Oxychlorination

In organic chemistry oxychlorination is a process for making C-Cl bonds. In contrast with direct use of Cl₂, oxychlorination uses hydrogen chloride in combination with oxygen.^[1] This process is attractive industrially because hydrogen chloride is less expensive than chlorine.^[2]

Mechanism

The reaction is usually initiated by copper(II) chloride (CuCl₂), which is the most common catalyst in the production of 1,2-dichloroethane. In some cases, CuCl₂ is supported on silica in presence of KCl, LaCl₃, or AlCl₃ as cocatalysts. Aside from silica, a variety of supports have also been used including various types of alumina, diatomaceous earth, or pumice. Because this reaction is highly exothermic (238 kJ/mol), the temperature is monitored, to guard against thermal degradation of the catalyst. The reaction is as follows:

CH₂=CH₂ + 2 CuCl₂ → 2 CuCl + ClH₂C-CH₂Cl

The copper(II) chloride is regenerated by sequential reactions of the cuprous chloride with oxygen and then hydrogen chloride:

½ O₂ + 2 CuCl → CuOCuCl₂

2 HCl + CuOCuCl₂ → 2 CuCl₂ + H₂O

Applications

The most common substrate for this reaction is ethylene:

CH₂=CH₂ + 2 HCl + ½ O₂ → ClCH₂CH₂Cl + H₂O

Oxychlorination is of special importance in the making of 1,2-dichloroethane, which is then converted into vinyl chloride. As can be seen in the following reaction, 1,2-dichloroethane is cracked:

ClCH₂CH₂Cl → CH₂=CHCl + HCl

2 HCl + CH₂=CH₂ + ½ O₂ → ClCH₂CH₂Cl + H₂O

The HCl from this cracking process is recycled by oxychlorination. The fact that the reaction is self-supplied is one of the reasons that industry uses oxychlorination instead of direct chlorination.^[3]

References

1. M. Rossberg et al. “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim. doi:10.1002/14356007.a06_233.pub2
2. Marshall, K. A. 2003. Chlorocarbons and Chlorohydrocarbons, Survey. Kirk-Othmer Encyclopedia of Chemical Technology
3. "Oxy | Oxychlorination." Oxy | Oxychlorination. N.p., n.d. Web. 10 Oct. 2012.