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 ΔHD‡-ΔHH‡ is large (13 kJ mol–1) and AH/AD 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.