Carbon is second only to hydrogen in the number of compounds that it forms. Most of these compounds which also contain hydrogen are termed organic and in general, these are not treated here. In this entry we discuss the inorganic chemistry and compounds of carbon. Carbon occurs naturally in the Earth’s crust as graphite and diamond and also occurs in carbonates, such as limestone, CaCO₃, in many rocks and minerals. The element principally exists in two isotopic forms, ¹²C (~99 %) and ¹³C (~1 %). Despite it’s low abundance, ¹³C is an important isotope magnetic properties of it’s nucleus and acts as a an effective natural marker for the characterisation of carbon containing compounds in ¹³C nuclear magnetic resonance (NMR) spectroscopy. Another isotope also exists, ¹⁴C which only occurs in living organisms. It is formed in the atmosphere by the action of cosmic rays on ¹⁴N nuclei and ends up as ¹⁴CO₂. Since the level of cosmic rays are pretty much constant, this labelled carbon dioxide maintains a steady concentration in the atmosphere and is taken up by plants thereby incorporating ¹⁴C atoms in their structures. Because of this all plant and animals have a minute but measurable quantity of this isotope (1.2 x 10^-10 % of total carbon). The nuclei are radioactive with a half life of 5730 years. Since the ¹⁴C content of an organism is continually regenerated during it’s life but tops in death, the radioactive decay of the element can be used to find it’s age. Radio-carbon dating therefore finds applications in archaeology for the dating of human remains, fabrics and leathers and other organic materials to a practical limit of 50,000 years.

The chemistry of carbon is almost totally covalent in nature though some ionic species are known. The C⁴⁺ is not observed but C^4- may exist in compounds which are called carbides formed with highly electropositive metals. An important feature of the chemistry of carbon and the basis of organic chemistry, is the propensity of the element for catenation. This is it’s ability to form bonds to itself leading to highly stable chain or rings. This is chiefly due to the high strength of the C-C single bond (356 kJ mol^-1). Along with the other first row non-metals carbon can form C=C double bonds and C≡C triple bonds through sideways overlap of atomic p orbitals and these can be incorporated into the carbon-carbon bonded frameworks of it’s compounds giving rigidity since these bonds can’t rotate. The combined effects of catenation and multiple bond formation also allows the formation of planar cyclic aromatic* compounds such as benzene. Other elements such as sulphur and silicon can also catenate but far less effectively due to much weaker bonds (Si-Si 226 kJ mol^-1). Multiple bonding for silicon is also highly unstable due to poor π overlap of it’s p orbitals. Because of these properties of carbon, it is not hard to see why nature uses carbon as the element of choice to give structure to it’s biomolecules.

Allotropes of Carbon

There are three general allotropes (natural structural forms of the element) of carbon; graphite, diamond and the fullerenes.

Graphite

Graphite is the most thermodynamically stable allotrope of carbon. The structure consist of sheets of sp³ hybridised carbon atoms in interlocking hexagons (see figure 1). The remaining p orbital of each carbon atom engages in π bonding and results in the π electron of each atom being deocalised over the whole sheet. This is the source of graphite’s ability to conduct electricity. Sheets of these hexagonal arrays of atoms stack on top of each other and are able to slide over each other which is why graphite is useful in pencils. It is found in two forms, α-graphite and β-graphite which differ in the relative arrangement of atoms in of neighbouring sheets overlap.

Figure 1: Diagram of the structure of a layer or graphite. Each carbon-carbon bond has partial double bond character as the π electron of each atom is delocalised over the whole sheet.

Graphite is able to form what are known as intercalation compounds in which ions or atoms of other material are incorporated between the graphite sheets. Heating under reduced pressure in the presence of alkali metals, for instance yield material with metal atoms between the layers. The first reported material of this type was the potassium intercalation compound C₈K in 1926. The intercalation compound of lithium have found application in batteries. Despite being the most stable it is more reactive than diamond and will react with oxidising agent and reactive gases like fluorine. Both diamond and graphite have extremely high melting points at around 4100 K.

Diamond

Diamond is marginally less thermodynamically stable than graphite by just under 3 kJ mol^-1 and but is much less reactive. In the structure, each carbon atom is sp³ hybridised and four bonds to four other carbon atoms arranged tetrahedrally around it. Because of this more rigid structure diamond is a much harder substance than graphite. Diamond can be made synthetically from graphite by heating to 3000 K under a pressure of about 125,000 atmospheres using a metal catalyst such as chromium, iron or platinum which find uses in machining tools.

Fullerenes

Otherwise known as buckminsterfullerenes or “bucky balls”, fullerenes are a class of carbon allotrope that was only recently discovered (c. 1990). They are produced by hitting graphite with powerful lasers or an electrical arc. have very interesting structures. If we imagine a portion of a graphite like sheet but with one of the hexagonal rings being replaced by a five-membered ring, bordered by five interlocking six-membered rings we have a curved rather than flat sheet. If we have several five membered rings in the structure bridged by six-membered rings the opposing edges of the sheet will meet and we get a spherical or near spherical molecule resembling a football. Fullerenes come in a variety of forms with different numbers of carbon atoms, the most commonly studied being the perfectly spherical C₆₀. The π electrons in the structure are not fully delocalised however but act chemically more like isolated alkenes. Because they have a cage like nature, formation in the presence of other compounds ca lead to their encapsulation. Example include gasses like helium and metal ranging from potassium to uranium. By altering the conditions under which bucky balls are formed we can form rod like objects like graphite sheets rolled into tubes. These nanotubes are an exciting area of materials chemistry with potential application as molecular wires and as hydrogen storage devices for hydrogen powered vehicles.

