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

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 [Delta H-double dagger = 32(1) kcal mol(-1), Delta S-double dagger = 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) x 10(-4) s(-1) + 3.5(2) x 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.