REACTIONS OF OH AND 0- RADICALS
Electron Spin Resonance and Pulse Radiolysis Studies of the Reactions of OH and 0- Radicals with Aromatic and Olefinic Compounds] by P. Neta,*2 Morton Z. H ~ f f m a nand , ~ M. Simic4 Radiation Research Laboratories and Center for Special Studies, Mellon Institute of Science, Carnegie-Mellon University Pittsburgh, Pennsylvania 16215, Department of Chemistry, Boston University, Boston, Massachusetts 02216, Department of Zoology, University of Texas, Austin, Texas 78712, and Pioneering Research Laboratory, U . S. A r m y Natick Laboratories, Natick, Massachusetts 01760 (Received October 6 , 1071) Publication coats assisted by Carnegie-Mellon University and the U . S. Atomic Energy Commission
The reactions of OH and 0- radicals with several aromatic or unsaturated organic compounds in irradiated aqueous solutions have been studied using both electron spin resonance and pulse radioIysis techniques. Each compound examined could react with the radical by addition to a double bond or aromatic ring or by hydrogen abstraction from an aliphatic chain. The relative yields of the radicals produced by the two possible mechanisms were compared at different pH values where either OH (pK, = 11.9) or 0 - is the main reacting radical. The results indicate that the rate of addition of 0- to an aromatic ring or to a double bond is much lower than that of OH, whereas the rates of hydrogen abstraction are comparable. For example, phenylacetic acid undergoes addition of OH to the ring but its reaction with 0 - involves H abstraction from the CH2 group. Similar results were obtained with diphenylacetic acid, o-, m-, and p-methylbenzoic acids, p-methoxybenzoic acid, benzyl alcohol, p-benzenedimethanol, p-phenylenediacetic acid, thymine, and crotonic acid. These findings also suggest the use of 0- for the production of radicals containing a double bond or an aromatic ring which cannot be produced by the reactions of OH or H since these latter radicals add to the unsaturated site in preference to abstraction.
Introduction (Eastman Organic Chemical), benzyl alcohol (Baker Analyzed Reagent), o-, m-,and p-methylbenzoic acid Differences between the reactivities of the neutral (Matheson Coleman and Bell and Aldrich), p-methand the dissociated forms of the hydroxyl radical (OH oxybenzoic acid (K & K Laboratories), p-benzenedi0-, pK = 11.9,611#) have been demonstrated in methanol (Aldrich), p-phenylenediacetic acid (Aldrich), several systems. Whereas most inorganic anions exthymine (Cyclo Chemical), and crotonic acid (Matheamined have been found to react with 0- much more for the pulse radiolson Coleman and Bell). Solutions slowly than with OH,7-9 cations react more r a ~ i d l y , ~ were prepared in triply distilled water ysis experiments and oxygen reacts only with 0- and with a very high and for the esr experiments in doubly distilled water. rate constant.1° Studies on organic compounds are The pH of solutions was adjusted using Baker Analyzed scarce, but generally 0- has been found to react more potassium hydroxide. At pH
