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

Article abstract of DOI:10.1021/ja00408a024

Kinetics of alkoxide-promoted dehydrofluorination reactions are reported for the series C6H5CH2CH2F (I), C6H5CH2CHF2 (II), C6H5CH2CF3 (III), and C6H5CH2CF2CF3 (V).Rates and activation parameters <(k * 10³ (M^-1 s^-1)(50 deg C), ΔH^* (kcal mol^-1), and ΔS^*, (eu)> are respectively: (a) using potassium tert-butoxide in tert-butyl alcohol, I (1.88 * 10^-4, 19.1, -16.8), II (1.18 * 10^-3, 20.3, -9.3), III (2.15 * 10^-3, 22.1, -2.4), V (3.93 * 10^-2, 16.9, -12.7); and (b) using sodium ethoxide in ethanol, I (1.32 * 10^-6, 25.6, -6.5), II (5.28 * 10^-7, 29.7, 3.0), III (3.45 * 10^-7, 32.6, 12.5), V (5.41 * 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, -CF2Cl, -CHF2, -CF2CF3) and variation of ΔS^* values for these reactions are also discussed in terms of a two-step mechanism.