Silicates (SiO4)-4: salts of silicic acids:

(1) Silicates is the general term applied for the solids with silicon – oxygen bonds.

(2) Silicates are regarded as the salts of silicic acid, H₄SiO₄. All the silicates are comprised of SiO₄ units.

(3) The Silicates units have a tetrahedral structure formed as a result of sp3 hybridization. Silicon atom has its complete octet but each oxygen atom is still short of one electron to complete its octet. They can complete their octet by taking up 4 electrons from a metal, getting converted to an anion [SiO4]^-4

(4) All the solids Silicates contain silicate ion (SiO4)^4- as the basic structural unit.

(5)The silicate ion is tetrahedral in structure and when the one or more oxygen atoms between such tetrahedrons, a complex structure arise.

(6) All the silicates units are non planer

(7) The Si atom in Silicates unit is covalently bonded to 4 oxygen atoms. Each oxygen atom possesses a formal negative charge. Hence each tetrahedral unit has a formal charge of –4. When linked together, the extended units are also negatively charged. Presence of other metallic ions such as Ca²⁺or Mg²⁺ are necessary for electrical neutrality.

(8) The covalent Si-O bond, having a bond enthalpy of 466 kJ mol^-1 is particularly strong compared with the C-C bond which has a bond enthalpy of 347 kJ mol^-1. The linkage -Si-O-Si-O- is very stable and instead of existing as discrete units of (SiO4)^4- ions, the silicates tend to form chains, sheets or networks.

 CLASSIFACATION OF SILICATES:

The silicates may be classified in to following groups chain silicates, ring silicates, cyclic silicates, sheet silicates, three – dimensional silicates depends on the way in which the (SiO4)4- tetrahedral units are linked together.

(1) Ortho silicates or Neso Silicates (SiO4)^4-: 

Number of oxygen shared by tetrahedron is Zero(0).Orthosilicates are salts of orthisilicic  acid

Example; Zircon ( ZrSiO₄ ), Phenacite (Be₂SiO₄) , Willemite (Zn₂SiO₄),  

(2) Pyro silicates or Sorosilicates or Disisilicates (Si₂O₇^-6) :    

(1) Number of oxygen atoms shared per tetrahedron is 1(one).

(2)Total number of shared oxygen towards one Si atom is ½

(3)Total contribution of all oxygen towards one Si atom is 3.5

(4) In pyrosilicates, one tetrahedron shares its one oxygen atom with other tetrahedron on they are salts of pyrosilicic acids .

Example: Thortveitite (Se₂Si₂O₇); lanthanoids disilicates (Ln₂[Si₂O₇] and Hemimorphite  Zn₄ (OH)₂[Si₂O₇].

(3) Meta silicates: 

 (A) Linear chain Meta Silicates [(SiO₃)_n ^-2n  ] :

In chain silicates each tetrahedron shares its  two  oxygen atoms with other tetrahedron atom such that a linear endless chain of silicates is formed , however in terminating chain silicates (n) tetrahedron  shares one oxygen.

Example: Spodumene (LiAlSiO₃)₂ ; Wollastonite [Ca₃(SiO₃)₃] Enstatite [Mg₂SiO₃)₃] and Diopsite [ CaMg(SiO₃)₂]

(B) Cyclic meta silicates [(Si_nO_3n) ^-2n  ] :

(1) Number of oxygen atoms shared per tetrahedron is 2(Two).

(2) Total number of shared oxygen towards one Si atom is one (1)

(3) In cyclic silicates each tetrahedron shares its two oxygen atoms with other tetrahedron atom such that a cyclo silicates is formed.

(4) There are several cyclo silicate is known like n= 3, 4, 5 ,6 but  3 and 6 is most common.

Example: Wollasponite [ Ca₃(Si₃O₉)] , Benitoite [BaTi(Si₃O₉)] , Beryl [Be₃Al₂(Si₃O₉)₂]

(4) Double chain Silicates or Amphiboles  (Si₄O₁₁)^-6n :

(1) Number of Silicon atoms in basic units is 2 .

(2) Total number of oxygen atoms in basic unit is 5.5.

(3) Number of oxygen atoms shared per tetrahedron is 3+2/2=2.5

(4) In this type silicates two strands of chain silicates are linked to each other by sharing oxygen atoms, Amphiboles are an asbestos mineral which contains magnesium.

