Proton-Transfer Reactions. 2. Effects of Internal Return on Reactivity Difference between Alkoxide-Promoted Eliminations in tert-Butyl Alcohol and Ethyl Alcohol

Kinetics of alkoxide-promoted dehydrofluorination reactions are reported for the series C6H5CH2CH2F (I), C6-H5CH2CHF2 (II), C6H5CH2CF3 (III), and C6H5CH2CF2CF3 (V). Rates and activation parameters [(k X 103 (M-1 s-1) (50 °C), ∆H* (kcal mol-1), and ∆S*, (eu)] are respectively: (a) using potassium tert-butoxide in tert-butyl alcohol, I (1.88 X 10-4, 19.1, -16.8), II (1.18 X 10-3, 20.3, -9.3), III (2.15 X 10-3, 22.1, -2.4), V (3.93 X 10-2 3, 16.9, -12.7); and (b) using sodium ethoxide in ethanol, I (1.32 X 10-6, 25.6, -6.5), II (5.28 X 10-7, 29.7, 3.0), III (3.45 X 10-7, 32.6, 12.5), V (5.41 X 10-5, 27.7, 7.6). The variation in tert-butoxide:ethoxide ratios of 140 (I), 2200 (II), 6200 (III), and 730 (V) are discussed in terms of a two-step mechanism with varying amounts of internal return for II, III, and V. Differences in reactivity for groups attached to the benzylic carbon (-CF3, -CF2C1, -CHF2, -CF2CF3) and variation of AS* values for these reactions are also discussed in terms of a two-step mechanism.