Q7(c). Silicones: preparation, properties and uses
What are silicones? How are they obtained? Discuss the important properties and applications of organosilicon polymers.
Answer in brief
Silicones are polymers with a backbone of alternating silicon and oxygen atoms and organic groups on silicon, –[R₂SiO]ₙ–. They are made by hydrolysing organochlorosilanes from the Rochow process; they are thermally stable, water repellent and inert, and are used as oils, greases, rubbers and resins.
Key points
- chains, rings or networks of –Si–O–Si– bonds with R (usually CH₃, also C₆H₅) on each Si. Repeat unit R₂SiO, analogous in formula to a ketone R₂CO (hence the name), but polymeric, because Si does not form stable Si=O double bonds.
- CH₃Cl over silicon with a copper catalyst gives methylchlorosilanes, mainly (CH₃)₂SiCl₂, separated by distillation: n MeCl + Si/Cu → Me_(n)SiCl_(4-n)
- Hydrolysis: (CH₃)₂SiCl₂ + 2H₂O → (CH₃)₂Si(OH)₂ + 2HCl
- the silanediol loses water: n (CH₃)₂Si(OH)₂ → [(CH₃)₂SiO]_(n) + n H₂O, giving chains or rings.
The full model answer
Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.
UNIT–R₂SiO–STARTCH₃Cl + Si/CuCONTROLMe₃SiCl ends, MeSiCl₃ cross-links
What they are
- Polyorganosiloxanes: chains, rings or networks of –Si–O–Si– bonds with R (usually CH₃, also C₆H₅) on each Si. Repeat unit R₂SiO, analogous in formula to a ketone R₂CO (hence the name), but polymeric, because Si does not form stable Si=O double bonds.
How they are obtained
- Rochow (direct) process: CH₃Cl over silicon with a copper catalyst gives methylchlorosilanes, mainly (CH₃)₂SiCl₂, separated by distillation: n MeCl + Si/Cu → Me_(n)SiCl_(4-n)
- Hydrolysis: (CH₃)₂SiCl₂ + 2H₂O → (CH₃)₂Si(OH)₂ + 2HCl
- Condensation: the silanediol loses water: n (CH₃)₂Si(OH)₂ → [(CH₃)₂SiO]_(n) + n H₂O, giving chains or rings.
- Controlling the product: (CH₃)₃SiCl gives (CH₃)₃SiOH, which caps a chain end (chain stopper; mixing ratio sets the chain length). CH₃SiCl₃ gives CH₃Si(OH)₃, which cross-links chains into resins.
Properties
- Thermal and chemical stability: the strong Si–O and Si–C bonds and the outer layer of methyl groups make silicones stable to heat and oxidation and inert to most reagents.
- Water repellent: the methyl groups face outwards; silicones are very hydrophobic and have low surface tension.
- Viscosity changes little with temperature (unlike hydrocarbon oils), and the flexible, helical Si–O–Si chains stay rubbery at low temperature. They are good electrical insulators and non-toxic.
- Form depends on chain length and cross-linking: oils (short chains), greases and rubbers (long chains, light cross-linking), hard resins (much cross-linking).
Applications
- Lubricants and inert fluids for high and low temperatures, hydraulic (brake) fluids; silicone greases and sealants.
- Water-repellent sprays for fabrics, shoes and masonry; varnishes and electrical insulation; silicone rubber for gaskets and implants.
- Personal-care products (shampoos, conditioners, shaving foams) and antifoaming agents.
Application. silicone rubber and the polyphosphazenes are the two "inorganic rubbers": both stay flexible at low temperature because the Si–O–Si and P–N–P links are highly flexible.
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All parts of Question 7
- 7(a)(i) Vaska's compound: structure and oxidative addition of HCl and O₂15 marks
- 7(a)(ii) Sodium nitroprusside from K₄[Fe(CN)₆]; reaction with S²-10 marks
- 7(b) Cytochromes: prosthetic group, haemoglobin, function10 marks
- 7(c) Silicones: preparation, properties and uses15 marks