueous solution

Abstract

The reactions of SO₄^•- with several substituted benzenes having the general formula, C₆H_5-nX_nY (where X = F, Cl or Br and Y = CH₃, CH₂Cl, CHCl₂, CF₃ or OCH₃), have been investigated in aqueous solution by pulse radiolysis. The transient absorption spectra exhibit maxima at 315-330 nm and additional peaks at 270-290 nm with chlorotoluenes and weak peaks around 400 nm with chlorobenzene and 3-chlorofluorobenzene. Only in the case of 3-chloroanisole is the observed spectrum different, exhibiting two distinct peaks at 290 and 475 nm. The second-order rate constants for the reaction of SO₄^•- range from about 10⁸ for 2-chlorobenzotrifluoride to 10¹⁰ dm³mol^-1s^-1 for 3-chloroanisole. It is concluded from the Hammett treatment (ρ ⁺=- 1.6) that the reaction mechanism involves both direct electron transfer and addition-elimination reactions. The intermediate radical cation is hydrolysed to give the corresponding ρ OH adduct absorbing at 315-330 nm except in the case of 3-chloroanisole where it is stabilized. The formation of a benzyl-type radical by direct H abstraction by SO₄^•- from the CH₃ group and/or deprotonation of the radical cation is an additional process whose extent is determined by the relative position of the CH₃ group, the order being para > ortho≈meta with monochlorotoluenes. The transient species absorbing around 400 nm is assigned to the phenoxyl-type radical. The differences in reaction mechanism between SO₄^•- and ^•OH attack are discussed.