Organosilicon compounds. L. Pyrolysis of phenyldimethylsilane

Abstract

The predominant products of pyrolysis of phenyldimethylsilane, studied in a through-Aow reactor, have proved to be diphenyldimethylsilane, 9,9-dimethyl-9-silafluorene, and polymers: benzene and toluene are formed to a small extent. For comparison we have studied the kinetics of pyrolysis of p-tolyldimethylsilane, giving rise to the analogous p-methylsubstituted products, and of diphenyldimethylsilane, which produces benzene , toluene, phenyldimethylsilane and polymers. On the basis of the results obtained a chain radical mechanism of pyrolysis of phenyldimethylsilane is proposed and discussed.The thermal stability of silicon hydrides has thus far been paid little attention. The literature l describes the pyrolyses of silane, disilane, higher silanes and ethylsilanes (CzHs)n SiH 3 -n , but not of siJanes with Si-aryl bonds. Theoretically the stability of silanes should be in a direct relation to the dissociation energies of the bonds present in the molecule of the compound considered. Unfortunately, the dissociation energies of the bonds Si-R where R = CH 3 , C6HS and H have not been safely determined or measured at all. Measured values of the dissociation energies of the bonds present in the molecules of phenyldimethylsilane and tolyldimethylsilane are listed in Table]. They suggest that in the initiation stage of pyrolysis of these compounds there will probably occur cleavage of the bonds between silicon and phenyl, hydrogen or methyl, the bond Si-C6Hs being probably the first to split. As the dissociation energy of the bond Si-C6Hs has been calculated from the combustion heat of tetraphenylsilane, the value obtained (51'3 kcal/mol) is most probably distorted by the possible mesomeric shifts, whose effect in phenyldimethylsilane is probably weaker than in tetraphenylsilane. Thus it is difficult to predict which of the silicon bonds considered will prove the most labile.