Przemyslaw Pruszynski

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

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.

Kinetics of proton and deuteron transfer from α,4-dinitrotoluene to 1,8-bis(dimethylamino)naphthalene in alcoholic solvents

Evidence is presented to show that the reaction between α,4-dinitrotoluene and 1,8-bis(dimethylamino)naphthalene, in either propan-2-ol or t-butyl alcohol as solvent, is a proton transfer from the carbon acid to the nitrogen base to give an ion pair. The second-order rate constants for the proton and deuteron transfer reactions have been determined over the temperature range 10–45 °C in the two solvents. The reactions are relatively slow and are characterized by enthalpies of activation of 10–14 kcal mol–1(42–58 kJ mol–1) and entropies of activation of –17 to –31 cal mol–1 K–1(–71 to –130 J mol–1 K–1). The isotopic rate ratios are small (kH/kD < 3), although ΔHD‡–ΔHH‡ is normal (ca. 1 kcal mol–1), with compensation by positive values for ΔSD‡–ΔSH‡.