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Concept 05 / 09The p-Block Elements

Compounds of Carbon and Silicon: Oxides, Silica, Silicones, Silicates and Zeolites

Carbon monoxide and carbon dioxide: preparation, structure, properties and toxicity; silicon dioxide as a network solid; silicones from alkylchlorosilanes; the classification of silicates by sharing of SiO4 tetrahedra; zeolites as molecular sieves and catalysts.

Figure 1
Carbon monoxide. Formed by incomplete combustion of carbon, or in the laboratory by dehydrating formic acid with concentrated H2SO4\mathrm{H_{2}SO_{4}} at 373 K373\,\mathrm{K}: HCOOH→H2O+CO\mathrm{HCOOH \rightarrow H_{2}O + CO}. Commercially as water gas (C+H2O→CO+H2\mathrm{C + H_{2}O \rightarrow CO + H_{2}}) or producer gas (CO+N2\mathrm{CO + N_{2}}, from air over hot coke). Colourless, odourless, almost insoluble, neutral. Structure :C≡O:\mathrm{:C \equiv O:} (one σ\sigma, two π\pi bonds, a lone pair on carbon). It is a powerful reducing agent (Fe2O3+3CO→2Fe+3CO2\mathrm{Fe_{2}O_{3} + 3CO \rightarrow 2Fe + 3CO_{2}}) and forms metal carbonyls by donating the lone pair on carbon. It is highly poisonous: it binds haemoglobin about 300300 times more strongly than oxygen, forming carboxyhaemoglobin and blocking oxygen transport. Carbon dioxide. Formed by complete combustion, and in the laboratory from CaCO3+2HCl→CaCl2+CO2+H2O\mathrm{CaCO_{3} + 2HCl \rightarrow CaCl_{2} + CO_{2} + H_{2}O}. Linear (spsp carbon, two C=O bonds, 115 pm115\,\mathrm{pm}, zero dipole moment). Weakly acidic in water: CO2+H2O⇌H2CO3\mathrm{CO_{2} + H_{2}O \rightleftharpoons H_{2}CO_{3}}. Solid CO2\mathrm{CO_{2}} (dry ice) sublimes at 195 K195\,\mathrm{K} and is used as a refrigerant. It is used in photosynthesis and is a major greenhouse gas; its rising concentration contributes to global warming. Silicon dioxide (silica). A covalent 3D network in which each Si is tetrahedrally bonded to four O atoms, and each O bridges two Si atoms. Crystalline forms: quartz, cristobalite, tridymite. Very unreactive (high Si-O bond enthalpy), but attacked by HF and NaOH: SiO2+4HF→SiF4+2H2O\mathrm{SiO_{2} + 4HF \rightarrow SiF_{4} + 2H_{2}O}; SiO2+2NaOH→Na2SiO3+H2O\mathrm{SiO_{2} + 2NaOH \rightarrow Na_{2}SiO_{3} + H_{2}O}. Uses: quartz as a piezoelectric material (clocks), silica gel as a drying agent and chromatography support, kieselguhr in filtration. Silicones. Synthetic organosilicon polymers with the repeating unit −(R2SiO)−\mathrm{-(R_{2}SiO)-} (see figure). Made by hydrolysis of alkylchlorosilanes followed by condensation: (CH3)2SiCl2+2H2O→(CH3)2Si(OH)2+2HCl\mathrm{(CH_{3})_{2}SiCl_{2} + 2H_{2}O \rightarrow (CH_{3})_{2}Si(OH)_{2} + 2HCl}; the silanediols then polymerise, losing water. (CH3)3SiCl\mathrm{(CH_{3})_{3}SiCl} caps the chain ends (chain length control); CH3SiCl3\mathrm{CH_{3}SiCl_{3}} gives cross-linked networks. The methyl groups make silicones water-repellent; the Si-O-Si backbone makes them thermally stable, chemically inert and good electrical insulators. Uses: sealants, greases, electrical insulation, waterproofing fabric, surgical and cosmetic implants. Silicates. All contain SiO44−\mathrm{SiO_{4}^{4-}} tetrahedra, linked by sharing 00, 11, 22, 33 or 44 corner oxygen atoms: orthosilicates (isolated SiO44−\mathrm{SiO_{4}^{4-}}, zircon), pyrosilicates (Si2O76−\mathrm{Si_{2}O_{7}^{6-}}), cyclic silicates ((SiO3)n2n−\mathrm{(SiO_{3})_{n}^{2n-}}, beryl), chain silicates (pyroxenes; double chains, amphiboles and asbestos), sheet silicates ((Si2O5)n2n−\mathrm{(Si_{2}O_{5})_{n}^{2n-}}, mica, talc, clay), and 3D silicates (sharing all four corners, silica itself). Replacing some Si by Al gives aluminosilicates such as feldspar and zeolites. Glass and cement are man-made silicates. Zeolites. 3D aluminosilicates with open cages and channels of molecular dimensions, acting as molecular sieves: only molecules of suitable size can enter. ZSM-5 converts alcohols directly into gasoline; hydrated zeolites are used as ion exchangers to soften water. Key Takeaways: - CO: neutral, reducing, forms carbonyls, binds haemoglobin; CO2\mathrm{CO_{2}}: linear, acidic, greenhouse gas. - SiO2\mathrm{SiO_{2}} is a network solid; attacked by HF and hot alkali. - Silicones: R2SiCl2\mathrm{R_{2}SiCl_{2}} gives chains, R3SiCl\mathrm{R_{3}SiCl} ends them, RSiCl3\mathrm{RSiCl_{3}} cross-links. - Silicates are classified by how many O atoms each SiO4\mathrm{SiO_{4}} tetrahedron shares. - Pitfall: CO2\mathrm{CO_{2}} is a gas while SiO2\mathrm{SiO_{2}} is a high-melting solid, because Si cannot form stable pπp\pi-pπp\pi double bonds with O. Brain-Teaser 1: What is the role of (CH3)3SiCl\mathrm{(CH_{3})_{3}SiCl} and CH3SiCl3\mathrm{CH_{3}SiCl_{3}} in making silicones? Solution: Hydrolysis of (CH3)3SiCl\mathrm{(CH_{3})_{3}SiCl} gives (CH3)3SiOH\mathrm{(CH_{3})_{3}SiOH} with only one OH, which can join only one end: it terminates chains and controls molecular mass. CH3SiCl3\mathrm{CH_{3}SiCl_{3}} gives three OH groups, allowing branching and cross-linking into a 3D resin. Brain-Teaser 2: What is the charge on a cyclic silicate anion containing six SiO4\mathrm{SiO_{4}} tetrahedra, each sharing two oxygens? Solution: Each tetrahedron has two bridging O (each counted half) and two terminal O: effective formula SiO32−\mathrm{SiO_{3}^{2-}}. Six units give Si6O1812−\mathrm{Si_{6}O_{18}^{12-}}, the anion in beryl, Be3Al2Si6O18\mathrm{Be_{3}Al_{2}Si_{6}O_{18}}. Brain-Teaser 3: Why is CO poisonous while CO2\mathrm{CO_{2}} is not, though both contain carbon and oxygen? Solution: CO has a lone pair on carbon and empty π∗\pi^{*} orbitals; it binds strongly to the iron of haemoglobin (both donating and accepting electrons), about 300300 times more strongly than O2\mathrm{O_{2}}, blocking oxygen transport. CO2\mathrm{CO_{2}} has no such binding ability and is merely an asphyxiant at very high concentration.

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Compounds of Carbon and Silicon: Oxides, Silica, Silicones, Silicates and Zeolites — DPP

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