Chemical Properties

Carbon is a fairly inert element and most of its modifications may only be reacted under rather harsh conditions. Nevertheless the entire Organic Chemistry, known exclusively to deal with its compounds alone, is founded on the chemistry of carbon. This apparent contradiction is resolved by the simple compounds being hard to obtain from the elements, but any further reaction being quite easy to achieve. They succeed in impressive variety, mainly due to the manifold ways of carbon bonding with itself (chains, rings, single and multiple bonds, etc.).

Carbon' s chemistry is governed by its position in the fourth main group. In contrast to the higher elements in this group it does not tend to exert only two out of its four valencies. The maximum connectivity, as present, for example, in diamond, is 4. The octet rule is strictly obeyed in covalently bound carbon, yet there are several coordination compounds with up to eight bonding partners. On these occasions, carbon is engaged in three .center two-electron bonds (for example, Al2 (CH3)2). They are, however, only possible with more electropositive coordination partners. Due to its medium position in the periodic table, carbon reacts with oxygen as well as with hydrogen and may adopt any oxidation number from +4 to -4. Methane (CH4) and carbon dioxide (CO2) represent the extremes of this range.

Carbon monoxide and carbon dioxide, respectively, are obtained from reacting carbon with water or oxygen at a sufficiently high temperature. The latter determines the course of the reaction as well as the present amounts of oxygen or water vapor do. The different enthalpies of the oxidative steps may be explained by the destruction of the crystal lattice required in the reaction of solid carbon to give CO. No energy has to be applied for this process in the second step from CO to CO2, so more heat is released here.

Strong heating of carbon with gaseous sulfur yields carbon disulphide (CS 2) in an endothermal reaction. This may be further reacted with elemental chlorine to give carbon tetrachloride.

With metals and elements such as boron or silicon (in general, with less electronegative elements) carbon forms carbides. Consequently carbon is the electron acceptor in these compounds. There are three different types: salt- iike, metallic and covalently bound carbides.

Salt-like carbides are obtained with the electropositive metals from main groups 1 to 3, and with certain lanthanides and actinides. The characteristic feature of this type is the presence of carbon anions. There are methanides (containing C4-), acetylenides ( C2-) and allenides ( Cj-). Methanides have been described for aluminum and beryllium; they yield methane upon hydrolysis. The acetylenides, that produce ethine when reacted with water, hold isolated [C=C]2--ions in their lattice. Elements from the first main group and subgroup form structures with a composition of M - (C2) , whereas it is MC . with those from the second main group and subgroup. With trivalent metals a stoichiometry of M. (C2)3 is obtained (M = Al, La, Ce, Pr, Tb). The most important of acetylenides is calcium carbide. It is produced on a million-tons-scale to generate acetylene for welding or, by reacting it with atmospheric nitrogen, to give calcium cyanamide (CaCN2), which is a valuable raw material for the fertilizer industry. As for the allenides, there are only Li4 C3 and Mg2C3 known to date. These contain isolated [C=C=C]4- - ions and release propyne upon hydrolysis.

Elements from subgroups 4 to 6 form metallic carbides that feature a series of distinct metallic properties (conductivity, metallic luster). Where the atomic diameter of the metal is more than 2.7 A, the carbon atoms may sit in the octahedral gaps of the host lattice. If all of these sites are occupied, a compound with the composition MC (M = Ti, Zr, Hf, V, Nb, Ta, Mo, W) is obtained regardless of

C(s) + 0.5O2 ^ CO (AH = -110.6 kJmol-1) CO + 0.5O2 ^ CO2 (AH = -283.2 kJmol-1

the metal's preferred valency. Usually they exhibit a cubic dense packing. Filling only 50% of the octahedral gaps yields carbides with a stoichiometry of M2C (M = V, Nb, Ta, Mo, W), normally with a hexagonal dense packing. The different types of intercalary carbides show in parts remarkable properties: their melting points lie between 3000 and 4000 °C, chemically they are largely inert, and their hardness comes close to that of diamond. Especially tungsten carbide finds widespread application as material for heavy-duty tools. Furthermore there are metallic carbides of third period metals from the subgroups 6 to 8 whose atomic diameter is less than 2.7 A. Their respective octahedral gaps are too small to harbor carbon atoms, and so the metal lattice is distorted on carbide formation. At the same time the carbon's coordination number increases, giving rise to a stoichiometry of M3C (occasionally also M - C2, M5C2, M7C3 - etc.; M = Cr, Mn, Fe, Co, Ni). Herein the carbon atoms usually are trigonal-prismatically surrounded by metal atoms. Cementite (Fe3C, an important structural constituent of steel) is an exponent of this class of carbides.

Elements with an electronegativity similar to that of carbon yield covalent carbides. They also feature great hardness. Most of all, silicon carbide has several applications for mechanically stressed objects (grinding and cutting tools) or as material for high-temperature transistors, light-emitting diodes, infrared radiators, etc. It is produced, for example, according to Acheson by the direct reaction of coke and quartz sand in an electric furnace. Boron as well forms several covalent carbides with stoichiometries of B- 2C3, B13C2 - and B-4C, which are employed for steel borination, as neutron absorber in nuclear reactors, or for vehicle armoring.

Carbon forms a multitude of compounds with the halogens. Typical stoichiometries are CX4, C2X6, C2X4, and C2X2. Commercially important examples for this class of compounds include carbon tetrachloride, FCCs (fluoro-chloro-carbons) and PTFE (polytetrafluoroethylene, teflon). The FCC's significance has decreased, though, because of their detrimental effect on the ozone layer and the resulting ban of their use.

With nitrogen, carbon forms a series of nitrides with a composition of (CN)n (n = 1, 2, x, where x can be any large number except among which the cyanogen (n = 1) is stable at high temperatures only. Paracyan (CN)* is obtained from dicyan by polymerization.

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  • lia
    How Al2(C2)3 is a methanide?
    1 year ago

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