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 (KH/KD 2.8) but not for azulene. These results are consistent with the expected A–SE2 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 (pKa–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.