Silanes, Silicon Halides, Silicates | Chemistry Optional Notes for UPSC PDF Download

Silanes

• It is an inorganic compound with chemical formula, Si H₄.
• It is a colourless, flammable gas with a sharp, repulsive smell. 
• It is the silicon analogue of methane.

Preparation

• Silane is produced from magnesium silicide and hydrochloric acid:
  
• It can also be synthesized by the reactions between silicon, hydrogen and silicon tetrachloride:
  
• A third route includes the reduction of silicon dioxide (SiO₂) in presence of aluminium and hydrogen:
  

Structure

• It has sp³ hybridisation
• The molecular structure of the silanes directly affects their physical and chemical characteristics. 
• The Si atoms in silanes are always sp³ hybridised, that is the valence electrons are said to be in four equivalent orbitals derived from the combination of the 3s orbital and the three 3 p orbitals. 
• Three orbitals, which have identical energies and are arranged spatially in the form of a tetrahedron. 
• It has 109.5 ° bond angle between the Si - H bonds. 
• A silane molecule has only Si- H and Si- Si single bonds.
• Si- H bonds are formed from the overlap of sp³ orbital of silicon with 1s - orbital of hydrogen. 
• Si- Si bonds are formed by the overlap of two sp3 orbitals on different silicon atoms. 
• The bond length is 746.0 pm for a Si- H bond and 233pm for Si- Si bond. 
• The spatial arrangement of the bonds is similar to that of the four sp3 orbitals, when they are arranged tetrahedrally with an angle of 109.5 ° between them.

Chemical properties

Uses

• It is used as coupling agents to adhere glass Fibre to a polymer Matrix for stabilizing the composite material.
• Used to couple a Bio inter layer on a Titanium implant. 
• Use as water repellents.

Silicon Halides

[Si X₄] [X = Cl, Br, I, F]

Silicon Tetrabromide [Si Br₄]

• It is an inorganic compound with the formula Si Br₄.
• It is a colourless liquid with suffering odour. 
• Undergoes hydrolysis readily with the liberation of HBr.
• It resembles closely as Si Cl₄ in all its properties.

Synthesis

• It is synthesized by the reaction of Si with HBr at 6000C.
   
• Reduction: SiBr₄ can be readily reduced by hydrides or complex hydrides.
  

Reaction with alcohols and amines

Reaction with grignard reagents

Grignard reaction with metal alkyl halide are particularly important reaction due in their production of organosilicon compounds which can be converted in Silicones
Si(OR) 4 nRMgX RnSiBr 4 nMgX Br Organosilicon magnesium

Uses

• The pyrolysis of SiBr₄ has the advantage of deposition silicon and faster rate.
• Pyrolysis of SiBr₄ for followed by treatment with NH₃ yeild Silicon nitride coating a hard compound used for ceramic, scalants and production of many cutting tools.

SiF₄: silicon tetrafluoride

• It is mainly occur in the volcanic fumes containing significant amount which on hydrolysis forms of hexa fluoro silicic acid.

Preparation

It is prepared by heating BaSiF₆, above 300° C

Uses

• It is used in microelectronics and organic synthesis. 
• SiI₄: Silicon tetraiodide
  

Physical properties

• Colourless liquid at room temperature
• SiCI4: Silicon tetrachloride

Preparation

• It is prepared by passing dry Cl₂ over heated amorphous silicon or magnesium silicate (Mg₂Si)
  
• It is also prepared by passing a mixture of dry Cl₂ and Si₂Cl₄ on SiO₂
  

Silicates

• They are the largest most interesting and complicated class of minerals than any other minerals.
• Approximately 30% of all minerals are silicates and some geologist estimate that 90% of the earth crust is made up of silicate SiO₄ ^4- base minerals. 
• Silicate is based on the basic chemical unit SiO₄ ^4- tetrahedron shaped and anionic group. 
• The central Silicon Ion has a charge of positive four which each oxygen has a charge of negative two (-2) and thus Si-O bond is equal to one half the total bond energy of oxygen. 
• This condition level the oxygen with the the option of bonding to another silicone ion and therefore linking one SiO₄ ^4- tetrahedron to another.

Classification

• Orthosilicates 
• Pyrosilicates 
• Ring and chain silicates 
• Double chain silicates 
• Silicates with sheet structure
• Silicates with 3D fragments

Structure and Bonding

• The main structural unit of silicate is Td cluster connecting one silicon and four O atom.
• The size of SiO₄ ^4- tetrahedral is relative stable with Si-O Bond length verifying from 0.161 to 0.164nm. 
• The strength of Si-O bond is pretty high that provides the thermal stability and chemical resistance of the majority of silicate compound. 
• The Tetrahedral Cluster can be polymerized (i.e.) linked to each other through the bridging oxygen atoms. 
• They are able to form polymers by means of linkage with one, two, three or four neighbouring tetrahedra, forming siloxane, Si - O - Si bonds. 
• Other ions can be located in the silicate lattices such as Li⁺, Na ⁺, K⁺, Be²⁺, F etc. 
• Some cations such as Al, B, Be, are able to isomorphically substitute Si atoms in the Si-O tetrahedral. 
• However, most of them are located out of the anionic framework and play the role of “charge balanced cations” and they are all six- coordinates.

Uses

• It is used as thermal insulators. 
• Used to make oscillators, used in watches, radios and pressure gages. 
• Used to make glass and ceramics. 
• Used to form microchips for watches