Phenol Photonitration and Photonitrosation upon Nitrite Photolysis in basic solution

Nitrophenols have been detected in some Antarctic lakes, the water of which is basic and rich in nitrate, nitrite and other nutrients. Nitrate or nitrite photolysis could be a possible reaction to explain the presence of these compounds. This work presents evidence for the formation of 2-nitrophenol (2NP), 4-nitrophenol (4NP) and 4-nitrosophenol (4NOP) upon UV irradiation of phenol and nitrite in aerated basic solutions.

The pH dependence of the 2NP initial formation rate is different from those of 4NP and 4NOP. The dependence of the first mainly reflects the phenol/phenolate equilibrium, with phenol yielding 2NP at a higher rate than phenolate. In the case of 4NOP, the initial formation rate vs pH has a maximum at pH 9.5. The pH dependence of 4NOP formation rate suggests that three pathways are likely to operate: nitrosation of undissociated phenol by N₂O₃, prevailing at pH<8.7, nitrosation of phenolate by N₂O₃, prevailing in the pH interval 8.7–10.8, and reaction between phenoxyl radical and ^?NO, prevailing at pH>10.8. Phenol nitrosation by N₂O₃ is favoured when phenol is negatively charged (phenolate), but it is also disfavoured at alkaline pH values, owing to the depletion of N₂O₃ (the nitrosating agent) by basic hydrolysis. Differently from 2NP, the initial formation rate vs pH of 4NP is very similar to that of 4NOP, suggesting that 4NP may originate from the oxidation of 4NOP. Moreover, while in neutral and acidic solutions the formation rate of 2NP is slightly higher than that of 4NP, in the pH interval 8–12 the formation of 4NP is much more rapid than that of 2NP. This indicates that the pH of natural waters influences the ratio of nitroisomers.