Use of Kinetic Isotope Effects in Mechanism Studies. 4. Chlorine Isotope Effects Associated with Alkoxide-Promoted Dehydrochlorination Reactions

Article abstract of DOI:10.1021/ja00345a043

The method of measuring k³⁵/k³⁷ for the alkoxide-promoted deprotiochlorination and dedeuteriochlorination reactions has been applied to C6H5CⁱH(CH3)CH2Cl (I-Cl), C6H5CⁱHClCH2Cl (V), and C6H5CⁱHClCF2Cl(III).For reactions occuring with ethanolic sodium ethoxide the following values for k³⁵/k³⁷ have been measured: I-Cl-h, 1.00590 + 0.00013, and I-Cl-d, 1.00507 +/- 0.00036 at 75 deg C; V-h, 1.00908 +/- 0.00008, and V-d, 1.00734 +/- 0.00012 at 24 deg C; III-h, 1.01229 +/- 0.00047, and III-d, 1.01003 +/- 0.00024 at 0 deg C.Methanolic sodium methoxide promoted eliminations gave similar results for V and III: V-h, 1.00978 +/- 0.00020, and V-d, 1.00776 +/- 0.00020 at 21 deg C; III-h, 1.01255 +/- 0.00048, and III-d, 1.01025 +/- 0.00043 at 20 deg C.The results for I-Cl are most consistent with an E2 mechanism, while the results for III and V suggest a multistep reaction sequence.The Arrhenius behavior of ethoxide-promoted dehydrochlorination of V-h gives a good linear correlation between -10 and 50 deg C, with an E_a^H = 21.26 +/- 0.05 kcal/mol and ln A^H = 29.29 +/- 0.09.Rate constants measured for 55, 60, 65, and 70 deg C show increasing negative deviation from the slope.Similar behavior was not observed for V-d which resulted in E_a^D = 21.57 +/- 0.28 and ln A^D = 28.37 +/- 0.28 when rate constants over a 50 deg C range (20 - 70 deg C) were used.The ΔE_a^D-H = 0.31 and A^H/A^D = 2.5 for an observed k^H/k^D = 4.24 at 25 deg C were similar for results obtained for III.The possibility that the high values of k³⁵/k³⁷ for both III and V could result from a chlorine isotope effect associated with the proton transfer step of an E1cB mechanism is discussed.