CARBON-DIOXIDE MODULATION OF HYDROXYLATION AND NITRATION OF PHENOL BYPEROXYNITRITE

We have examined the formation of hydroxyphenols, nitrophenols, and th e minor products 4-nitrosophenol, benzoquinone, 2,2'-biphenol, and 4,4 '-biphenol from the reaction of peroxynitrite with phenol in the prese nce and absence of added carbonate. In the absence of added carbonate, the product yields of nitrophenols and hydroxyphenols have different pH profiles. The rates of nitration and hydroxylation also have differ ent pH profiles and match the trends observed for the product yields. At a given pH, the sum of the rate constants for nitration and hydroxy lation is nearly identical to the rate constant for the spontaneous de composition of peroxynitrite. The reaction of peroxynitrite with pheno l is zero-order in phenol, both in the presence and absence of added c arbonate. In the presence of added carbonate, hydroxylation is inhibit ed, whereas the rate of formation and yield of nitrophenols increase. The combined maximum yield of o- and p-nitrophenols is 20 mol% (based on the initial concentration of peroxynitrite) and is about fourfold h igher than the maximal yield obtained in the absence of added carbonat e. The o/p ratio of nitrophenols is the same in the presence and absen ce of added carbonate. These results demonstrate that hydroxylation an d nitration occur via two different intermediates. We suggest that the activated intermediate formed in the isomerization of peroxynitrous a cid to nitrate, ONOOH, is the hydroxylating species. We propose that intermediate 1, O=N-OO-CO2-, or secondary products derived from it, is (are) responsible for the nitration of phenol. The possible mechanism s responsible for nitration are discussed. (C) 1997 Academic Press.