Azide ion trapping and lifetime in aqueous solution of a primary carbenium ion stabilized by a 2-imidazoyl ring

Abstract

2-Chloromethyl-1-methylimidazole undergoes a pH-dependent aqueous hydrolysis with the neutral substrate being the reactive species, and the imidazole-protonated form (pKa = 5.7) unreactive. Addition of sodium chloride retards the hydrolysis, evidence that there is a free carbenium ion intermediate (the common ion effect). The rate constant ratio Kcl/Kw for the reactions of this cation with the added chloride and with the solvent is 7.4 M−1. Further evidence for a free cation is the observation of the 2-azidomethyl product when the hydrolysis is carried out with sodium azide present, but with no change in the rate constant. The Kaz/Kw ratio as determined by product analysis is 1.1 × 102 M−1 With the assumption that kaz represents a diffusion-controlled reaction and has a value of 7 × 109 M−1 s−1, the rate constant kw for the reaction of the cation with solvent is 6 × 107 s−1. A comparison with azide–water selectivity ratios reported for other cations shows that the imidazole-stabilized primary cation of this study is relatively long-lived. A possible explanation for this is given, in terms of the extensive resonance delocalization of the positive charge in this cation. Keywords: solvolysis, carbenium ion, heterocycle.