Formation of nitrosamines in alkaline conditions: a kinetic study of the nitrosation of linear and cyclic secondary amines by nitroalkanes

Abstract

A study has been made of the nitrosation of sixteen secondary amines, six alkylamines (dimethylamine, diethylamine, dipropylamine, diisopropylamine, dibutylamine, diisobutylamine) and ten cyclic secondary amines (2-methylaziridine, azetidine, pyrrolidine, piperidine, 2-methylpiperidine, homopiperidine, heptamethyleneimine, piperazine, 1-methylpiperazine and morpholine) by nitropropane and nitrobutane in a strongly basic medium ([OH^–]= 0.1 mol dm^–3). The nitrites were not formed in situ(i.e. in the actual bulk of the reaction medium) but rather were isolated, purified and used in pure form. The rate equation (i) was found. v=k_2obs[amine][nitrite](i)

The fitting of the experimental results to the Taft correlation points to a nucleophilic attack on nitrite esters by the amines. Analysis of the log k₂/pK_a and log k₂/E_i(v) correlations indicates orbital control of the reactions studied. These results, together with the fact that the reactivity of the different amines diminishes ostensibly when the values of the ¹³C–H nuclear spin coupling constant in the series of corresponding cycloalkanes increase, show that the overall hybridization of the nitrogen atom in the cycle changes from sp² in the triangular nucleophile methylaziridine to sp³ in larger cycles.

The results obtained at different temperatures and with water–tetrahydrofuran media, together with a study of isotope effects suggest that these reactions occur through a highly ordered transition state and that the role of solvation should not be overlooked.