The effect is understood in terms of classical hyperconjugation depicted in structure 3 in scheme 1 or in terms of molecular orbital overlap 1 which is a stabilizing overlap between the empty p-orbital of the carbocation and the filled sigma molecular orbital of the silicon to carbon bond. .
The alpha-silicon effect is the destabilizing effect of a silicon atom next to a reaction center with a partial positive charge.
In a pioneering study by Frank C. Whitmore ethyltrichlorosilane (scheme 2) was chlorinated by sulfuryl chloride as chlorine donor and benzoyl peroxide as radical initiator in a radical substitution resulting in chloride monosubstitution to some extent in the α-position (28%, due to steric hindrance of the silyl group) and predominantly in the β-position. By adding sodium hydroxide to the α-substituted compound only the silicon chlorine groups are replaced but not the carbon chlorine group. Addition of alkali to the β-substituted compound on the other hand leads to an elimination reaction with liberation of ethylene.
In another set of experiments (scheme 3) the chlorination is repeated with n-propyltrichlorosilane The α-adduct and the γ-adduct are resistant to hydrolysis but the chlorine group in the β-adduct gets replaced by a hydroxyl group.
The silicon effect is also manifest in certain compound properties. Trimethylsilylmethylamine (Me3SiCH2NH2) is a stronger base with a pKa of 10.96 for the conjugate acid than the carbon analogue neopentyl amine with pKa 10.21. In the same vein trimethylsilylacetic acid (pKa 5.22) is a poorer acid than trimetyl acetic acid (pKa 5.00).