Deuterium Kinetic Isotope Effects in the Gas-Phase SN2 Reactions of Solvated Fluoride Ions with Methyl Halides

Rate constants and deuterium kinetic isotope effects (KIEs) are measured for gas-phase nucleophilic substitution (SN2) reactions of solvated fluoride ions of F-(methanol) + CH3X (X = Br, I), F-(isopropyl alcohol) + CH3I, and F-(hydrogen fluoride) + CH3I at 300 K. The isotope effects are determined as the rate constant ratio k H/k D for specifically deuterated reactants, that is, methanol (CH3OH, CD3OH, CH3OD, and CD3OD), isopropyl alcohol (i-C3H7OH and i-C3H7OD), hydrogen fluoride (HF and DF), and methyl halides (CH3X and CD3X). The data reveal identical trends to those previously observed for F-(water) + CH3X (O'Hair, R. A. J.; Davico, G. E.; Hacaloglu, J.; Dang, T. T.; DePuy, C. H.; Bierbaum, V. M. J. Am. Chem. Soc. 1994, 116, 3609). The SN2 reactivities decrease as reaction exothermicity decreases (CH3I > CH3Br > CH3Cl) and as the nucleophile is solvated. Moderate inverse kinetic isotope effects (k H/k D < 1) are observed for the deuteration of the methyl halide, whereas substantial inverse KIEs are measured for the deuteration of the hydroxyl group of the solvent (or the deuteration of hydrogen fluoride). Moderate inverse KIEs are also measured for the deuteration of the methyl group in methanol. The observed trends and magnitudes of the isotope effects are rationalized qualitatively in terms of the SN2 transition-state structure and bonding interactions analogous to those in the F-(H2O) + CH3X system.