Chemistry:Carbon compounds

Short description: Chemical substances containing carbon

Carbon compounds are defined as chemical substances containing carbon. More compounds of carbon exist than any other chemical element except for hydrogen. Organic carbon compounds are far more numerous than inorganic carbon compounds. In general bonds of carbon with other elements are covalent bonds. Carbon is tetravalent but carbon free radicals and carbenes occur as short-lived intermediates. Ions of carbon are carbocations and carbanions are also short-lived. An important carbon property is catenation as the ability to form long carbon chains and rings.

Allotropes of carbon

The known inorganic chemistry of the allotropes of carbon (diamond, graphite, and the fullerenes) blossomed with the discovery of buckminsterfullerene in 1985, as additional fullerenes and their various derivatives were discovered. One such class of derivatives is inclusion compounds, in which an ion is enclosed by the all-carbon shell of the fullerene. This inclusion is denoted by the "@"symbol in endohedral fullerenes. For example, an ion consisting of a lithium ion trapped within buckminsterfullerene would be denoted Li⁺@C₆₀. As with any other ionic compound, this complex ion could in principle pair with a counterion to form a salt. Other elements are also incorporated in so-called graphite intercalation compounds.

Carbides

Carbides are binary compounds of carbon with an element that is less electronegative than it. The most important are Al₄C₃, B₄C, CaC₂, Fe₃C, HfC, SiC, TaC, TiC, and WC.

Metal	Structure of pure metal	Metallic radius (pm)	MC metal atom packing	MC structure	M2C metal atom packing	M2C structure	Other carbides
titanium	hcp	147	ccp	rock salt
zirconium	hcp	160	ccp	rock salt
hafnium	hcp	159	ccp	rock salt
vanadium	bcc	134	ccp	rock salt	hcp	h/2	V4C3
niobium	bcc	146	ccp	rock salt	hcp	h/2	Nb4C3
tantalum	bcc	146	ccp	rock salt	hcp	h/2	Ta4C3
chromium	bcc	128					Cr23C6, Cr3C, Cr7C3, Cr3C2
molybdenum	bcc	139		hexagonal	hcp	h/2	Mo3C2
tungsten	bcc	139		hexagonal	hcp	h/2

Organic compounds

It was once thought that organic compounds could only be created by living organisms. Over time, however, scientists learned how to synthesize organic compounds in the lab. The number of organic compounds is immense and the known number of defined compounds is close to 10 million. However, an indefinitely large number of such compounds is theoretically possible. By definition, an organic compound must contain at least one atom of carbon, but this criterion is not generally regarded as sufficient. Indeed, the distinction between organic and inorganic compounds is ultimately a matter of convention, and there are several compounds that have been classified either way, such as: COCl₂, CSCl₂, CS(NH₂)₂, CO(NH₂)₂. With carbon bonded to metals the field of organic chemistry crosses over into organometallic chemistry.

Carbon-oxygen compounds

There are many oxides of carbon (oxocarbons), of which the most common are carbon dioxide (CO₂) and carbon monoxide (CO). Other less known oxides include carbon suboxide (C₃O₂) and mellitic anhydride (C₁₂O₉). There are also numerous unstable or elusive oxides, such as dicarbon monoxide (C₂O), oxalic anhydride (C₂O₄), and carbon trioxide (CO₃).

There are several oxocarbon anions, negative ions that consist solely of oxygen and carbon. The most common are the carbonate (CO₃²⁻) and oxalate (C₂O₄²⁻). The corresponding acids are the highly unstable carbonic acid (H₂CO₃) and the quite stable oxalic acid (H₂C₂O₄), respectively. These anions can be partially deprotonated to give the bicarbonate (HCO₃⁻) and hydrogenoxalate (HC₂O₄⁻). Other more exotic carbon–oxygen anions exist, such as acetylenedicarboxylate (O₂C–C≡C–CO₂²⁻), mellitate (C₁₂O₉⁶⁻), squarate (C₄O₄²⁻), and rhodizonate (C₆O₆²⁻). The anhydrides of some of these acids are oxides of carbon; carbon dioxide, for instance, can be seen as the anhydride of carbonic acid.

