Micellar effects upon the reaction of azide ion with N-alkyl-2-chloropyridinium ions

Abstract

The reaction of N-alkyl-2-chloropyridinium ions (1a–f; alkyl = Me, n-hexyl, n-decyl, n-dodecyl, n-tetradecyl, n-hexadecyl) with N₃^– is affected by cationic micelles of cetyltrimethylammonium ion (CTAX, X = Cl^–, Br^–, N₃^–), which inhibit the reaction of ion (1a) at low [surfactant] but catalyse it at high [surfactant]. First-order rate constants, k_ψ, for the reaction of ion (1b) continue to increase with increasing [surfactant] but, for reaction of the hydrophobic substrates (1c–f) at a given [NaN₃], k_ψ goes through maxima with increasing [surfactant]. Reaction in CTABr is always faster than in CTACl, but the difference decreases with increasing substrate hydrophobicity so the rate–surfactant profiles do not fit the pseudophase, ion-exchange model. For reactions of substrates (1c–f) with N₃^–, k_ψ increases with [substrate] in the absence of surfactant and it also increases with [CTAN₃]. The rate enhancement for reaction of ion (1f) in water is greater than that of ion (1a) by a factor of ca. 1.2 × 10⁴, and this is probably due to self-micellization of 2-chloro-N-hexadecylpyridinium ion. Second-order rate constants for reactions of ions (1a–f) with N₃^– in MeCN–water (1:1 w/w) are very similar.