The chemistry of nitroso compounds. Part 12. The mechanism of nitrosation and nitration of aqueous piperidine by gaseous dinitrogen tetraoxide and dinitrogen trioxide in aqueous alkaline solutions. Evidence for the existence of molecular isomers of dinitrogen tetraoxide and dinitrogen trioxide

Abstract

Detailed quantitative results are reported for the interaction of aqueous piperidine in aqueous 0.1M-NaOH at 25° with gaseous N₂O₄ and N₂O₃. Both reagents rapidly give substantial amounts of N-nitrosopiperidine, plus smaller amounts of N-nitropiperidine in the case of N₂O₄, in addition to hydrolysis products such as NO₂^–. All these reactions are considered to occur predominantly in the aqueous phase and to be complete in a few seconds. With excess amine, yields of N-nitrosopiperidine reach maximum values corresponding to 100% for N₂O₃ but only ca. 50% for N₂O₄. The yield of N-nitropiperidine from N₂O₄, however, shows no maximum even at the highest [Piperidine]. The dependence of product yields on initial [Piperidine] and [N₂O_x] suggests that N-nitrosopiperidine formation follows Rate =k_p[Piperidine][N₂O_x]. The concurrent hydrolysis of N₂O₃ and N₂O₄ is not significantly catalysed by HO^– and is considered to involve H₂O only. On a molar basis, piperidine is more reactive than H₂O towards nitrosation by N₂O₃ and N₂O₄ by factors of 3 300 and 2 000, respectively. The results are discussed in relation to the existence of two molecular isomers for both N₂O₃ and N₂O₄, and the mechanisms by which these entities react with amines. For N₂O₄, the more stable symmetrical O₂N–NO₂ is considered to form only N-nitropiperidine, probably via a four-centre transition state: N-nitrosopiperidine results from concurrent reaction by the less stable ON–ONO₂ isomer formed in aqueous solution by dimerisation of NO₂ from the gaseous phase. For N₂O₃(which is fully dissociated in the gaseous phase) recombination of NO with NO₂ in aqueous solution produces the less stable, symmetrical ON–ONO rather than the more stable ON–NO₂ isomer present in aqueous HNO₂. The existence of two isomers explains the higher reactivity of gaseous N₂O₃ towards weakly basic amines. N-Nitrosopiperidine formation with gaseous N₂O₃ results predominantly from nucleophilic attack by the amine on the ON–ONO isomer. Analysis of the data suggests that the formation of both ON–ONO₂ and ONONO from their radical components in solution may be the rate-limiting step for the reactions leading to N-nitrosopiperidine.