Hydrogen isotope effects in the inversion of (–)-menthone in mixtures of water and dimethyl sulphoxide

Abstract

The rate of inversion of ordinary and deuteriated (–)-menthone has been measured in solutions of sodium hydroxide in six solvents containing 0–75 mole percent of dimethyl sulphoxide. The kinetic isotope effect has a maximum value of k^H/k^D= 6·5 in a solvent containing 30 to 40 mole percent dimethyl sulphoxide, suggesting that the proton is approximately half transferred in the transition state. In agreement with this, published values of H_– have been used to show that the hydroxide ion and the anion of menthone have approximately the same basic strength in solvents of the above composition, and the variation of rate with H_– corresponds to a Brönsted exponent β= 0·48.