Silanes
Encyclopedia
Silanes are chemical compound
s of silicon
and hydrogen
, which are analogues of alkane
hydrocarbons. Silanes consist of a chain of silicon atoms covalently bonded
to each other and to hydrogen atoms. The general formula of a silane is SinH2n+2. Much of the early work establishing that silicon does indeed form an homologous series of hydrides analogous to the alkanes
, albeit to a much smaller extent, was conducted by Alfred Stock
and Carl Somiesky. Although monosilane and disilane were already known, Stock and Somiesky discovered, beginning in 1916, the next four members of the SinH2n+2 series, up to n = 6, and they also documented the formation of solid phase polymeric silicon hydrides (vide infra).
, Si3H8 is trisilane, and so forth. There is no need for a prefix for one; SiH4 is called either monosilane or simply silane
. Silanes can also be named like any other inorganic compound; in this naming system, silane is named silicon tetrahydride. However, with longer silanes, this becomes cumbersome.
Branched silanes also exist. The radical
·SiH3 is termed silyl, ·Si2H5 is disilanyl, and so on. Trisilane with a silyl group attached to the middle silicon atom is named silyltrisilane. The nomenclature parallels that of alkyl radicals.
Circular structures, called cyclosilanes, also exist. These are analogous to the cycloalkane
s. Solid phase polymeric silicon hydrides called polysilicon hydrides
are also known. When hydrogen in a linear polysilene polysilicon hydride is replaced with alkyl
or aryl
side-groups, the term polysilane is used.
Unsaturated silicon hydride molecules called silenes contain the trivalent (doubly-bonded) silicon atom, >Si= (e.g. methylene silane or silaethene, H2Si(CH2). They cannot be isolated under normal conditions. The silylenes contain the divalent Si atom (e.g. SiH2, SiF2). Most silylenes are unstable at normal temperatures and can only be synthesized in the gas phase at elevated temperatures or isolated in a low-temperature matrix.
. These reactions may be classified as heterogeneous acid-base chemical reactions since the silicon ion can serve as a Brønsted–Lowry base capable of accepting protons. An example is the reaction between magnesium silicide
, Mg2Si, and hydrochloric acid.
In general, the alkaline-earth metals form silicides with various stoichiometry
. However, in all cases these substances react with Brønsted–Lowry acids to produce some type of hydride of silicon that is dependent on the Si anion connectivity in the silicide. The possible products include silane and/or higher molecules in the homologous series, a polymeric silicon hydride, or a silicic acid. Hence, MSi with their zigzag chains of Si anions (containing two lone pairs of electrons on each Si anion that can accept protons) yield the polymeric hydride (SiH2)x.
, with their decomposition reaction rate increasing with increases in the number of silicon atoms in the molecule. This makes preparation and isolation of SinH2n+2 molecules with n greater than about 8 difficult. Greater catenation
of the Si atoms can be obtained with the halides (SinX2n+2 with n = 14 for the fluorides) because of pi back bonding from the halogen p orbitals to the Si d orbitals, which compensates for the electron withdrawal from Si towards the halogen that occurs through the sigma bonding.
Silanes can also incorporate the same functional groups as alkanes, e.g. –OH, to make a silanol
(an analogue of alcohol
) or a halogen
to make a silicon halide (an analogue of alkyl halide
). There is (in principle) a silicon analogue for all carbon alkanes derivatives.
s to a polymer matrix, stabilizing the composite material
. They can also be used to couple a bio-inert layer on a titanium
implant
. Other applications include water repellents, masonry
protection, control of graffiti
, applying polycrystalline silicon
layers on silicon wafers when manufacturing semiconductors, and sealants.
Silane (SiH4) and similar compounds containing Si—H bonds are used as reducing agents in organic and organometallic chemistry.
