The chemistry of nitroso-compounds. Part III. The nitrosation of substituted benzenes in concentrated acids

Abstract

Rates of nitrosation are reported for benzene, toluene, diphenyl ether, anisole, and phenol in 10·4M-HClO₄ at 52·9 °C, for benzene in 12·2M-H₂SO₄, and for [²H₆]benzene in 12M-D₂SO₄. The effective reagent under these conditions is NO⁺ and substitution occurs predominantly at the para-position of monosubstituted substrates. Benzene reacts about 50 times more rapidly in H₂SO₄ than in HClO₄ of the same H₀ acidity, and about 8·5 times more rapidly than [²H₆]benzene in D₂SO₄, which shows that H⁺-loss from the Wheland intermediate is ratelimiting. Rates of nitrosation are moderately increased by electron-donating substituents and log (partial rate factors) for the para-position correlate with σ⁺ parameters to give ρ=–6·9. This relatively low ρ value is consistent with the rate-limiting step suggested. The reactivity of NO⁺ is discussed and it is shown to be at least 10¹⁴ times less reactive than NO₂⁺. The significance of the findings to recent explanations of aromatic reactivity in nitrations is also discussed.