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 (S N 2) reactions of solvated fluoride ions of F - (methanol) + CH 3 X (X = Br, I), F - (isopropyl alcohol) + CH 3 I, and F - (hydrogen fluoride) + CH 3 I at 300 K. The isotope effects are determined as the rate constant ratio k H / k D for specifically deuterated reactants, that is, methanol (CH 3 OH, CD 3 OH, CH 3 OD, and CD 3 OD), isopropyl alcohol ( i -C 3 H 7 OH and i -C 3 H 7 OD), hydrogen fluoride (HF and DF), and methyl halides (CH 3 X and CD 3 X). The data reveal identical trends to those previously observed for F - (water) + CH 3 X (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 S N 2 reactivities decrease as reaction exothermicity decreases (CH 3 I > CH 3 Br > CH 3 Cl) 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 S N 2 transition-state structure and bonding interactions analogous to those in the F - (H 2 O) + CH 3 X system.