Compounds of carbon with oxygen and sulfur

Key points: Carbon monoxide is a key reducing agent in the production of iron and a common ligand in d-metal chemistry; carbon dioxide is much less important as a ligand and is the acid anhydride of carbonic acid; the sulfur compounds CS and CS₂ have similar structures to their oxygen analogues. Carbon forms CO, CO₂, and the suboxide, O=C=C=C=O (Table 14.3). The uses of CO include the reduction of metal oxides in a blast furnace (Section 5.16) and the shift reaction (Section 10.4) for the production of H₂: where we deal with catalysis, we describe the conversion of carbon monoxide to acetic acid and aldehydes. The CO molecule has very low Brønsted basicity and negligible Lewis acidity towards neutral electron-pair donors. Despite its weak Lewis acidity, however, CO is attacked by strong Lewis bases at high pressure and somewhat elevated temperatures. Thus, the reaction with OH^- ions yields the formate ion, HCO₂^-:

Similarly, the reaction with methoxide ions (CH3 O) yields the acetate ion, CH₃CO₂. Carbon monoxide is an excellent ligand towards d-metal atoms in low oxidation states (Section 22.5). Its well-known toxicity is an example of this behaviour: it binds to the Fe atom in hemoglobin, so excluding the attachment of O₂, and the victim suffocates. An interesting point is that H₃BCO can be prepared from B₂H₆ and CO at high pressures in a rare example of the coordination of CO to a simple Lewis acid. A complex of similar stability is not formed by BF₃; this observation is consistent with the classification of BH₃ as a soft acid and BF3 as a hard acid. Carbon dioxide is only a very weak Lewis acid. For example, only a small fraction of molecules are complexed with water to form H₂CO₃ in acidic aqueous solution but, at higher pH, OH coordinates to the C atom, so forming the hydrogencarbonate (bicarbonate) ion, HCO₃. This reaction is very slow; yet the attainment of rapid equilibrium between CO₂ and HCO₃ is so important to life that it is catalysed by a Zn-containing enzyme carbon dioxide hydratase (carbonic anhydrase, Section 27.9a). The enzyme accelerates the reaction by a factor of about 109. Carbon dioxide is one of several polyatomic molecules that are implicated in the greenhouse effect. In this effect, a polyatomic molecule in the atmosphere permits the pas sage of visible light but, because of its vibrational infrared absorptions, it blocks the immediate radiation of heat from the Earth. There is strong evidence for a significant increase in atmospheric CO₂ since the industrialization of society. In the past, nature has managed to stabilize the concentration of atmospheric CO₂, in part by precipitation of calcium carbon ate in the deep oceans, but it seems that the rate of diffusion of CO₂ into the deep waters is too slow to compensate for the increased influx of CO₂ into the atmosphere (Box 14.3). There is convincing evidence for increasing concentrations of the greenhouse gases CO₂, CH₄, N2 O, and chlorofluorocarbon bons, and it is clear that they are having an impact on global temperatures. One method of slowing the rate of increase in atmospheric CO₂ is carbon dioxide sequestration (Box 14.4), in which CO₂ is captured from industrial flues by reaction with amines. It is then released and liquefied by compression and pumped underground often back into gas or oil wells in order to drive out further oil or gas. From an economic perspective, an important reaction is CO₂ with ammonia to yield ammonium carbonate, (NH₄)₂CO₃, which at elevated temperatures is converted directly to urea, CO(NH₂)₂, a fertilizer, a feed supplement for cattle, and a chemical intermediate. Another important use of CO2 is in the soft drinks industry, where it dissolves under pressure to give a pleasant acidic taste of carbonic acid, H₂CO₃, and comes out of solution in the form of bubbles when the pressure is released. In organic chemistry, a common synthetic reaction is that be tween CO₂ and carbanion reagents to produce carboxylic acids. In the crucial biological process known as the Calvin cycle, CO₂ is ‘fixed’ (to the extent of 100 Gt per year) into organic mol ecules by reaction with the electron-rich C=C double bond of a pentose enolate ligand coordinated to a Mg₂ ion in the enzyme known as ‘Rubisco’.

Metal complexes of CO₂ are known (13), but they are rare and far less important than the metal carbonyls. In its interaction with a low-oxidation state, electron-rich metal centre, the neutral CO₂ molecule acts as a Lewis acid and the bonding is dominated by electron donation from the metal atom into an antibonding π orbital of CO₂. An important use of superfluid CO₂ (that is, highly com pressed carbon dioxide but above its critical temperature) is as a solvent. Applications range from decaffeination of coffee beans to its use in chemical synthesis in place of conventional solvents as an important part of the strategy for implementing the procedures of ‘green chemistry’. The sulfur analogues of carbon monoxide and carbon dioxide, CS and CS₂, are known. The former is an unstable transient molecule and the latter is endoergic (∆_fG^O 65 kJ mol 1). Some complexes of CS (14) and CS₂ (15) exist,

and their structures are similar to those formed by CO and CO₂ . In basic aqueous solution, CS₂ undergoes hydrolysis and yields a mixture of carbonate ions, CO₃^-2, and trithio carbonate ions, CS₃^-2.

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مواضيع ذات صلة

Phosphine, arsane, and stibine

Hydrazine and hydroxylamine

Ammonia

Phosphides

Azides

Nitrides

Nitrogen activation

Microporous solids

Aluminosilicates

Compounds with nitrogen

Oxides of germanium, tin, and lead

Tin and lead halides