The Dehalogenation of Haloalkanes on SiO2-supported Metals

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The dehalogenations of haloethanes and halopropanes over metals (Cu, Ag, Au, Fe, Co, Ni, Pd, and Pt) supported by silica gel were studied by means of the microcatalytic pulse technique in order to elucidate the mechanism of dehalogenation. The dechlorination reactivities of cis- and trans-1,2-dichloroethylenes and the dechlorination products from dl- and meso-2,3-dichlorobutanes were observed from the viewpoint of stereoselectivity. The order of catalytic activities among the metals for the dehalogenation of haloethanes was: Pt>Cu≈Ni>Ag≈Co≈Pd>Fe≈Au. Dehalogenation proceeded exclusively in the 1,2-dihaloethanes. However, dehydorhalogenation took place in the 1,2-dihalopropanes to an appreciable extent when the product distributions of dehydrohalogenation were similar to those on solid acids. Both 1,1,2-trichloroethane and 1,2-dichloropropane were more reactive than 1,2-dichloroethane. 1,2-dichloroethylenes from 1,1,2,2-tetrachloroethane and 2-butenes from dl- and meso-2,3-dichlorobutanes were nearly in an equilibrium ratio of cis/trans forms. From a comparison of these results with those of the dehydrohalogenation on solid acids, it is concluded that the dehalogenation of haloalkanes on metals proceeds through a radical-step-by-step mechanism.
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