“Surprising kinetics” or the major role of trace quantities in radical chain mechanisms

Although hydrogen abstraction from 1,1,1,2‐tetrachloroethane should produce only one kind of radical, CCl 3 CHCl, the presence of small amounts of trichloroethene, as impurity as well as product of reaction, induces the formation of the a priori unexpected CHCl 2 CCl 2 radicals. The importance of this radical in the reaction pathways of the chlorination and the chlorine induced dehydrochlorination of CCl 3 CH 2 Cl, under different experimental conditions, is computed and discussed. For example, at 380 K, and for small partial pressures of chlorine (2 torr), the CHCl 2 CCl 2 radicals account for more than 70% of the formation of the reaction products after only 2% reaction. The major role of trace quantities in long chain radical reactions is emphasized. © 1993 John Wiley & Sons, Inc.