Factors influencing kinetic isotope effects in some proton-transfer reactions in aprotic solvents

Abstract

Kinetic studies have been carried out on the proton-transfer reactions of (i) 4-nitrobenzyl cyanide (4-NBC) with tetramethylguanidine (TMG) and (ii) 4-nitrophenylphenylcyanomethane (4-NPPCM) with 1,8-bis(dimethylamino)naphthalene (DMAN) in acetonitrile.

The reaction (i) has a moderate enthalpy of activation (ΔH^‡= 25 kJ mol^–1) and a negative entropy of activation (ΔS^‡=–98 J mol^–1 K^–1). The deuterium kinetic isotope effect is large.

The reaction (ii) of the more hindered molecule 4-NPPCM and DMAN has similar activation parameters (ΔH^‡= 26 kJ mol^–1, ΔS^‡=-140 J mol^–1 K^–1), but the kinetic isotope effect is considerably larger; also ΔH_D^‡-ΔH_H^‡ is large (13 kJ mol^–1) and A_H/A_D is small, indicating a considerable tunnelling effect. Evidence that the large isotope effects are not artefacts of hydrogen exchange is presented. The tunnelling factor is estimated.