Dehydrobromination of bromoalkanes over cabosil and alkali-ion-exchanged cabosilsI. A stereochemical study

Abstract

The stereochemistry of dehydrobromination of 2-bromobutane and 2,3-dibromobutane has been determined over cabosils, amorphous silicon dioxide prepared by a dry process, to which various amounts of Cs and Li were loaded by an ion-exchange method. Stereochemistry, product distribution, and Arrhenius parameters were essentially the same as those obtained for silica gels prepared by an ordinary wet process. This fact confirmed the idea that the steric course is controlled by the acid-base properties of catalysts: The steric course of dehydrobromination of 2-bromobutane is mainly anti mode over basic solid surfaces and syn mode over acidic surfaces. The reaction rates via anti mode increased in proportion to the number of Cs ions loaded on silica. Basic sites responsible for anti elimination are likely the oxide ions associated with electron-donating Cs ion. A possible mechanism of anti elimination, which does not necessitate any particular pore structure, was discussed on the basis of the experimental results.