The chemistry of nitroso-compounds. Part X. Acid catalysed nitrosation of azulene and 1-nitroazulene

Abstract

Rates of nitrosation in dilute aqueous HCIO₄ and HCI under various conditions are reported for azulene (to form the 1-nitroso-derivative) and 1-nitroazulene. Both reactions show a first-order dependence on substrate, nitrous acid, and solvent acid concentrations, and for azulene the reaction is further catalysed by chloride ion. A primary kinetic hydrogen isotope effect is observed for 1-nitroazulene (K_H/K_D 2.8) but not for azulene. These results are consistent with the expected A–S_E2 pathway characteristic of aromatic electrophilic substitution, where encounter of the reactants is rate limiting for azulene, and proton loss from the Wheland intermediate is rate limiting for 1-nitroazulene. Activation parameters for azulene (ΔH 33 ± 5 kJ mol^–1; ΔS 45 ± 7 J K^–1 mol^–1) are consistent with an encounter-controlled rate. These findings confirm recent deductions from the nitrosation of indoles that highly reactive substrates (pK_a–3.5) react on encounter with NOCI and H₂ONO⁺, and suggest that the high reactivity of indoles is not a consequence of reaction at the heterocyclic nitrogen atom.