Carbides

Carbon will form binary compounds with most elements called carbides. These generally fall into three classes; ionic, covalent and interstitial. Ionic carbides are those formed with highly electropositive metals from the alkali and alkaline earth metals and also the lanthanides and metals like aluminium. These contain metal cations and anionic carbon in the form C^4- or [C≡C]^2-. These compounds are excedingly reactive with water giving methane or ethyne, for example

Al₄C₃ + 12 H₂O → 4 Al(OH)₃ + 3 CH₄

CaC₂ + 2 H₂O → Ca(OH)₂ + HC≡CH

Covalent carbides are formed with elements such as boron and silicon. These material are extremely hard and inert. Silicon carbide, SiC, has a diamond structure in which tetrahedral carbon atoms are bonded to four silicon atoms which in turn are tetrahedral and each bonded to four carbons.

Interstitial carbides are materials where carbon atoms occupy the spaces between metal atoms (interstitial spaces) in metal lattices. Some of these are reactive material but other, such as tungsten carbide (WC), are extremely hard inert materials with very high melting points. Tungsten carbide finds frequent use in machining tools.

Oxides of Carbon, Carbonates and Carbonic Acid

Carbon monoxide, CO, is a colourless, odourless gas that is formed through incomplete combustion due to lack of oxygen. It is flammable and will form carbon dioxide if burnt in air. It is made on industrial scales as a feedstock for other chemical processes and mixtures with hydrogen called synthesis gas is important for the production of methanol. The nature of the bonding between the oxygen and carbon in this molecule make it able to interact and form strong complexes with transition metals and this is the reason why it is so toxic. Carbon monoxide will readily bind to the Fe ions in heamoglobin, the mammalian oxygen transport protein, with a much greater affinity than oxygen.

Carbon dioxide is the full combustion product of carbon compounds and is also a volcanic gas and the product of respiration in pants and animals. Carbon dioxide is a solid material (more commonly known as dry ice) up to –78 °C upon which it sublimes* rather than forming a liquid. An important use of carbon dioxide is it’s use as a supercritical fluid* for the de-caffienation of coffee.

When carbon dioxide dissolves in water it is in equilibrium with a small quantity of carbonic acid, H₂CO₃

CO₂ + H₂O ↔ HCO₃^- + H⁺↔ H₂CO₃

Carbon forms a wide range of carbonate compounds containing the CO₃^2- or HCO₃^- ions. Most carbonates, apart from those of the alkali metals, are insoluble in water and precipitate from aqueous solution as anyone living in a hard water area will know well. The most common carbonate is calcium carbonate, or limestone. Carbonates will react with acids to give carbon dioxide and water in the reverse of the carbonic acid formation reaction, ie

Na₂CO₃ + 2 HCl → CO₂ + H₂O + 2 NaCl

Carbon-Sulphur Compounds

The simplest sulphur compound of carbon is carbon disulphide, CS₂. It is a pale yellow toxic liquid and is structurally analogous to carbon dioxide. In contrast to carbon dioxide however, it is much more reactive and is flammable. It is sometimes used as a solvent for some specific chemical reaction where there isn’t an alternative. It is produced via the reaction of methane and elemental sulphur over silica (SiO₂) or alumina (Al₂O₃) at around 1000 °C.

CH₄ + 4 S → CS₂ + 2 H₂S

CS₂ will undergo reaction with a variety of nucleophiles. It will reaction with the ion HS^- producing trithiocarbonate CS₃^2-, with alcohols in the presence of a base to from compounds called xanthates and with amines to give compounds called thiocarbamates (see figure 2)

CS₂ + S^2-→ CS₃^2-

CS₂ + NaOH + ROH → (RO-CS₂)Na + H₂O

CS₂ + R₂NH → R₂N-CS₂H

Figure 2: Structures of xanthates, thiocarbamates and a typical coordination mode of these compounds in metal complexes.

The –CS₂ motif of xanthates and thiocarbamates are analogous to carboxylates and can form metal complexes in the same way. Carbon disulphide can also form complexes with metals, the first example of this was discovered by the group of the Nobel prize winner Geoffrey Wilkinson. CS₂ was added to a solution of the platinum complex [Pt(P(C₆H₅)₃)₃] to give [Pt(CS₂ )(P(C₆H₅)₃)₂] in which the CS₂ ligand is bent (figure 3).

Figure 3: Structure of the square planar complex [Pt(CS₂ )(P(C₆H₅)₃)₂].

Carbon does not form a free sulphur analogue or carbon monoxide as CS is an unstable radical (contains unpaired electrons) which will react with just about anything. It does however exist as a ligand in a variety of metal complexes. The analogous selenium and tellurium compounds have also been observed. The carbon dioxide analogues S=C=Se and S=C=Te are also known.

