SOLVATION CHANGES ACCOMPANYING PROTON-TRANSFER FROM A CARBON ACID TO ALKOXIDE BASES AS REVEALED BY KINETIC ISOTOPE EFFECTS

Primary hydrogen isotope effects are reported for the elimination of 4-nitrophenol from 4-(4-nitrophenoxy)-2-butanone in methanolic methoxide at 20-degrees-C and in aqueous hydroxide at 25-degrees-C. The isotope effects do not change significantly when the isotopic composition of the solvents is changed: (k(H)/k(D))MeOH = 6.48 +/- 0.22, (k(H)/k(D))MeOD = 6.40 +/- 0.24; and (kH/kD)HOH = 7.54 +/- 0.11, (k(H)/k(D))DOD = 7.48 +/- 0.21. Kinetic solvent isotope effects (using the undeuterated substrate) are k(MeOD)/k(MeOH) = 2.195 +/- 0.064 and k(DOD)/k(HOH) = 1.526 +/- 0.029. The experimental results are compared with model vibrational analysis calculations and about 60 literature examples of related solvent isotope effects on proton-transfer rates. The observations of primary hydrogen isotope effects that are unchanged upon isotopic substitution in protonic sites of the solvent were used to conclude that solvent reorganization is not coupled to proton transfer in the reaction coordinate.