Pulse radiolysis study of the reactions of SO ˙–4 with some substituted benzenes in aqueous solution

Abstract

The reactions of SO^˙–₄ 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 SO^˙–₄ 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 ˙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 SO^˙–₄ 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 SO^˙–₄ and ˙OH attack are discussed.