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Group IV Elements. Carbon. The element with the most compounds known Most of the compounds are considered to be organic What defines an organic compound? Compound is made by an organism and found in nature as part of an organism or stemming from an organism Inorganic carbon compounds.
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Carbon • The element with the most compounds known • Most of the compounds are considered to be organic • What defines an organic compound? • Compound is made by an organism and found in nature as part of an organism or stemming from an organism • Inorganic carbon compounds
Carbon • The electronic structure of Carbon: • 1s22s22p2 • The C4+ ion does not exist • A unique feature of Carbon is its self binding properties. • It can form single, double and triple bonds, and forms stable chains. • The C_C bond is thermodynamically very stable and strong.
Carbon • Chemistry and Physical Properties of Diamond, Graphite, Fullerenes, adn Carbides • Diamond – Graphite: differ in their properties because of the differences in the arrengement and bonding • Density of Diamond is higher, graphite is more stable • Graphite can be transformed into Diamond at 3000K adn pressures above 125kbar. AS catalysts Cr, Fe or Pt. • Chemial reactivity of Diamond is lower then graphite or amorphous carbon.
Carbon • Graphite: • Amorphous carbon: many forms are microcrystalline forms of graphite. • Physical properties of these materials are mainly determined by their surface area. • The large surface forms: • Absorb large amounts of gases and solutes. Carbon, doted with Pd, Pt are uysed as industrial catalysts.
Carbon • Fullerenes: • C60 Buckminster fullerene or Buckyball fullerene. • Birch reduction: • Unsaturation of C60 by Li in NH3/t-BuOH • The other chemistry of fullerenes is with organic molecules (we are not going through this).
Carbon • C60 can react towards transition metals an η2 connection to Pt or Ir is made by the π electrons of a C=C group
Carbon • Partial Halogenation can be achieved with reaction with Cl2 or Br2 • C60 + Br2 C60Br2 or C60 Br4 • At 150 deg C Br2 can be recovered • C60 + Cl2 C60Cln (n=24 at average) • At t>400deg C dechlorinate. • Alkali metals results in black materials (K+)3C603-. • Films of C60 doped with K,Rb or Cs metal vapor are superconducting. RbnC60 is 30 K.
Carbon • Carbides • Solid compounds with elements other than H are generally called carbides. • Classification of Carbides: • Ionic Carbides • Alkali and alkaline earth metals and Al.
Carbon • Interstitial Carbides • Reactions with transition metals. The metals occupy tetrahedral holes in the close packed arrays of metal atoms. • Materials are very hard, conducting, and have high melting points (3000-4800 deg C). • WC is hard, used for machining steel. • Cr,Mn,Fe,Co,Ni are between ionic and interstitial. Hydrolysed by watere and dilute acids.
Carbon • Covalent Carbides: • Si, B form SiC B4C hard, chemically inert. SiC has diamond like structure in which C and Si are tetrahedrally surrounded by four of the other kind of atoms.
Carbon • Carbon Monoxides • A mixture of CO and Hydrogen is used as synthesis gas in the synthesis of Methanol. • Carbon monoxide is formally the anhydride of formic acid (HCO2H). • Most important property is its ligand towards transition metals. • Only Fe and Ni react directly with CO (toxic).
Carbon • Carbon dioxide • Found in the atmosphere, volcanic gases and in some spring water. • Large scale released from fermentation processes, limstone calcination, combustion of C and Carbon compounds. • Involved inn Photosynthesis. • Solid CO2 (-78.5 deg C) • Carbonic acid
Carbon • Carbonic acid: • Hydration occurs pH<8 direct hydration
Carbon • pH>10 direct attack with OH-
Carbon • Cyanides and Related Compounds • Most important inorganic cemistry is the C-N bonds. • The most important species are Cyanide, Cyanate, Thiocyanate ions. • Cyanogen (CN)2 • Flammable gas
Carbon • Cu catalysed production of Cyanogen • Dry (CN)2 is made from
Carbon • Hydrogen cyanide • HCN is a covalent molecule • Can dissociate in aqueous solution • Poisonous colourless gas • High dielectric constant • Due to hydrogen bonding • Liquid HCN can polymerize • Aqueous solutions polymerize inducer is UV light. • HCN, water, Ammonia pentamerizes to adenine.
