Electrophilic Bromination of Ethylene and Ethylene-d4. A Combined Experimental and Theoretical Study

Abstract

The deuterium kinetic isotope effect (dkie) for the electrophilic bromination of ethylene-h4 and ethylene-d4 in methanol and dichloroethane (DCE) at 25 °C has been determined using mass spectrometry. The dkie's are inverse, that in methanol being kH/kD = 0.664 ± 0.050 and that in DCE being kH/kD = 0.572 ± 0.048. A product study of the bromination of trans-ethylene-d2 in dichloroethane indicates that the addition is trans. Computations of the expected equilibrium deuterium isotope effect (EIE) for the process C2H4 + Br+ ⇌ C2H4−Br+ using density functional theory indicate that the EIE is also inverse at KH/KD = 0.63. Detailed analyses of the molar partition functions and the zero-point energies for the various vibrational modes in the ground and ion states indicate that the major contributor to the EIE is the creation of a new mode in the ion, termed the CH2-symmetric twist, that arises from the loss of the rotational freedom about the C−C axis in ethylene. In the absence of this new mode, the computed EIE is normal, KH/KD = 1.12. The computations also indicate that the ion state undergoes very little rehybridization of the carbons, the sum of the H−C−H and H−C−C angles at each carbon being 357.3°. A discussion is presented concerning the detailed sequence of events contributing to the reaction mechanism in both solvents and how each of these might contribute to the dkie.