are organic compounds
containing carbon silicon bonds
. Organosilicon chemistry
is the corresponding science exploring their properties and reactivity.
Like carbon, the organically bound silicon is tetravalent and tetrahedral. Carbon-silicon bonds are not found in any biomolecule. The first organosilicon compound, tetraethylsilane was discovered by Charles Friedel and James Crafts in 1863 by reaction of tetrachlorosilane with diethylzinc. The carbosilicon silicon carbide is an inorganic compound.
Carbon silicon bonds compared to carbon carbon bonds are longer (186 pm
vs. 154 pm) and weaker with bond dissociation energy
vs. 607 kJ/mol . The C–Si is somewhat polarized towards carbon due to its higher electronegativity
(C 2.55 vs Si 1.90). One manifestation of bond polarization in organosilanes is found in the Sakurai reaction
. In oxidative couplings silicon is represented by the Hiyama coupling
. Certain alkyl silanes can be oxidized to an alcohol
in the Fleming-Tamao oxidation
Certain allyl silanes can be prepared from allylic ester such as 1 and monosilylcopper compounds such as 2 in .
In this reaction type silicon polarity is reversed in a chemical bond with zinc and a formal allylic substitution on the benzoyloxy group takes place.
The chemistry of silanes such as tetramethylsilane is comparable to that of alkanes in many aspects such as thermal stability. The β-silicon effect describes the stabilizing effect of a β-silicon atom on a carbocation with many implications for reactivity.
More notably bonds of silicon to oxygen
are much shorter and stronger (809 compared to 538 kJ/mol) than that of those of carbon to oxygen. The polarization in this bond increases towards oxygen. Examples are silyl acetals
RR'Si(OR)2, the siloxanes
and the polymeric polysiloxanes
. Silyl ethers
are extensively used as protective groups
. Only silicon bonds to fluorine
are stronger and that is why the fluorine source TASF
(or more commonly TBAF
) is useful in deprotection. The favorable formation of Si–O bonds drive many organic reactions
such as the Brook rearrangement
and Peterson olefination
Another manifestation is the highly explosive nature of the silicon pendant Si(CH2ONO2)4 and Si(CH2N3)4 of pentaerytritol tetranitrate :
A single crystal of this compound, first synthesized in 2007 even detonates when in contact with a teflon spatula and in fact made full characterization impossible. Another contributor to its exothermic decomposition (inferred from much safer in silico experimentation) is the ability of silicon in its crystal phase to coordinate to two oxygen nitrito groups in addition to regular coordination to the four carbon atoms. This additional coordination would make formation of silicon dioxide (one of the decomposition products) more facile.
Organosilyl halides are important reagents
in organic chemistry notably trimethylsilyl chloride
SiCl. A classic method called the Flood reaction
for the synthesis of this compound class is by heating hexaalkyldisiloxanes R3
with concentrated sulfuric acid
and a sodium halide
. Other relevant silyl halides are dichloromethylphenylsilane
The silicon to hydrogen bond is longer than the C–H bond (148 compared to 105 pm) and weaker (299 compared to 338 kJ/mol). Hydrogen is more electronegative
than silicon hence the naming convention of silyl hydrides. Silyl hydrides are very reactive and used as reducing agents
for example PMHS
In one study triethylsilylhydride is used in the conversion of an phenyl azide to an aniline :
In this reaction ACCN is a radical initiator and an aliphatic thiol transfers radical character to the silylhydride. The triethylsilyl free radical then reacts with the azide with expulsion of nitrogen to a N-silylarylaminyl radical which grabs a proton from a thiol completing the catalytic cycle:
Aqueous workup then gives aniline.
Silyl hydrides can even take up the reduction of robust molecules such as carbon dioxide (to methane) :
Although it takes a very complex catalyst system.
Silyl hydrides react with various unsaturated substrates such as alkenes
to new organosilicon compounds in hydrosilylation
. In the reaction of triphenylsilyl hydride
the reaction product is a trans or cis
or the geminal
vinyl silane, for example :
In the related silylmetalation, a metal replaces the hydrogen atom.
Organosilicon compounds unlike their carbon counterparts do not have a rich double bond
chemistry due to the large difference in electronegativity. Existing compounds with organosilene
Si=C bonds are laboratory curiosities such as the silicon benzene analogue silabenzene
, and Si=Si bond containing disilenes
are the silicon pendants of pyrroles
and of current academic interest due to their electroluminescence
and other electronic properties . Siloles are efficient in electron transport. They owe their low lying LUMO
to a favorable interaction between the antibonding sigma
silicon orbital with a antibonding pi orbital
of the butadiene
Unlike carbon, silicon compounds can be coordinated to five atoms as well in a group of compounds ranging from so-called silatranes
to a uniquely stable pentaorganosilicate :