Reactions at high pressure. Part 6. Anomalous volumes of activation, and kinetic isotope effects in the ionisation of 2-nitropropane

Abstract

Rates of iodination of 2-nitropropane, catalysed by 2,4,6-trimethylpyridine have been measured in aqueous t-butyl alcohol at 25° at pressures of 1–1 285 bar and similarly for 2-nitro[2-²H]propane. It is found that oxygen affects the measured rates and, if the reaction is carried out under nitrogen, rates are faster and the kinetic isotope effect, k_H/k_D 16, less than previously reported. There is still a large tunnelling contribution and this is found to be pressure dependent, tending towards 7. The volumes of activation for ¹H and ²H compounds are different, –31 and –40 cm³ mol^–1 respectively; it is believed the former is anomalous. The large negative value of ΔV₀^‡ is consistent with a high degree of proton transfer and almost maximum charge development in the transition state.