Reaction of phenol with nitrite ion: pathways of formation of nitrophenols in environmental waters

When sodium nitrite is spiked into phenolic waters, 2-nitrophenol is produced immediately while 4-nitrophenol forms in minor yield after an induction period. The reaction occurs at ambient temperature under acidic, neutral and alkaline conditions. The reaction also occurs in the dark. The nature and yield of the products formed in the dark are analogous to that obtained under ordinary light. Results show that the formation of 2-nitrophenol is first order in nitrite, however, within a narrow range of concentrations. No such order of reaction could be determined for 4-nitrophenol, the yield of which increases in presence of excess nitrite ions and at high nitrite to phenol ratios. Results of this study indicate that NO2− is the active species in the reaction, attacking the phenol ring at the ortho position and at the same time undergoing a para-substitution in excess nitrite atmosphere. Neither any photo-induced oxidation nor any nitronium ion, NO2+ formation is involved as active species in the nitration of phenol. Our study indicates that in environmental waters a simple contact of nitrite ion with phenolic wastewaters under ambient conditions may form mononitrophenols, which are toxic to aquatic species.