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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) SiClunder 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

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