Some important carbonates are Ag₂CO₃, BaCO₃, CaCO₃, CdCO₃, Ce₂(CO₃)₃, CoCO₃, Cs₂CO₃, CuCO₃, FeCO₃, K₂CO₃, La₂(CO₃)₃, Li₂CO₃, MgCO₃, MnCO₃, (NH₄)₂CO₃, Na₂CO₃, NiCO₃, PbCO₃, SrCO₃, and ZnCO₃.

The most important bicarbonates include NH₄HCO₃, Ca(HCO₃)₂, KHCO₃, and NaHCO₃.

The most important oxalates include Ag₂C₂O₄, BaC₂O₄, CaC₂O₄, Ce₂(C₂O₄)₃, K₂C₂O₄, and Na₂C₂O₄.

Carbonyls are coordination complexes between transition metals and carbonyl ligands. Metal carbonyls are complexes that are formed with the neutral ligand CO. These complexes are covalent. Here is a list of some carbonyls: Cr(CO)₆, Co₂(CO)₈, Fe(CO)₅, Mn₂(CO)₁₀, Mo(CO)₆, Ni(CO)₄, W(CO)₆.

Carbon-sulfur compounds

Important inorganic carbon-sulfur compounds are the carbon sulfides carbon disulfide (CS₂) and carbonyl sulfide (OCS). Carbon monosulfide (CS) unlike carbon monoxide is very unstable. Important compound classes are thiocarbonates, thiocarbamates, dithiocarbamates and trithiocarbonates.

carbon monosulfide	carbon disulfide	carbonyl sulfide
Inorganic carbon-sulfur compounds

Carbon-nitrogen compounds

Small inorganic carbon – nitrogen compounds are cyanogen, hydrogen cyanide, cyanamide, isocyanic acid and cyanogen chloride.

	composition		Molar mass (g/mole)	Boiling point °C	Melting point °C
cyanogen	(CN)2		52.03	−21	−28
hydrogen cyanide	HCN		27.03	25–26	−12 – -14
cyanamide	CN2H2		42.04	260 (decomp.)	44
isocyanic acid	HNCO		43.03	23.5	−86
cyanogen chloride	CNCl		61.47	13	−6
chlorosulfonyl isocyanate	CNClO3S		141.53	107	−44
cyanuric chloride	(NCCl)3		184.41	192	154
Inorganic carbon-nitrogen compounds

Paracyanogen is the polymerization product of cyanogen. Cyanuric chloride is the trimer of cyanogen chloride and 2-cyanoguanidine is the dimer of cyanamide.

Other types of inorganic compounds include the inorganic salts and complexes of the carbon-containing cyanide, cyanate, fulminate, thiocyanate and cyanamide ions. Examples of cyanides are copper cyanide (CuCN) and potassium cyanide (KCN), examples of cyanates are potassium cyanate (KNCO) and silver cyanate (AgNCO), examples of fulminates are silver fulminate (AgOCN) and mercury fulminate (HgOCN) and an example of a thiocyanate is potassium thiocyanate (KSCN).

Carbon halides

The common carbon halides are carbon tetrafluoride (CF₄), carbon tetrachloride (CCl₄), carbon tetrabromide (CBr₄), carbon tetraiodide (CI₄), and a large number of other carbon-halogen compounds.

Carboranes

A carborane is a cluster composed of boron and carbon atoms such as H₂C₂B₁₀H₁₀.

Alloys

Some other common alloys that are based on iron and carbon include anthracite iron, cast iron, pig iron, and wrought iron.

In more technical uses, there are also spiegeleisen, an alloy of iron, manganese, and carbon; and stellite, an alloy of cobalt, chromium, tungsten, and carbon.

Whether it was placed there deliberately or not, some traces of carbon is also found in these common metals and their alloys: aluminum, chromium, magnesium, molybdenum, niobium, thorium, titanium, tungsten, uranium, vanadium, zinc, and zirconium. For example, many of these metals are smelted with coke, a form of carbon; and aluminum and magnesium are made in electrolytic cells with carbon electrodes. Some distribution of carbon into all of these metals is inevitable.

References

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