PULSE-RADIOLYSIS STUDY OF THE REACTIONS OF SO4-RADICAL-ION WITH SOME SUBSTITUTED BENZENES IN AQUEOUS-SOLUTION

The reactions of SO4.- with several substituted benzenes having the general formula, C6H5-nXnY (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 10(8) for 2-chlorobenzotrifluoride to 10(10) dm3 mol-1 s-1 for 3-chloroanisole. It is concluded from the Hammett treatment (rho+ = -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 SO4.- from the CH3 group and/or deprotonation of the radical cation is an additional process whose extent is deter-mined by the relative position of the CH3 group, the order being para > ortho almost-equal-to 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.