Absolute rate constants for reaction of Sod.-with substituted benzenes and benzoates have been determined by pulse radiolysis. The values are found to range from about 5 X lo9 M-' s-I for anisole to < I O 6 M-' s-I for nitrobenzene. A correlation of the rate constants with the Hammett substituent constant u gave p = -2.4 for both series of compounds. It is concluded that the reaction takes place by an electron transfer from the ring to so4*-.The reaction of the sulfate radical anion, SO4.-, with several aromatic compounds has been shown by ESR to result in the production of hydroxycyclohexadienyl radicak2 In the case of methoxy derivatives radical cations have been observed3 and in the case of several carboxy derivatives decarboxylation has been found to take place, resulting in the production of phenyl radicah4q5 These observations can be explained by various mechanisms. The hydroxycyclohexadienyl radicals can be produced by addition to the ring followed by hydrolysis as suggested by Norman et al.? or by electron transfer from the ring to the SOY-, as suggested by several authors,6-8 followed by hydroxylation with water or OH-. The radical cations from anisoles can also be produced by direct electron transfer3 or via addition followed by elimination. Decarboxylation can occur through direct oxidation of the carboxyl group, as is the case with aliphatic carboxylic acids,2s8 or again following oxidation of the ring.5 It appeared that examination of the effect of substituents on the rate constant for reaction of s04.-with aromatic compounds might shed some light on the mechanism.Absolute rate constants for reactions of SO4--radical can be determined by kinetic spectrophotometric pulse radiolysis as described p r e v i o u~l y .~-~~ The pseudo-first-order decay rate of the SO4.-absorption is measured in the presence of increasing concentrations of the aromatic compound and the second-order rate constant calculated from a linear plot of the rate vs. the concentration. The rate constants were found to vary from 5 X lo9 M-I s-l for anisole down to

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Publication costs assisted by the Department of EnergyThe products of the reactions of SO4-with some benzoic acid derivatives have been examined by ESR using steady-state radiolysis and photolysis to produce SO4-and by optical absorption spectrophotometry in pulse radiolysis experiments. The optical absorption spectrum of the hydroxycyclohexadienyl radical following reaction of benzene with SO4-is shown to be formed with

Publication costs assisted by Carnegie-Meilon University and the U. S. Energy Research and Development AdministrationESR spectra of a number of phenyl and 2-pyridyl radicals have been detected in aqueous solution. Two methods of radical production were used, namely, reaction of SO4.-produced by photolysis of &Os2-with aromatic carboxylate ions and reaction of eaq-produced by radiolysis with aromatic bromides. Most radicals had only carboxyl groups as further substituents. The proton hyperfine constants of the phenyl radicals, a, -17, um -6, and up -2 G, are readily assigned on the basis of effects of the various substitutions and agree well with previous determinations for radicals in the solid state. With four phenyl radicals containing two to four carboxyl groups it was also possible to detect all of the possible 13C containing isomers at natural abundance. A hyperfine constant of -135 G for the carbon at the radical site confirms the u nature of phenyl radical. The other ring 13C hyperfine constants are uoc -7.5, urnc -13, and upc -1.5 G.Both 14N and proton hyperfine constants were determined for the 2-pyridyl radicals. Only the 14N value (-27 G) was known previously. The g factors of phenyl and 2-pyridyl radicals are low (near or below the free electron value) reflecting their u nature. The presence of carboxyl groups in a phenyl radical affects the g factor in an additive fashion with increments of 13, -2, and -3 units in the fifth decimal place for ortho, meta, and para substituents, respectively. The observation of phenyl and 2-pyridyl radicals demonstrates in a direct way that these radicals are formed in the two above-mentioned reactions. (One example each of a 4-pyridyl and a pyrazyl radical were also produced and studied.) Phenyl and 2-pyridyl radicals with no ortho carboxyl group were found to be very reactive toward addition to another aromatic molecule and were best detected using low concentrations (5 X lov4 M ) of aromatic bromide. Spectra of the unsubstituted phenyl and 2-pyridyl radicals were obtained in this way from the corresponding bromides. At higher solute concentrations such as used for the decarboxylation reactions these radicals react further to form adducts of the cyclohexadienyl type. Radicals with an ortho carboxyl group (for example, 2-carboxylphenyl radical produced from phthalate) are much less reactive toward addition and could be studied at higher concentrations of the starting compound. Reaction of SO4--with acids in which three (or more) adjacent carboxyl groups are present leads preferentially to loss of a central or interior carboxyl group. To provide data for comparison with earlier indirect studies, a number of adducts of phenyl and pyridyl radicals to CH2=N02-and trimesate were studied and their hyperfine constants determined. In the latter cases it was found that the ortho and meta protons in the phenyl radical which has added (i.e., the side group in the final cyclohexadienyl radical) can produce splittings of -0.05 and -0.3 G, respectively. The radicals detected in exp...