Nitration of pyridine by dinitrogen pentoxide, a study of the reaction mechanism†

Abstract

The nitration of pyridine and substituted pyridines by dinitrogen pentoxide (DNP) has been studied. The reaction of DNP with pyridine in either liquid SO₂ or an organic solvent produces N-nitropyridinium nitrate (4). On reaction of this with aqueous solutions of SO₂ or NaHSO₃ three transient species are formed: N-nitro-1,4-dihydropyridine-4-sulfonic acid (5), N-nitro-1,2-dihydropyridine-2-sulfonic acid (6) and 1,2,3,6-tetrahydro-3-nitropyridine-2,6-disulfonic acid (7). Compounds 6 and 7 may be sulfite esters instead of sulfonic acids. Compound 5 reacts by a first order reaction [ΔH^‡ = 32(1) kcal mol^–1, ΔS^‡ = 31(4) cal K^–1 mol^–1] and 3-nitropyridine is formed. Compound 6 is rapidly transformed to 7 which reacts by a first order pH dependent reaction {k_obs = 1.9(4) × 10^–4 s^–1 + 3.5(2) × 10^–2[H⁺] M^–1 s^–1} to give 3-nitropyridine. From the available evidence the reactions were either intramolecular or took place in a solvent cage. Two mechanisms are found to be in accordance with the reported evidence: the nitro group either migrated as a nitronium ion in a solvent cage or by a sigmatropic shift. The results from the nitration of a series of dimethylpyridines support the sigmatropic shift migration mechanism.