The direct synthesis of methylchlorosilanes: New aspects concerning its mechanism

New results are given regarding the mechanism of the chemical process of copper alloyed silicon with methyl chloride (the `direct process'). As indicated by Photo-EMF measurements, carried out with doped silicon samples the reactivity of silicon significantly depends on the type of the doping with elements like phosphorus (n-type) tin, boron, indium (p-type). In-situ trapping experiments with 2,3-dimethylbutadiene are consistent with the creation of silylene intermediates SiMeCl and SiCl₂ . Theselectivity of their competitive insertion steps can be controlled by the doping type and concentrations of the doping elements, especially the phosphorus/tin ratio criterion. n-Type doping favors the silylene insertion into the C-Cl bond due to the electronic silylene stabilization on the silicon surface. In case of p-type doping silylene insertion into Si-Cl bond is favored more intensively leading to the formation of disilanes.