Substrate and solvent hydrogen isotope effects in the pyridine- and 2,6-lutidine-catalysed iodination of diethyl ketone (pentan-3-one)

Abstract

The relative rates of base-catalysed iodination of pentan-3-one and [2,2,4,4-²H₄]pentan-3-one are 4·0 : 1 for pyridine catalysis and 6·8 : 1 for 2,6-lutidine catalysis in ordinary water at 25°. The exaltation of the isotope effect for the sterically hindered base parallels previous observations of hydrogen isotope effects with nitroalkanes and shows that it is confined neither to one class of substrates nor to proton transfers between bases of similar strength.

The reactions proceed ca. 20% more slowly in D₂O as solvent than in ordinary water. There are indications that this solvent isotope effect is slightly dependent on the isotopic nature of the proton (deuteron) transferred in the process. The solvent isotope effect is substantially independent of the nature of the catalyst base. It is suggested that this argues against the importance of non-equilibrium solvation of the transition state as a cause of the observed variation in the primary isotope effects.