P. Neta and Richard W. Fessenden
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constant (within reasonable limits) to a particular neutralization reaction. These considerations also apply to the absolute yields of conductivity changes obtained from the pulse experiments, In addition to the ions resulting from the existing equilibria (eq 1 and 2, and to a minor extent also from others4,5), the reaction products including the Hg2C12 (the latter according to its solubility prod-
uct) contribute to the final conductivity to an uncalculable ektent.
Acknowledgment. The authors are very grateful to Mrs. M. Schoner for experimental assistance. This work has been supported by funds made available from the Verband der Chemischen Industrie.
lectron Spin Resonance Study of Radical Anions from Aromatic Carboxylic Acids' Nets* and Richard W. Fessenden Radiation Research Laboratories, Center tor Special Studies and Department of Chemistry, Mellon institute of Science, Carncyie-Mellon University, Pittsburgh, Pennsylvania 75273 (Received September 25, 7972) Publication costs assisted by the Carnegie-Mellon University and the U S . Atomic Energy Commission
Radical anions produced by the reaction of hydrated electrons with benzoic, phthalic, isophthalic, terephthalic, trimesic, pyromellitic, mellitic, and biplienyldicarboxylic acids in aqueous solutions have been studied by esr. Solutions containing the aromatic compound together with a scavenger for OH radicals were irradiated by 2.8-MeV electrons while flowing through the esr cavity and the spectra were recorded under steady-state conditions. The acid-base properties of the radicals have been examined and in most cases several different stages of protonation were identified. All protonations of the radical anions were found to take place on the carboxyl groups and not on the ring. The loss of the last dissociable proton from each electron adduct was found to occur a t a pH much higher (at least 4 units) than in the case of the parent molecules. The presence of two ortho carboxyl groups was found to result in a strong hydrogen-bonded bridge which, in the case of phthalate, does not dissociate even at pH 14.
Introduction An electron spin resonance study of radical anions from aromatic carboxylate ions in liquid ammonia has been reported.2 The spectra were interpreted in terms of radicals with one more unit of negative charge than in the parent carboxylate ion. These radicals, when produced in aqueous solutions, are expected to protonate at low pH values. In a recent pulse radiolysis study of the radical anion from benzoate it was concluded that two successive protonations take place on the carboxyl group with pK, values of 5.3 and 12.0.3 An esr study of this radical could reveal additional information on its structure. The present study was undertaken for this reason and was extended to the polycarboxy derivatives. In recent studies4,s of radical anions from olefinic carboxylate ions a major difference has been found between radicals from maleate and fumarate, The cis isomer forms a strong intrainolecular hydrogen bond which resists dissociation even in 1 M base,4 whereas the trans isomer is completely dissociated at p H above 11. In parallel with these findings one expects the radical anion from phthalate to form a strong hydroen bridge between the two carboxyl groups with a high pK, while lower pK, values are predicted for the other isomers. These predictions will be examined both in the dicarboxybenzenes and also in pyromellitic and mellitic acids. The Journal of Physical Chemistry, Vol. 77,No. 5, 1973
Experimental Section Benzoic acid was a Baker Analyzed Reagent, phthalic and isophthalic acids were obtained from Eastman, the biphenyldicarboxylic acids from Chemicals Procurement Laboratories, and the other acids from Aldrich. All these compounds were of the purest grade commercially available and were used without further purification. tertButyl alcohol was of Mallinckrodt AR grade. Sodium formate and all the inorganic compounds were Baker Analyzed Reagents. Solutions were prepared in water which was distilled and the vapor passed with oxygen through a silica tube at 600" to destroy all organic impurities. The pH was adjusted using KOH, HC104, NazB407, KHzP04, and KzHP04, the latter three being used as buffers. Solutions were deoxygenated by bubbling with pure nitrogen and were irradiated with 2.8-MeV electrons directly in the esr cavity. All the other details of the experiment were as described previously.6 (1) Supported in part by the U. S . Atomic Energy Commission.
(2) A. R. Buick, T. J. Kemp, G . T. Neal, and T. J. Stone, J. Chem. SOC. A . 2227 (1970). ( 3 ) M. Simic and M. Z . Hoffman, J. Phys. Chem.. 76,1398 (1972). (4) P. Neta and R . W. Fessenden, J. Phys. Chem.. 76,1957 (1972). (5) N. H . Anderson, A. J. Dobbs, D. J. Edge, R. 0. C . Norman, and P. R. West,J. Chem. SOC.6, 1004 (1971). (6) I
