Beta-silicon effect 4739376 174191257 2007-11-27T19:19:24Z Christian75 1306352 Reference changed to dx.doi.org 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.<ref name='Butterworth'>Silicon in Organic Synthesis'' Colvin, E. Butterworth: London 1981</ref> 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. [[Image:SiliconHyperconjugation.png|center|400px|Scheme 1. Silicon hyperconjugation]]. 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]]<ref>''Organo-silicon Compounds. II.1 Silicon Analogs of Neopentyl Chloride and Neopentyl Iodide. The Alpha Silicon Effect'' Frank C. Whitmore, Leo H. Sommer [[J. Am. Chem. Soc.]]; '''1946'''; 68(3); 481-484. [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1946/68/i03/f-pdf/f_ja01207a036.pdf Abstract]</ref><ref>''Organo-silicon Compounds. III.1 - and -Chloroalkyl Silanes and the Unusual Reactivity of the Latter'' Leo H. Sommer, Frank C. Whitmore [[J. Am. Chem. Soc.]]; '''1946'''; 68(3); 485-487. [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1946/68/i03/f-pdf/f_ja01207a037.pdf Abstract]</ref> ethyltrichlorosilane (''scheme 2'') was [[Halogenation|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<ref>''The Reactivity with Alkali of Chlorine-Carbon Bonds Alpha, Beta and Gamma to Silicon'' Leo H. Sommer, Edwin Dorfman, Gershon M. Goldberg, [[Frank C. Whitmore]] [[J. Am. Chem. Soc.]]; '''1946'''; 68(3); 488-489. [http://dx.doi.org/10.1021/ja01207a038 Abstract]</ref> 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. [[Image:SiliconHyperconjugationWhitmore.png|center|600px|Scheme 3. Beta silicon effect]] The silicon effect is also manifest in certain compound properties. Trimethylsilylmethylamine (Me<sub>3</sub>SiCH<sub>2</sub>NH<sub>2</sub>) is a stronger [[base (chemistry)|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).<ref name='Butterworth'/> == References == <references/> [[Category:Physical organic chemistry]]