The chemistry of nitroso-compounds. Part IV. Acid-catalysed nitrosation of para-substituted phenols

Abstract

Rates of nitrosation at 0 °C are reported for p-cresol in 0·01–9·3M-HClO4, for [2,6-2H2]-p-cresol in 0·01–5·7M-HClO4 and for four other para-substituted phenols in 6·8M-HClO4. Only 2-nitro-products are obtained and these result from an initial rate-limiting nitrosation followed by rapid oxidation rather than from a direct nitration reaction. The rate for p-cresol is virtually constant below 0·5M-HClO4, but thereafter increases rapidly with acidity to reach a maximum in 6·8M-HClO4; the reaction also shows a substantial deuterium isotope effect. These facts suggest that an A–SE2 mechanism via a dienone intermediate is involved, in which expulsion of the proton is slow. Rates of nitrosation are increased moderately by electron-donating para-substituents. The Hammett plot for these data is linear with ρ=–6·2. Thus low ρ values appear to be characteristic of aromatic nitrosation and other aromatic electrophilic substitutions in which proton loss from the Wheland intermediate is slow.