NITRATION OF PYRIDINE BY DINITROGEN PENTOXIDE, A STUDY OF THE REACTION-MECHANISM

The nitration of pyridine and substituted pyridines by dinitrogen pent oxide (DNP)has been studied, The reaction of DNP with pyridine in eith er liquid SO2 or an organic solvent produces N-nitropyridinium nitrate (4), On reaction of this with aqueous solutions of SO2 or NaHSO3 thre e 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), D elta S-double dagger = 31(4) cal K-1 mol(-1)] and 3-nitropyridine is f ormed, 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 evi dence the reactions were either intramolecular or took place in a solv ent cage, Two mechanisms are found to be in accordance with the report ed evidence: the nitro group either migrated as a nitronium ion in a s olvent cage or by a sigmatropic shift. The results from the nitration of a series of dimethylpyridines support the sigmatropic shift migrati on mechanism.