The Direct Synthesis of Organohalogensilanes.The Induction Period and the Radicalic Mechanism

摘要

At industrial scale organohalogensilanes can be prepared by direct synthesis.The synthesis involves the passage of organochloride (EX) gas over solid silicon,in a reaction catalysed by copper.The organochloride does not react readly with pure silicon,and only after a certain induction period does chlorosilane production begin.Owing to the lenghty induction period,the reaction between silicon and organochloride is considered erratic and irreproductible.Since the discovery of the direct synthesis more than fifty years ago,a great deal of effort has been levelled at trying to identify the reaction mechanism and elucidating those conditions whereby RfiiCL is the only product.The first mechanism proposed to account for the formation of organohalogensilanes was that proposed by Rochow and Hurd [1] in 1945,who concluded that the purpose of the copper was to transport the halogen from the alkyl chloride and make the methyl group available to silicon in the form of methylcopper.The next step is formation of silicon monochloride and regeneration of the copper catalyst.The silicon monochloride is then easily methylated by methycopper or methyl radicals to form at first a surface compound =SiClCH_3 which can then be further methylated by the same species until all four silicon valences are satisfaied,at which point the compound can be desorbed.Analogous mechanisms are thought to govern also the formation of other organohalosilanes,such as ethylchlorosilanes and phenylchlorosilanes,and even the formation of organohalogermanes.