TUNNELING IN ELIMINATION-REACTIONS - TESTS OF CRITERIA FOR TUNNELING PREDICTED BY MODEL-CALCULATIONS

Isotope effects in E2 reactions of mixtures of ArCL2CH2X and ArCLTCH2X (L = H or D) were determined by isotope fractionation methods. The temperature dependences of the secondary isotope effects, kHH/kHT (the subscript represents the transferred and the superscript represents the nontransferred atom) were abnormal as predicted by model calculations with tunneling when Ar = Ph and X = NMe3 in EtO-/EtOH (AaH/AaT = 0.705 ± 0.024) and when Ar = p-ClPh and X = OTs in t-BuO-/t-BuOH (AaH/AaT = 0.704 ± 0.023). When Ar = Ph and X = Br in t-BuO-/t-BuOH, the temperature dependence is not definitely abnormal (AaH/AaT = 0.927 ± 0.040), but the result can be shown still to be consistent with tunneling. The secondary effects kDD/kDT at 50 °C for Ar = Ph, X = NMe3 in EtO-/EtOH and Ar = p-ClPh, X = OTs in t-BuO-/t-BuOH were 1.0314 ± 0.0099 and 1.0274 ± 0.0077, respectively, and remain well below the kHH/kHT values (1.2042 ± 0.0149 and 1.1907 ± 0.0122, respectively), when converted to KHH/kHT values by KHH/kHT = (KDD/KDT)3.26, observations again diagnostic of tunneling. Efforts to apply the same criterion to primary isotope effects, KDD/KTD and kHH/KTH, were inconclusive. © 1990, American Chemical Society. All rights reserved.