Silanes

The term is silanes according to the IUPAC rules for a group of substances of chemical compounds that consist of a silicon backbone and hydrogen. Similar groups of substances are germane and alkanes. Silanes may have a branched (iso- and neo- silanes) or unbranched (n- silanes) structure. The empirical formula of acyclic ( open-chain, also catena- silanes mentioned ) silanes Si n H 2n 2 is. Ring-shaped silicon-hydrogen compounds called cyclosilanes ( empirical formula: Si n H 2n ).

• 4.1 Chemical reactions

History

Friedrich Wöhler had in 1857 produced by the decomposition of silicon- aluminum with hydrochloric acid for the first time a silane. He had observed that the liberated during this reaction, hydrogen was contaminated with another substance. This is the conclusion he came, because the resulting gas violently reacted with oxygen in the air and exploded. Hydrogen itself reacts only at elevated temperature or in the presence of catalysts (see oxyhydrogen sample).

1902, the idea of ​​silicon hydrides has been revived. Henri Moissan succeeded in the detection of mono-silane by proteolysis of lithium silicide. Since 1916, Alfred Stock, a professor of inorganic chemistry in Karlsruhe dealt intensively with the Silanwasserstoffchemie. He hoped to win by similar decompositions longer-chain liquid " silicon Gasolines". With the help of magnesium silicide ( Mg2Si ) and acid was initially successful floor in the representation of the lower gaseous silanes Monosilane, disilane, and trisilane tetrasilane. Since these silanes are termed " Stock'sche silanes ". Stock gave the whole group of materials also the common name silanes.

Alfred Stock, designed to represent the silanes A glass apparatus that allowed working under complete exclusion of air. He used a glass flask half filled with aqueous sulfuric acid, this acid was stirred and poured the ground, gray magnesium silicide portions so. Here, the silicide decomposed under heat and formation of gaseous hydrogen and silanes. The gases led floor now in a glass apparatus, which he cooled from the outside. The cooling was adjusted so that the hydrogen and monosilane were not condense. He hoped that through the cooling that any resulting long-chain silanes would condense due to their higher boiling points. In fact, he managed to win a water-clear liquid, which was composed of three chain- silanes. However, further derivatization of these compounds had no success.

Nomenclature

The file name extension is analogous to the alkanes. Each name ending with the suffix -silane, such as the alkanes, at ends (methane, ethane, propane, butane, and so on ). Otherwise, the number of silicon atoms is as a Greek numeral in the appointment with a: Monosilane ( a silicon atom ), disilane ( two silicon atoms ), trisilane etc. To further differentiate it provides depending on the structure of the molecule, one of the descriptors n-, iso-, neo - or cyclo - the connection name forward.

Structure

Konstitutionsisomerie

Contains a silane four or more silicon atoms, so are different arrangements, or more precisely Constitutions, conceivable. One speaks of Konstitutionsisomerie. The isomers are distinguished by various physical characteristics such as melting and boiling points.

Homologous series of silanes

Silanes are silicon - homologs based on a carbon skeleton alkanes. However, it is much less silanes produced as there are hydrocarbons. Are previously known silanes having up to 8 silicon atoms

The homologous series of linear, unbranched stems from silanes of the general formula H-( SiH2 ) n-H with n = 1, 2, 3, ...

Isomers and cyclic silanes

Similar to the alkanes (from tetrasilane ) is observed at the higher silanes, the occurrence of structural isomers. There are known branched and cyclic silanes.

Properties

The lowest silanes - Monosilane and disilane ( Si2H6 ) - are gaseous. From trisilane ( Si3H8 ) silanes occupy a liquid state.

Chemical reactions

Silanes are very unstable in contrast to the homologous alkanes. They are synthesized only under exclusion of air. The low silanes, ie, the silanes with one to four silicon atoms are very unstable and are subject to spontaneous combustion in air, explode and burn spontaneously to form silicon dioxide and water.

The reactivity decreases with increasing chain length. Already pentasilane unresponsive autonomously with the oxygen content of the air. From heptasilane silanes are no longer spontaneous self-igniting.

At 300 ° C thermolysis takes place at the higher silanes. They fall into Monosilane, polysilanes and other polymer products ( (SiH <2) x). Higher temperatures lead to the disintegration of the elements.

Silanes can also react with nitrogen by decompose at temperatures of about 500 ° C and the resulting silicon reacts with nitrogen. And microwave radiation can be used for the decomposition. This reaction is, for example, used for producing thin layers of silicon nitride or silicon nanocrystals.

In water at a pH above 7 is silanes decompose to silica and hydrogen:

Production and representation

Best viewed the silanes are in the form of so-called crude silane mixture by means of decomposition of magnesium silicide ( Mg 2 Si ) under acidic conditions and the exclusion of air.

The selective synthesis of mono-silane is possible by the method of Sunderineyer in a molten salt as the reaction medium. The starting materials are Tetrachlorosilane and lithium hydride as a hydrogen:

Trisilane and higher silanes are accessible in good yields from monosilane by means of an electric discharge.

Arise, depending on the synthesis conditions, among other Polysilen ( SiH2 ) ∞, the silicon homologs of the alkenes, or Polysilin (SiH ) ∞ (compare alkynes ). Compounds with intermediate stoichiometry ( ( reflect essence ) ∞, 1 ≤ n ≤ 2 ) and ring-shaped oligo- and polysilanes are possible.

Derivatives of silanes

Derivatives ( derivatives) of the silanes are formally by replacement (substitution) of the hydrogen atoms by halogens, oxygen, nitrogen and carbon or these groups of elements contained.

These are, the chlorosilanes monochlorosilane ( SiH3Cl ), dichlorosilane ( SiH2Cl2 ), trichlorosilane ( SiHCl3 ) and Tetrachlorosilane ( SiCl4 ).

By replacing the hydrogen in the silanes by organic radicals, we obtain organosilicon compounds that are perceived by IUPAC as derivatives of the silicon.

Other important compounds are substituted silanes, such as the largely chemically inert tetramethylsilane as well as the number of chloromethylsilanes, especially the dichlorodimethyl which serve as starting materials for the manufacture of silicone.

Compared with the corresponding carbon compounds only the silanols and siloxanes are significant stable silicon compounds.

Use

Trichlorosilane is an intermediate for the production of high-purity silicon integrated circuit ( microchip ). From chlorosilanes and chloroalkylsilanes be prepared by reacting in an oxyhydrogen flame so-called pyrogenic silicas produce, an important filler for plastics. Special designated as functional silanes, organosilanes are used for surface functionalization, which is also referred as silanization.

Higher, not self-igniting silanes were discussed for their use as rocket fuel. The high price and the lower compared to hydrazine or hydrogen-oxygen mixtures specific impulse speak against silanes. However, higher liquid silanes such pentasilane could be suitable as non-toxic additives to increase the combustion efficiency of conventional rocket fuels.

Sources and footnotes

• Substance Group
• Silane