PHOTONITRATION AND PHOTONITROSATION OF PHENOLIC DERIVATIVES INDUCED IN AQUEOUS-SOLUTION BY EXCITATION OF NITRITE AND NITRATE IONS

The excitation of nitrite and nitrate ions in the presence of phenolic compounds may induce oxidation, nitration and nitrosation. Oxidation occurs whatever the substituents and can be attributed to hydroxyl radicals formed in the photolysis of ions. Nitration and nitrosation are not observed in the presence of electron-withdrawing substituents. When nitrite ions are excited in the presence of phenol or resorcinol the observed para nitrosation can be attributed to nitrogen oxides formed in the photolysis of NO2-. It is enhanced by increasing nitrite concentration. From the influence of oxygen it is concluded that two mechanisms compete, one involving N2O3 and the other the radical adduct phenol...NO2.. This reaction does not occur with pyrocatechol and hydroquinone. With these substrates nitration is observed but mainly in the presence of oxygen. Reactions are more complex when nitrate ions are excited in the presence of phenol or resorcinol. The formation of nitro and nitroso derivatives is attributed to the nitrogen oxides NO2. and N2O4. With pyrocatechol and hydroquinone, nitrosation does not occur and nitration is favoured by deoxygenation. In environmental conditions nitration or nitrosation by excitation of NO2- are highly unlikely. Their occurence upon excitation of NO3- depends on the concentration of phenolic compounds and on the pH of the solution.