Halide Adsorption on Metal Surfaces

When methyl halide monolayers were generated on a Ni(lOO) surface at 100 K, CH3CI (2) desorbed unchanged, while CHjBr (3) was partly 

Detailed investigations into the mechanism of reaction between alkyl halides and metals have concentrated primarily on systems of great practical interest the Grignard reaction, the Direct Process Reaction, and a. few others. Whereas research reported in the preceding section involved solely pure gaseous halides and pure metals as single crystals, mechanistic investigations have involved a much wider variety of substrates and reaction conditions. When a solvent is involved, solvation of the metal product 

There has been some controversy over the nature of intermediates in the Grignard reaction (54-56). Available evidence indicates that organic radicals form through cleavage of the carbon-halogen linkage everyone agrees on this. Controversy centers on the question of whether the alkyl radical remains adsorbed on the metal surface ( A model ) or whether it diffuses into solution ( D model ). Evidence appears to be more consistent with the D model (56), but the question cannot be considered as settled. 

As previously mentioned, the Direct Process Reaction involves the reaction of (2) with silicon at elevated temperatures to form methyl-chlorosilanes (6-9)  

This equation is a major oversimplification a mixture of many silicon-containing products actually forms. Their proportions may be controlled by adjusting reaction conditions. In fact, research has concentrated on determining those conditions that favor formation of one specific product. Pure silicon is not used instead, copper must be added as a catalyst. The addition of trace metals such as zinc, aluminum, and/or tin tends to increase reaction rates and to favor the formation of dimethyldichlorosilane over other products (57). Two recent patents pertaining to the preparation of the silicon-copper catalyst have been issued (58,59). 

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