Use of Kinetic Isotope Effects in Mechanism Studies. 4.1 Chlorine Isotope Effects Associated with Alkoxide-Promoted Dehydrochlorination Reactions

The method of measuring k35/k37 for the alkoxide-promoted deprotiochlorination and dedeuteriochlorination reactions has been applied to C₆H₅CiH(CH₃)CH₂Cl (I-Cl), C₆H₅C'HC₁CH₂C1 (V), and C₆H₅CiHCICF₂Cl (III). For reactions occurring with ethanolic sodium ethoxide the following values for k35/k37 have been measured: I-Cl-h, 1.005 90 + 0.000 13, and I-Cl-rf, 1.005 07 ± 0.000 36 at 75 ºC; V-h, 1.00908 ± 0.00008, and W-d, 1.007 34 ± 0.00012 at 24 ºC; III-h, 1.012 29 ± 0.000 47, and lll-d, 1.01003 ± 0.000 24 at 0 ºC. 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 ºC; III-h, 1.01255 ± 0.00048, and III-d, 1.01025 ± 0.00043 at 20 ºC. 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 ºC, with an EaH = 21.26 ± 0.05 kcal/mol and In AH = 29.29 ± 0.09. Rate constants measured for 55, 60, 65, and 70 ºC show increasing negative deviation from the slope. Similar behavior was not observed for V-d which resulted in £a = 21.57 ± 0.28 and In A = 28.37 ± 0.28 when rate constants over a 50 ºC range (20-70 ºC) were used. The δ. EaD-H = 0.31 and AH/AD = 2.5 for an observed kH/kD = 4.24 at 25 ºC were similar for results obtained for III. The possibility that the high values of kis/k31 for both III and V could result from a chlorine isotope effect associated with the proton transfer step of an ElcB mechanism is discussed.