(5)  General formula of double chain silicates (Si₄O₁₁)^-6n

Example: Tremotites  [Ca₂Mg₅(Si₄O₁₁)₂(OH)₂] Crocidolite [Na₂Fe₃Fe₂(Si₄O₁₁)₂(OH)₂]

(5) Sheet Silicates or phyllo silicates [(Si₂O₅)_n]^-2n:

In Sheet Silicates, one tetrahedron shares its three (3) oxygen atom such that  a two dimensional non planer layer is formed , such type of silicates have greasy touch or lubricating action.

Example:  Clay [Al₂(Si₂O₅)(OH)_{4 ,}Talc or Soap Stone  [Mg₃(Si₂O₅)₂(OH)_{2 ,} Kaolinite  [Al₂(Si₂O₅)(OH)₄  Chrysotite (white Asbestos)  [Mg(Si₂O₅)(OH)₄

(6) 3D-Silicates:

Examples: Silica, Quarts, feldspar, Zeolite, Ultramarine

Related Questions:

(1) What is "Zeolite's" ?

(2) What are most common cyclic meta silicates?

(3) What are the Silicones? Give one example one cyclic Silicone?

SILICONES: (R2SiO WITH -Si-O-Si- LINKAGE):

(1) Silicones are organosilicon polymeric compounds containing repeated R₂SiO units and (-Si-O-Si-) linkage.

(2) The name is given silicone because their empirical formula is analogous to that of ketones (R₂CO).

(3)  The Silicones are form by hydrolysis of silicone tetra chloride (SiCl₄) .we know that CCl₄ do not hydrolyses by water at room temperature While SiCl₄  undergoes   water Hydrolysis  to corresponding OH group.  

                          CCl₄ +H₂O¾® no hydrolysis

                     But super heated steam gives phosgene gas.

                      CCl₄ + H₂O ¾®COCl₂ + 2HCl

(4) SiCl₄ under goes hydrolysis due to presence vacant d orbital and gives Silicic acid followed by dehydration gives 3D Silicates (SiO₂)

     

PREPARATION OF SILICONES:

It is two step process

Step: (1) Preparation of Organosilicon halides  as silicone intermediates.

Step: (2) hydrolysis of Organosilicon halides followed by condensation polymerisation.

Step: (1) [A] FROM FRIGNARD REAGENT:

Step: (1) [B] BY DIRECT HEATING PROCESS:

Note-The Yield of  above reaction is 50% R₂MgCl₂ and  50 % ( R₃MgCl + RMgCl₃), now these can seperated by fractional distilation.

Step: (2) HDROLYSIS FOLLOWED BY CONDENSATION:

(1) RMgCl₃ (R = Me or Ph): On hydrolysis  and followed by condensation give 3D network cross linked Silicoes It also provides the crosslinking among the chain making the polymer more hard and hence controlling the proportion of RSiCl₃ we can control the hardness of polymer. 

(2) R₂SiCl₂ (R = Me or Ph): On hydrolysis  and followed by condensation give linear as well as cyclic Silicones depending upon number of Silicon atomes.  Commercial silicon polymers are usually methyl derivatives and to a lesser extent phenyl derivatives. They are prepared by the hydrolysis of R₂SiCl₂ (R = Me or Ph).

Note-Silicones may have the cyclic structure also having 3, 4, 5 and 6 nos. of silicon atoms within the ring. Alcohol analogue of silicon is known as silanol.

Note-Cyclic Silicone have Sp3 oxygen and Silicon atoms so cyclic Silicones have following properties .they are non planer ,polar and have Back Bonding  and chair confermer.

(3) R₃SiCl (R = Me or Ph): On hydrolysis  and followed by condensation  so only dimmerisation take place due to presence of single OH group.

Note- R₃SiCl use in a certain proportion we can control the chain length of the polymer  due to this reason R₃SiCl is called as chain stopping unit.

INERTNESS OF SILICONES: DUE TO..

(1) Silicones are chemically inert due to back bonding between oxygen and Silicone atoms

(2) High bond energy of Si-C and Si-O bond also due to Back Bonding.

(3) Alkyl group constitute hydrophobic part which act as water repellent hence nucleophilic attack retarded .

USES OF SILICONES:

(1) Silicones are chemically inert, water repelling nature, heat resistance and having good electrical insulating properties.

(2) Silicones are used as sealants, greases, electrical insulators and for water proofing of fabrics, car polish, shoe polish and masonry works in buildings

(3) Silicones can be used as electrical insulator (due to inertness of Si-O-Si bonds)

(4) Silicones are used as antifoaming agent in sewage disposal, beer making and in cooking oil used to prepare potato chips.

(5) Silicones use as a lubricant in the gear boxes