Dimensiosolvatic effects. VI. Rates of ionization of 2-fluoro-4,4,5,5-tetramethyl-1,3-dioxolane: Dimensiosolvatic vs. hydrogen-bond effects

The title compound was prepared by treating 4,4,5,5-tetramethyl-1,3-dioxola n-2-yl acetate with cesium fluoride. Rates of ionization of this compound w ere determined in various solvents by the dynamic NMR technique. While effe cts due to polarity as well as solvent molecular size were observed in many solvents examined, the rates in solutions of which the solvent is capable of being an acceptor in hydrogen-bond formation with the substrate were unu sually small, and this effect is diminished in bulky solvents. Such results are attributed to stabilization of the original state due to hydrogen-bond formation with the acidic C(2) - H group of the substrate in those solvent s. The hydrogen-bond formation is hindered by the steric effects in bulkier solvents than in smaller solvents. The rates of contact ion pair and solve nt-separated ion pair formations should be independently determined in 2-fl uoro-4,4,5,5-tetramethyl-1,3-dioxolane, but so far no clear example which s hows that the rate-limiting step is the solvent intervention has been found , even though various bulky solvents were examined.