The mechanism of nitrosation of 4-nitrophenol with aqueous nitrous acid

The nitrous acid catalysed nitration of 4-nitrophenol to give 2,4-dinitrophenol in aqueous solution has been studied under a variety of conditions. At low concentrations of nitrous acid the rate of reaction is dependent upon the concentration of nitrous acid but the rate becomes independent of nitrous acid at high concentrations of nitrous acid when the latter is in excess of 4-nitrophenol concentration. This saturation phenomenon is attributed to the conversion of 4-nitrophenol to its corresponding nitrite ester which then undergoes an acid-catalysed rearrangement to give the accepted normal nitroso -complex. Other kinetic parameters, such as salt effects, acidity dependence, and kinetic isotope effects are very similar under the conditions of high and low concentrations of nitrous acid