“Magic sand
” exposes regular sand to trimethylhydroxysilane vapors to make the sand waterproof.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s of silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, which are analogues of alkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
hydrocarbons. Silanes consist of a chain of silicon atoms covalently bonded
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
to each other and to hydrogen atoms. The general formula of a silane is SinH2n+2. Much of the early work establishing that silicon does indeed form an homologous series of hydrides analogous to the alkanes
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
, albeit to a much smaller extent, was conducted by Alfred Stock
Alfred Stock
Alfred Stock was a German inorganic chemist. He did pioneering research on the hydrides of boron and silicon, coordination chemistry, mercury, and mercury poisoning...
and Carl Somiesky. Although monosilane and disilane were already known, Stock and Somiesky discovered, beginning in 1916, the next four members of the SinH2n+2 series, up to n = 6, and they also documented the formation of solid phase polymeric silicon hydrides (vide infra).
Nomenclature
There exists a regular nomenclature for silanes. For linear silanes, each silane's name is the word -silane preceded by a numerical prefix (mono-, di-, tri-, tetra-, etc.) for the number of silicon atoms in the molecule. Thus Si2H6 is disilaneDisilane
Disilane is a chemical compound with chemical formula Si2H6 that was identified in 1902 by Henri Moissan and . Moissan and Smiles reported disilane as being among the products formed by the action of dilute acids on metal silicides...
, Si3H8 is trisilane, and so forth. There is no need for a prefix for one; SiH4 is called either monosilane or simply silane
Silane
Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...
. Silanes can also be named like any other inorganic compound; in this naming system, silane is named silicon tetrahydride. However, with longer silanes, this becomes cumbersome.
Branched silanes also exist. The radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
·SiH3 is termed silyl, ·Si2H5 is disilanyl, and so on. Trisilane with a silyl group attached to the middle silicon atom is named silyltrisilane. The nomenclature parallels that of alkyl radicals.
Circular structures, called cyclosilanes, also exist. These are analogous to the cycloalkane
Cycloalkane
Cycloalkanes are types of alkanes that have one or more rings of carbon atoms in the chemical structure of their molecules. Alkanes are types of organic hydrocarbon compounds that have only single chemical bonds in their chemical structure...
s. Solid phase polymeric silicon hydrides called polysilicon hydrides
Polysilicon hydride
Polysilicon hydrides x are silicon backbone polymeric solids. The polysilicon hydrides are generally colorless or pale-yellow/ocher powders that, like silanes, are easily hydrolyzed and ignite readily in air.-Synthesis:The silanes are much less thermally stable than alkanes and they are...
are also known. When hydrogen in a linear polysilene polysilicon hydride is replaced with alkyl
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
or aryl
Aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....
side-groups, the term polysilane is used.
Unsaturated silicon hydride molecules called silenes contain the trivalent (doubly-bonded) silicon atom, >Si= (e.g. methylene silane or silaethene, H2Si(CH2). They cannot be isolated under normal conditions. The silylenes contain the divalent Si atom (e.g. SiH2, SiF2). Most silylenes are unstable at normal temperatures and can only be synthesized in the gas phase at elevated temperatures or isolated in a low-temperature matrix.
Synthesis
The reactions of metal silicides with dilute acids produce mixtures of monosilane, along with higher member of the homologous series. If the gaseous products are collected and condensed (by cooling) into a solid, and then liquified, the components can be separated by fractionationFractionation
See also: Fractionated spacecraftFractionation is a separation process in which a certain quantity of a mixture is divided up in a number of smaller quantities in which the composition changes according to a gradient. Fractions are collected based on differences in a specific property of the...
. These reactions may be classified as heterogeneous acid-base chemical reactions since the silicon ion can serve as a Brønsted–Lowry base capable of accepting protons. An example is the reaction between magnesium silicide
Magnesium silicide
Magnesium silicide, Mg2Si, is an inorganic compound consisting of magnesium and silicon. As a powder magnesium silicide is dark blue or slightly purple in color. Silicon dioxide, SiO2, found in sand and glass, when heated with magnesium forms magnesium oxide, and, if an excess of magnesium is...
, Mg2Si, and hydrochloric acid.
