Kinetic study of the nitrosation of 3-substituted indoles

Kinetic studies of the nitrosation reaction of three 3-substituted indoles (3-methylindole, indol-3-yl acetate and indole-3-acetic acid) show that the final state is an equilibrium between the reactants (nitrous acid and indole derivative) and the 1-nitroso derivative. Values of the rate constants for the nitrosation of the three indoles and denitrosation of the corresponding nitrosoindoles as well as values of the equilibrium constants have been obtained. The almost complete insensitivity of the reaction rates to medium acidity, the absence of catalysis by the usual catalysts of nitrosation (halides) and the high reactivity at low acidities, are in contrast to the kinetic characteristics of other N-nitrosation reactions. This atypical behaviour is discussed in terms of possible reaction mechanisms.