The Catalytic Dehydrohalogenation of Haloethanes on Solid Acids and Bases

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The elimination reactions of haloethanes and halopropanes on solid acids and bases, including a series of metal sulfates supported by silica gel, were studied at 300°C by means of the pulse technique in order to clarify the mechanism of the catalytic dehydrohalogenation. The trans-1,2-dichloroethylene/cis-1,2-dichloroethylene (trans-1,2/cis-1,2) selectivities from 1,1,2-trichloroethanes as well as the activities of the metal sulfates supported by silica gel were well correlated with the electronegativities of the metal ions. Generally speaking, the rates of the dehydrochlorination from chloroalkanes were nearly equal to those of the dehydrobromination from bromoalkanes on the solid acids, but the latter were larger than the former on solid bases. The reactivities of the alkylhalides on solid acids and bases were examined from the electronic view-point, and we tried to correlate them with some reactivity indexes. A kinetic isotope effect with regard to hydrogen was observed on solid bases, but not on solid acids, in the dehydrobromination of 1,2-dibromoethane. From these facts, it was concluded that the dehydrohalogenation proceeds through a carbonium-ion mechanism on solid acids and via an E2 step-by-step mechanism on solid bases.
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