ELECTRON-SPIN-RESONANCE AND PULSE-RADIOLYSIS STUDIES ON THE REACTION OF OH. AND SO4.- WITH 5-MEMBERED HETEROCYCLIC-COMPOUNDS IN AQUEOUS-SOLUTION

The reactions of several five-membered oxygen and nitrogen heterocyclics with OH. and SO4.- radicals have been investigated in aqueous solution using in-situ radiolysis and photolysis ESR, and optical and conductometric pulse radiolysis techniques for detection. OH. reacts with (the saturated) oxazolidone, imidazolidinone, hydantoin, and oxazoline derivatives by hydrogen abstraction, preferentially from the carbon adjacent to the nitrogen atom. With (the unsaturated) oxazoles and isoxazoles, OH. reacts by addition to the carbon at the 5-position of the ring to produce allylic radicals. In basic and acidic solutions of oxazole a ring opening process follows the OH. addition. SO4.- reacts with oxazoles and isoxazoles by addition to C5 yielding SO4- adducts, whereas with imidazole, pyrazole, and pyrrole derivatives, SO4.- gives rise to neutral, conjugated radicals with the unpaired electron delocalized over the entire ring, which are derived from the parent compounds by one-electron oxidation followed by deprotonation. In the case of N-methylimidazole, the radical cation is obtained. The absolute rate constants determined for the reaction of SO4.- with the five-membered heteroaromatics (k = 3 × 108 to 8 × 109 M-1 s-1) reflect the electrophilicity of the SO4.- radical. © 1990 American Chemical Society.