Halides and Oxohalides

Carbon form tetrahalides with all of the halogens from fluorines to iodine. The lightest of these is tetrafuoromethane, CF₄, and is an extremely stable gas due to very strong C-F bonds. It is commonly used in the etching of electronic circuit boards in industry and as a coolant. Carbon will also form compounds analogous to alkanes but in which every hydrogen atom is replaced by fluorine. These are highly inert substances, an important example being the polymer polytetrafluoroethylene (PTFE) otherwise known as teflon. Carbon tetrahchloride, CCl₄, is a toxic liquid that is most often used as a solvent but was also used to make freons, better known as chlorofluorocarbons or CFCs (such as CFCl₃, CF₂Cl₂, CF₃Cl). These non-toxic, non-flammable and highly chemically are excellent refrigerants and were widely used in 1960’s and 70’s but have since been outlawed because of their role in stratospheric ozone depletion. These have been temporarily replaced by HCFCs (which have hydrogen atoms in them to make them more reactive and break down harmlessly before being able to reach the ozone layer) and HFCs (which don’t contain chlorine) but these have their own problems in being potent greenhouse gases. Carbon tetrabromide, CBr₄, and carbon tetraiodide, CI₄ are much less thermally stable than the lighter tetrahalides. They are both solids, CBr₄ being pale yellow and CI₄ being bright red and crystalline. There also a series of mixed halides containing bromine called halons. These are non-flammable gases and are used as fire suppressants.

Carbon also forms a series of highly reactive trigonal planar oxohalide compounds (X₂C=O). The homohalides F₂C=O, Cl₂C=O and Br₂C=O are known and are gases at room temperature, though I₂C=O has not been made and is probably too unstable to exist. Cl₂C=O, otherwise known as phosgene, is a highly toxic gas that was briefly used as an ineffective chemical weapon during the first world war. It now a major industrial chemical for the production of other bulk chemicals. Mixed halides are also known, for example, FClC=O and FBrC=O, and though the diiodide is not known, the compound FIC=O. The oxohalides are reactive compounds and will react with nucleophiles (compounds with non-bonding lone pairs of electrons) such as water, hydroxide and amines. For example, phosgene reacts in the presence of ammonia to give urea as the major product.

Cl₂C=O + 2 NH₃→ (H₂N)₂C=O + 2 HCl

Carbon-Nitrogen Compounds

In addition to the vast array of nitrogen containing organic compounds, carbon also forms a wide range of inorganic compounds with C-N bonds. The most important of these compounds are the cyanides, cyanates and thiocyanates.

An important compound is cyanogen, (CN)₂. It is a linear molecule with the structure N≡C-C≡N and is a highly toxic and flammable gas and produces one of the hottest known flames. This is made industrially by overall gaseous reaction of hydrogen cyanide with oxygen using NO₂ as a catalyst.

2 HCN + NO₂→ (CN)₂ + NO + H₂O

NO + ½O₂→ NO₂

If pure then cyanogen is very stable but trace impurities lead it to polymerise under hetaing to around 500 °C. The C≡N fragment has been compared in it’s reactivity with the halogens. For example, it can dissociate into CN radicals like halogen X₂ molecules can and will also undergo a reaction with some metal compexs called oxidative addition. Here the C-C (or X-X) bond is broken by donation of two electron by the metal and two M-C bonds form raising the oxidation state of the metal by two units, hence the reaction is oxidative. For example

((C₆H₅)₃P)₄Pd⁰ + N≡C-C≡N → ((C₆H₅)₃P)₂Pd^II(C≡N)₂ + 2 P(C₆H₅)₃

Also like the halogen, cyanogen can undergo disproportionation* in basic solutions.

(CN)₂ + 2 OH^-→ CN^- + OCN^- + H₂O

Another important carbon-nitrogen compound is hydrogen cyanide, HCN. It is also an extremely poisonous compound and is only just a gas at room temperature having a boiling point of 25.6 °C. The relatively high boiling point for such a small molecule being dur to the presence of hydrogen bonding. HCN is thought to have been an important compound in the formation of biological molecule in the early primordial atmosphere of Earth. Under mild pressure and in the presence of water and ammonia it will spontaneously form adenine.

It is made industrially by the reaction of methane, ammonia and oxygen over a platinum metal catalyst.

2 CH₄ + 2 NH₃ + O₂→ 2 HCN + 6 H₂O

It has many industrial uses, for example, it is used in the production of feedstocks for nylon production. It is also used for the production of metal cyanides. HCN is weakly acidic and will partly dissociate in water to give H⁺ and CN^- ions and can therefore be neutralised by alkali metal hydroxides to give the metal hydroxide. The metal hydroxide can then be crystallised from solution.

HCN + NaOH → NaCN + H₂O

The cyanides of highly electropositive metals, eg the alkali metals, are very water soluble, however those of silver(I), mercury(I) and lead(II) are very insoluble. The CN^- ion will readily form complexes with many metals. It form complexes with metallic silver and gold and was used in the extraction of these metal from their lower grade ores.