Reaction of OH with pyridine. Pulse-radiolytic and product-analysis studies

The products obtained from the reaction of OH radicals, generated radiolytically, with pyridine under conditions where the intermediate 1-azahydroxycyclohexadienyl radicals are oxidized to isomeric hydroxypyridines have been examined by high-performance liquid chromatographic methods. From product-analysis data, the initial radiation chemical yields of o-, m-, and p-hydroxypyridines are 1.0, 3.8, and 0.3, respectively. The high yield of m-hydroxypyridine exemplifies the electrophilic nature of OH. The low yield of p-hydroxypyridine is attributed to the failure to quantitatively oxidize the 1-aza-p-hydroxycyclohexadienyl radical by the more powerful oxidant such as IrCl62-. The isomeric OH adducts of pyridine exhibit selectivity in reactivity with the two oxidants examined, and their reducing strength follows the order meta > ortho > para. From optical pulse radiolysis the second-order rate constant for the electron-transfer processes between hexachloroiridate and meta and ortho pyridine-OH adducts was determined to be 5.8 x 108 M-1s-1