Mechanisms for the formation of epoxide and chlorine-containing products in the oxidation of ethylene by chromyl chloride: a density functional study

The reaction between CrO2Cl2 and ethylene leading to the formation of epoxi de and chlorohydrin precursors or directly to 1,2-dichloroethane has been s tudied by density functional theory. The formation of the epoxide precursor (Cl-2(O)Cr-OC2H4) was found to take place via a [3+2] addition of ethylene to two Cr=O bonds followed by rearrangement of the five-membered diol to t he epoxide product. The alternative mechanisms involving a direct addition of oxygen to ethylene or the [2+2] addition of the olefin to a Cr=O bond we re found to have much higher activation energies. The formation of the chlo rohydrin precursor (Cl(O)Cr-OCH2=CHCl) was found to take place via a [3+2] addition to one Cr-Cl and one Cr=O bond. Pathways involving initial [2+2] a ddition to a Cl-Cl or Cr=O bond had much higher activation barriers. The ge neration of 1,2-dichloroethane is highly unfavorable with an endothermicity of 44.7 kcal/mol and an even higher activation barrier. It is suggested th at the formation of epoxide and chlorohydrin from the respective precursors requires the addition of H2O.