The chemistry of nitroso-compounds. Part VII. The first ‘fast’ proton transfer for an aromatic nitrosation
Rates of nitrosation are reported for phenol and 2-naphthol in aqueous carboxylic acid buffers over the pH range 1–ca. 5·5 at 25 °C. Substitution of phenol occurs predominantly at the para-position at rates which are pH independent, but general-base catalysed, below pH ca. 4·5, and comparative experiments with [4-2H]phenol show the existence of a substantial primary hydrogen isotope effect (kH/kD= 3·5). Above pH ca. 4·5, however, the kinetic dependences for phenol show significant changes. These differences are more easily seen with 2-naphthol, which undergoes nitrosation exclusively at the 1-position. Here reaction rates are pH independent, subject to general-base catalysis, and to substantial primary hydrogen isotope effects (kH/kD= 4·0) only below pH ca. 2. At higher pH, the rate is proportional to [H3O+], and both abse catalysis and isotope effects diminish substanitally. All these observations are consistent with a common A–SE2 reaction mechanism in which proton expulsion from a dienone intermediate is rate limiting for phenol at pH ca. 4·0, so this is the first-known aromatic nitrosation for which proton expulsion is rapid. Neither nitrous anhydride (N2O3) nor nitrosyl acetate (NOOAc) is sufficiently reactive to substitute the nucleus of phenol or 2-naphthol and reaction at pH 1–ca. 5 probably involves the nitrous acidium ion (H2NO2+); with halide ions present, additional reaction via nitrosyl halides occurs. The pH dependent rate for 2-naphthol excludes significant nitrosation of the 2-naphtholate ion up to pH ca. 5, but this species may preferentially interact at oxygen to form an unstable aryl nitrite. The implications of this deduction on the ambident nucleophilic properties of phenolic compounds, and the incursion of free radical substitution pathways, are also discussed.