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-h 4 and ethylene-d 4 in methanol and dichloroethane (DCE) at 25 °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 study of the bromination of trans-ethylene-d 2 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 K H/K D = 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, K H/K D = 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.

Synthesis and structures of π-allylpalladium(II) complexes containing bis(1,2,4-triazol-5-ylidene-1-yl)borate ligands. An unusual tetrahedral palladium complex

“…Similar couplings were also observed in chloroethene (ClHC ¼ CH 2 , Scheme 2), where J ac and J bc ¼ 7.0 and 14.6 Hz, respectively ( Fig. 1) [58]. As shown in Table 1, the chemical shifts of 1 H NMR peaks of the alkyl ligand in tetrahedral 2d is downfield shifted from those of square planar 2c.…”

Enantioselective Dihalogenation of Alkenes

“…In the case of adamantylidene adamantane, solid bromiranium salts are stable, can be isolated, and their structures have been determined by means of X‐ray crystallography [22] . Based on the observed diastereoselectivity and calculations, cyclic cations 12 are favored as long as the alkene is not substituted by an electron‐rich aromatic ring or another highly electron‐donating substituent [16, 23] . In this case, open‐chain β‐bromocarbenium ions 13 can be more stable than the cyclic form [18a, 24] .…”

Density Functional Studies of Halonium Ions of Ethylene and Cyclopentene

“…The ethylene part of the bromonium ion, 2, is near planar as the sum of the C-C-H, H-C-H, and C-C-H angles were computed equal to 357.3º and 357.2º for X=Cl or Br respectively. This sum was calculated equal to 357.3º for X=Br at the density functional level with effective core potentials [19] and 356.6º and 356.6º for X=Cl and Br respectively at the MP2 level. [10] Considerable discussion has been done in whether the three membered ring in bridged halonium ions is a σ -complex or a π -complex.…”