Substituent effects on proton tunnelling. Reaction between 2,4,6-trinitrotoluene and 1-substituted piperidines in acetonitrile

Abstract

The proton/deuteron-transfer reaction of 2,4,6-trinitrotoluene (TNT) with 1-methylpiperidine, 1-ethylpiperidine or 1-phenethylpiperidine in acetonitrile is a simple reaction forming a 1:1 ion pair. The proton- and deuteron-transfer reactions have been followed at 5–35 °C by a stopped-flow method. The isotope rate ratio k^H/k^D at 25 °C is 14.7–16.2, the activation-energy difference E_a(D)–E_a(H) is 9.2–10.9 kJ mol^–1 and the ratio of Arrhenius pre-exponential factors A(D)/A(H) is 2.5–4.0 for the forward reaction. Similar values of k^H/k^D, E_a(D)–E_a(H) and A(D)/A(H) are also found for the reverse reaction. All these values signify a tunnelling contribution to the proton-transfer reaction. An analysis based on an unsymmetrical parabolic potential barrier shows that as the bulkiness of the substituted group in the piperidines increases the top of the barrier becomes much rounder, and so tunnelling decreases.