Carbon • Production of HCN
Carbon • Cyanides • Transition metal cyanides are water insoluble • Ag and Au cyanides are of importance. Others are used analytically.
Carbon • Compounds with C-S bonds • CS2 is a very reactive molecule, is a liquid and is toxic. • Carbon disulfide is one of the samll molecules that readilyundergo insertion reactions.
Group IV elements • Si, Ge, Sn,Pb • Si most abundant element in Nature afdter O • Ge, Sn, Pb are rare elements • Sn,Pb have been known since long time, because they can be just melted out of their minerals • Ge was discovered after its existance has been predicted. • It is purified from coal and zinc ore concentrates
Group IV elements • Multiple bonding • It has been long thought that Si and the other elements of group IV do not form multiple bonds • But: • At 500 degC
Group IV elements • Although compounds are similar in stoichiometry they do not share structural or chemical similarity. • CO2 is a gas, SiO2 is a network forming molecule with each Si atom bonded to four O. • Silane alcohols when dehydrated give polymers, not like the alkenes from carbon.
Stereochemistry of Si • Compounds having oxidation state IV form tetrahedral compounds • But valence shell expansion using outer d-orbitals can occur (dsp3 or d2sp3) • 5 and 6 coordinate compounds are common. • 5 coordination:
Isolation and properties • Si is obtained by reductionof SiO2 with carbon or CaC2 in an electric furnace. • Ge is prepared by reduction with C or H2. • Si and Ge are mainly used in semiconductirs. • The element is first converted to a tetra chloride, reduced back to the metal by hydrogen at high temperatures. • After casting into rods it is refined by zone melting.
Sn and Pb are made by the reduction of the oxide or sulfide with carbon. • Si is rather ureactive. It reacts with halifdes and alkali giving silicates. • It is only attack by HF acid. • Ge is a bit more reactive. Dissolves in conc H2SO4 and HNO3. • Sn,Pb dissolve in several acids, react with halogens. Slowly react with cold alkali, fast with hot alkali.
Hydrides • Colorless gases. • Only SiH4 is of importance. It is used in the reaction for precursors of silicones. • Chlorides: • MCl4 gives colorless liquids • Exception Pb-- yellow.
Chlorides • The tetrachlorides are hydrolysed by water • To hydrous oxides • SiCl4 and GeCL4 are used in the synthesis of pure Si and Ge and in organic chemistry.
Oxygen compounds • SiO2 occurs as • Quartz and cristobalite. • Si is always tetrahedrally bound to 4 oxygens. • Quartz and criustobalite can be interconverted by heat. • Slow cooling of molten SiO2 will result in glass. • Basic character of the dioxides increases from Si to Pb. • No true hydroxiudes exist.
Complex compounds • Anionic complexes • The other F anions are readily hydrolysed by bases.
Complex compounds • Cationic complexes • Their main application is in organic chemistry and will not be discussed here.
The divalent state • Si • Divalent Si species are thermally unstable • But at 1100 degC: • In the frozen state it is stable, when thawing it gives fluorosilanes up to Si16F34
The divalent state • Ge: • GeF2 is a crystalline solid and stable. • It is obtained by reacting Ge with HF at 200 degC. • GeCl2 gives salts similar to the Sn ions. • Sn: • The most important compounds are: • SnF2 and SnCl2
The divalent state • Only Pb has a well defined low valent cationic chemistry. • Most Pb salts are only sparingly soluble in water, PbSO4 and PbCrO4 are insoluble.
