Aqueous-phase nitration of phenol by N2O5 and ClNO2

Nitrophenols are present in the atmospheric gas phase and in cloud and rainwater. Their formation via aqueous-phase reactions of phenol with the nitronium ion, NO2+, arising from N2O5 and ClNO2 partitioning into the aqueous phase, has been proposed but not verified experimentally. Here, we demonstrate for the first time that gaseous N2O5 and ClNO2 partitioning into dilute aqueous solutions of phenol yields 2- and 4-nitrophenol (and 4-nitrosophenol), but no dinitrophenol isomers. The rate of nitration does not vary significantly between 5 and 20 °C, presumably because of opposing temperature dependences in Henry's law partitioning and reaction rate coefficients. The rate coefficient for reaction of NO2+ with phenol could not be directly quantified but is evidently large enough for this reaction to compete effectively with the reaction between NO2+ and water and to provide a feasible route to nitrophenol production in the atmosphere.