Nucleophilic displacements in substituted pyridine N-oxides. Part II. Kinetics of the reactions between piperidine and 2- and 4-bromo-, 4-chloro-, and 2- and 4-nitro-pyridine 1-oxide in anhydrous ethanol

Abstract

Rates of the “dark” reactions of 2- and 4-bromo-, 4-chloro- and 2- and 4-nitro-pyridine 1-oxide with piperidine in ethanol have been measured and the Arrhenius parameters calculated and discussed.In the 4-substituted pyridine 1-oxides, mobility towards piperidine in the order NO2 [graphic omited] Br [graphic omited] Cl, but the energy of activation and log A term for the reaction of the 4-nitro-compound are appreciably lower than for those of the halogens. This is attributed to stabilisation of, and specific orientation imposed on, the transition state by hydrogen bonding between piperidine and the nitro-group.The very low energy of activation for the reaction of 2-nitropyridine 1-oxide is attributed to a high ground-state energy caused by interaction between the N-oxide oxygen and the nitro-group, in addition to stabilisation of the transition state by hydrogen bonding.