Kinetics of proton and deuteron transfer reactions of 4-nitrophenylnitromethane with triethylamine and tri-n-butylamine in acetonitrile solvent

The second-order rate constants for the proton and deuteron transfer reactions between 4-nitro-phenylnitromethane and the organic bases triethylamine and tri-n-butylamine have been determined at eight different temperatures between 0 and 35°C, using the stopped-flow technique. The four reactions are characterized by low enthalpies of activation and large negative entropies of activation. The isotope effects give values for (ΔD^≠_D–ΔH^≠_H) of 1.02 and 0.74 kcal mol^–1 and (ΔS^≠_D–ΔS^≠_H) of 1.2 and 0.9 cal mol^–1 K^–1 for triethylamine and tri-n-butylamine respectively. The rate ratios are small for primary deuterium effects; k_H/k_D for Et₃N at 0°C is 3.56 and for n-Bu₃N at 0°C is 2.41. These reaction constants are used to elucidate the type of energy barrier involved in the reaction and the nature of the transition state.