Carbon compounds

Carbon compounds encompass an immense and diverse group of chemical substances containing carbon, a unique element distinguished by its ability to form an extraordinary variety of stable bonds. Owing to its tetravalency, catenation and capacity to create single, double and triple covalent bonds, carbon forms more known compounds than any other element except hydrogen. These compounds display a vast range of structural and chemical properties, underpinning both organic and inorganic branches of chemistry.

General Characteristics of Carbon Bonding

Carbon’s ability to form four covalent bonds enables the creation of long chains, branching structures and rings. This property, known as catenation, is fundamental to the diversity of carbon chemistry. While stable carbon compounds overwhelmingly feature covalent bonds, reactive intermediates such as free radicals, carbenes, carbocations and carbanions occur transiently in numerous reactions. Their short lifetimes nonetheless play a critical role in organic mechanisms and synthetic chemistry.

Allotropes of Carbon and Their Chemical Significance

Carbon exists in several allotropes, each displaying distinct physical and chemical properties. The best-known allotropes include:

• Diamond, with a rigid three-dimensional network of carbon atoms.
• Graphite, composed of layered sheets with delocalised electrons.
• Fullerenes, including buckminsterfullerene (C₆₀), discovered in 1985, which prompted rapid advancement in inorganic carbon chemistry.

Fullerenes in particular have given rise to endohedral fullerenes, a class of inclusion compounds where ions or small atoms are encapsulated within the carbon cage. For example, Li⁺@C₆₀ represents a lithium ion trapped inside a C₆₀ sphere. Other elements may also form graphite intercalation compounds, in which ions or molecules are introduced between the layers of graphite.

Carbides

Carbides are binary compounds of carbon with elements less electronegative than carbon. They vary widely in structure and bonding, from ionic to covalent to metallic. Notable carbides include:

• Aluminium carbide
• Boron carbide
• Calcium carbide
• Iron carbide
• Silicon carbide
• Titanium carbide
• Tungsten carbide

Many of these materials exhibit high hardness, thermal stability and chemical resistance, making them valuable in industrial applications such as abrasives, cutting tools and structural materials.

Organic Compounds

Early chemists believed that organic compounds could only be produced by living organisms, a misconception overturned by the synthesis of urea in the nineteenth century. Since then the number of known organic compounds has grown to nearly ten million, with countless more theoretically possible. Organic compounds are characterised by the presence of carbon–hydrogen bonds, though exceptions and borderline cases occur. Examples ambiguously classified at various times include phosgene, thiourea and urea.
Organic chemistry overlaps with organometallic chemistry, which studies compounds featuring direct carbon–metal bonds. These materials are crucial in catalysis, materials science and synthetic methodologies.

Carbon–Oxygen Compounds

Carbon forms numerous oxides and oxygen-containing species. The most common are:

• Carbon dioxide (CO₂)
• Carbon monoxide (CO)

Less familiar oxides include carbon suboxide (C₃O₂) and mellitic anhydride (C₁₂O₉). Other oxides such as dicarbon monoxide (C₂O) and carbon trioxide (CO₃) are unstable and difficult to observe.
Carbon and oxygen also form several important oxocarbon anions, including:

• Carbonate (CO₃²⁻)
• Bicarbonate (HCO₃⁻)
• Oxalate (C₂O₄²⁻)

More complex species include squarate, mellitate and rhodizonate ions. Their corresponding acids range from highly unstable (carbonic acid) to relatively stable (oxalic acid). Carbonate salts exist for many metals and find extensive use in industry, water treatment and biological buffering.
Metal carbonyls, coordination compounds containing carbon monoxide as a ligand, are notable examples of covalent carbon complexes with transition metals. Examples include chromium hexacarbonyl, iron pentacarbonyl and nickel carbonyl.

Carbon–Sulphur Compounds

Prominent inorganic compounds of carbon and sulphur include:

• Carbon disulphide (CS₂), an industrial solvent.
• Carbonyl sulphide (OCS), found naturally in the atmosphere.

Carbon monosulphide (CS) is highly unstable. Various thiocarbonates, dithiocarbamates and related compounds also form important categories in coordination chemistry and materials science.

Carbon–Nitrogen Compounds

Carbon and nitrogen form numerous small inorganic molecules:

• Cyanogen (C₂N₂)
• Hydrogen cyanide (HCN)
• Cyanamide (NH₂CN)
• Isocyanic acid (HNCO)
• Cyanogen chloride (CNCl)

These may polymerise or trimerise, giving species such as paracyanogen and cyanuric chloride. Carbon–nitrogen chemistry also encompasses polyatomic ions including:

• Cyanide (CN⁻)
• Cyanate (OCN⁻)
• Fulminate (OCN⁻, in a distinct structural arrangement)
• Thiocyanate (SCN⁻)

Their salts are widely encountered in industrial, mining and laboratory contexts.

Carbon Halides

Carbon forms stable compounds with all halogens. Common examples include:

• Carbon tetrafluoride (CF₄)
• Carbon tetrachloride (CCl₄)
• Carbon tetrabromide (CBr₄)
• Carbon tetraiodide (CI₄)

Beyond these, a very large number of halogenated organic and inorganic carbon compounds exist, many of which are of environmental and industrial significance.

Carboranes

Carboranes are polyhedral clusters composed of boron, carbon and hydrogen atoms. An example is H₂C₂B₁₀H₁₀, representing a family of compounds known for their exceptional stability and utility in materials science and medicinal chemistry.

Carbon in Alloys

Carbon is a crucial alloying element in metallic systems, most notably steel, where varying carbon content produces a wide range of mechanical properties. Other iron–carbon alloys include cast iron, wrought iron and pig iron. Specialised alloys such as spiegeleisen (iron–manganese–carbon) and stellite (cobalt–chromium–tungsten–carbon) highlight carbon’s broad metallurgical importance.
Trace amounts of carbon appear in many metals as a consequence of smelting with carbon-based fuels or through the use of carbon electrodes in industrial processes. Even when not intentionally added, small quantities of carbon may enter metals such as aluminium, magnesium, titanium and vanadium during extraction and refining.