Beta-silicon effect

The beta-silicon effect also called silicon hyperconjugation in organosilicon chemistry is a special type of hyperconjugation and describes the stabilizing effect of a silicon atom placed in a position once removed (β) from a carbocation. A prerequisite is an antiperiplanar relationship between the two groups. Silicon hyperconjugation explains specific observations regarding chemical kinetics and stereochemistry of organic reactions with reactants containing silicon.

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).