T. Koerner et al., ELECTROPHILIC BROMINATION OF ETHYLENE AND ETHYLENE-D(4) - A COMBINED EXPERIMENTAL AND THEORETICAL-STUDY, Journal of the American Chemical Society, 120(23), 1998, pp. 5628-5636
The deuterium kinetic isotope effect (dkie) for the electrophilic brom
ination of ethylene-h(4) and ethylene-d(4) in methanol and dichloroeth
ane (DCE) at 25 degrees C has been determined using mass spectrometry.
The dkie's are inverse, that in methanol being k(H)/k(D) = 0.664 +/-
0.050 and that in DCE being k(H)/k(D) = 0.572 +/- 0.048. A product stu
dy of the bromination of trans-ethylene-d(2) in dichloroethane indicat
es that the addition is trans. Computations of the expected equilibriu
m deuterium isotope effect (EIE) for the process C2H4 + Br+ reversible
arrow C2H4-Br+ using density functional theory indicate that the EIE
is also inverse at K-H/K-D = 0.63. Detailed analyses of the molar part
ition functions and the zero-point energies for the various vibrationa
l modes in the ground and ion states indicate that the major contribut
or 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 c
omputed EIE is normal, K-H./K-D = 1.12. The computations also indicate
that the ion state undergoes very little rehybridization of the carbo
ns, the sum of the H-C-H and H-C-C angles at each carbon being 357.3 d
egrees. 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.