In general, the alkaline-earth metals form silicides with various stoichiometry
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
. However, in all cases these substances react with Brønsted–Lowry acids to produce some type of hydride of silicon that is dependent on the Si anion connectivity in the silicide. The possible products include silane and/or higher molecules in the homologous series, a polymeric silicon hydride, or a silicic acid. Hence, MSi with their zigzag chains of Si anions (containing two lone pairs of electrons on each Si anion that can accept protons) yield the polymeric hydride (SiH2)x.
Properties
The silanes (SinH2n+2) are much less thermally stable than alkanes (CnH2n+2) and they are kinetically labileLability
Lability refers to something that is constantly undergoing change or something that is likely to undergo change.-Chemistry:The term is used to describe a relatively unstable and transient chemical species...
, with their decomposition reaction rate increasing with increases in the number of silicon atoms in the molecule. This makes preparation and isolation of SinH2n+2 molecules with n greater than about 8 difficult. Greater catenation
Catenation
Catenation is the ability of a chemical element to form a long chain-like structure via a series of covalent bonds. Catenation occurs most readily in carbon, which forms covalent bonds with other carbon atoms. Catenation is the reason for the presence of a large number of organic compounds in nature...
of the Si atoms can be obtained with the halides (SinX2n+2 with n = 14 for the fluorides) because of pi back bonding from the halogen p orbitals to the Si d orbitals, which compensates for the electron withdrawal from Si towards the halogen that occurs through the sigma bonding.
Silanes can also incorporate the same functional groups as alkanes, e.g. –OH, to make a silanol
Silanol
Silanol, also known as silyl alcohol, is a chemical with formula SiH3OH. It is the simplest silicon alcohol, and is a heavy, volatile, colorless, flammable liquid. At room temperature it is a polar liquid...
(an analogue of alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
) or a halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
to make a silicon halide (an analogue of alkyl halide
Haloalkane
The haloalkanes are a group of chemical compounds derived from alkanes containing one or more halogens. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially and, consequently, are known under many chemical and...
). There is (in principle) a silicon analogue for all carbon alkanes derivatives.
Applications
Several industrial and medical applications exist for the simplest silane (SiH4) and functionalized silanes. For instance, silanes are used as coupling agents to adhere glass fiberFiberglass
Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...
s to a polymer matrix, stabilizing the composite material
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
. They can also be used to couple a bio-inert layer on a titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
implant
Implant (medicine)
An implant is a medical device manufactured to replace a missing biological structure, support a damaged biological structure, or enhance an existing biological structure. Medical implants are man-made devices, in contrast to a transplant, which is a transplanted biomedical tissue...
. Other applications include water repellents, masonry
Masonry
Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...
protection, control of graffiti
Graffiti
Graffiti is the name for images or lettering scratched, scrawled, painted or marked in any manner on property....
, applying polycrystalline silicon
Polycrystalline silicon
Polycrystalline silicon, also called polysilicon, is a material consisting of small silicon crystals. It differs from single-crystal silicon, used for electronics and solar cells, and from amorphous silicon, used for thin film devices and solar cells....
layers on silicon wafers when manufacturing semiconductors, and sealants.
Silane (SiH4) and similar compounds containing Si—H bonds are used as reducing agents in organic and organometallic chemistry.
“Magic sand
Magic sand
Magic sand, Moon Sand, Mars Sand, Space Sand,Sqand or Aqua Sand is a toy made from sand coated with a hydrophobic compound. The presence of this hydrophobic compound causes the grains of sand to adhere to one another and form cylinders when exposed to water...
” exposes regular sand to trimethylhydroxysilane vapors to make the sand waterproof.
See also
- SilaneSilaneSilane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...
, the simplest silicon-hydrogen compound with chemical formula SiH4 - SilanolSilanolSilanol, also known as silyl alcohol, is a chemical with formula SiH3OH. It is the simplest silicon alcohol, and is a heavy, volatile, colorless, flammable liquid. At room temperature it is a polar liquid...
s