The Electrical Effect of the Trimethylsilyl Group

Abstract

The magnitude and direction of the electrical effect of tetravalent silicon atoms in organosilicon compounds has not been definitely established. The difference in electronegativity between carbon and silicon on the Pauling scale (+0.7 unit) suggests that silicon should be fairly strongly electron-releasing with respect to carbon. Nonetheless, the low reactivity of trimethylsilylmethyl chloride toward ionizing reagents has been interpreted as indicating that tetravalent silicon atoms are “electronically unsaturated” relative to tetravalent carbon. The possibility exists that silicon atoms may tend to be more electron-attracting than otherwise anticipated since resonance forms involving more than four pairs of electrons in the outer atomic valence shells should be energetically more favorable for silicon than carbon. Actually, strong evidence has been obtained for important contributions of resonance forms involving pentacovalent silicon in the transition states of the displacement reactions of organosilicon halides.