Reaction of hydroxyl radicals with 2- and 4-pyridones in aqueous solution. An electron spin resonance and pulse radiolysis study

Abstract

OH radicals react with 2- and 4-pyridone (k = 4.4 x 10/sup 9/ and 3.6 x 10/sup 9/M/sup -1/s/sup -1/, respectively) and with 2- and 4-pyridonecarboxylic acids by preferential attachment to the 3 or 5 position of the pyridone ring. With the exception of 4-pyridone-2,6-dicarboxylic acid, the OH adducts thus formed undergo keto-enol tautomerization to yield OH adducts of the pyridol type (k greater than or equal to 10/sup 3/s/sup -1/). At pH greater than or equal to 10 the OH adducts deprotonate to yield the corresponding radical anions. The ionized OH adduct of 4-pyridone eliminates OH/sup -/(k = 1.8 x 10/sup 4/s/sup -1/) to yield the pyridin-4-oxyl radical which is able to oxidize ascorbate. The pyridin-4-oxyl radical can also be obtained from the OH adduct of 4-pyridone by H/sup +/ assisted dehydration (k = 2.5 x 10/sup 6/M/sup -1/s/sup -1/). Corresponding reactions involving the OH adduct of 2-pyridone were not observed.