On the effect of pH in aromatic photonitration upon nitrate photolysis

This paper studies the pH effect on the photonitration of catechol, 1-naphthol, naphthalene, and benzene. The pH trend is influenced by the generation of HNO2 and peroxynitrous acid (HOONO) upon nitrate photolysis. HNO2 can be involved in a direct and an indirect nitration process. Direct nitration follows the pH distribution of HNO2 (flexus around 3). Indirect nitration, possibly involving nitrosation + oxidation, would be highest around pH 3. HOONO can be involved in electrophilic nitration, where the initial formation rate of the nitroderivatives is proportional to [H+], or take part in nitration directly, in which case a less important pH effect in photonitration is observed. The relative importance of the various nitration pathways for each substrate determines the resulting pH effect in photonitration upon nitrate photolysis.