Mechanism and proton activating factors in base-induced β-elimination reactions of 2-(2-chloroethyl)pyridine

Abstract

The substrate 2-(2-chloroethyl)pyridine reacts in OH⁻/H₂O, 50 °C, μ = 1 M KCl by an elimination reaction with the formation of 2-vinylpyridine; the second order rate constant is k^N_OH = 4.59 × 10⁻⁴ dm³ mol⁻¹ s⁻¹. In acetohydroxamate–acetohydroxamic acid buffers, the elimination reaction competes with the S_N2 reaction of the acetohydroxamate nucleophile. Studies of acid–base catalysis at pH values ranging from 8.42 to 9.42 are in agreement with an E1cb irreversible mechanism, where carbon deprotonation occurs from the substrate protonated at the nitrogen of the pyridine ring (NH⁺+++++), even if it is present at very low concentrations with respect to the unprotonated substrate (N) under the reaction conditions. The value for the reactivity ratio between NH⁺+++++ and N is of the order of 10⁵. The strong reactivity of NH⁺+++++ is attributed to the high stability of the carbanion intermediate formed; this intermediate has an enamine structure.