Solvent isotope effects in the acidolysis and other electrophilic substitution reactions of penta-aquaorganochromium(III) ions in aqueous solution

Abstract

The rates of some reactions of penta-aquaorganochromium(III) ions with electrophilic reagents (HgMe⁺, Br₂, H₃O⁺, and water) have been measured in mixtures of H₂O and D₂O. The first two reagents effect substitution on carbon without the formal involvement of water, and the observed solvent isotope effect is accordingly ascribed to the isotopic change in the five water ligands and, for the reaction with bromine, in the solvation of the incipient bromide ion. With this information an attempt has been made to separate the isotope effects on proton transfer from the isotope effects on the water ligands for aquation reactions involving aqueous hydrogen ion or water as a proton (deuteron) donor. It is shown that the large solvent and product isotope effects observed in these reactions are then readily explained in terms of fractionation factor theory. The analysis leads to transition-state fractionation factors in the range observed for the extensively studied reactions involving a rate-limiting proton transfer to carbon in purely organic substrates.