Alone among the elements, carbon is able to form stable bonds with itself. As a result, it makes up complex molecules, some of which contain hundreds of thousands of carbon atoms. Because of this, carbon has its own branch of science called organic chemistry, the study of compounds that contain carbon. (It is called "organic" because it was originally the study of chemical composition of living things.) Carbon compounds include the hydrocarbons, which contain only carbon and hydrogen, but also compounds containing other elements too, such as nitrogen, oxygen, phosphorus and sulphur (carbon dioxide and the carbonates are not considered organic). Organic chemistry is vital to the study of life sciences: DNA, fats, carbohydrates and proteins are all organic compounds. The use of organic chemistry is also hugely important to our everyday lives: plastics, dyes, clothing, flavourings, scents, detergents, explosives, pharmaceutical, fuels, fertilizers, foodstuffs and many other products are all made from organic compounds.
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In 1828, the German scientist Friedrich Wöhler (1800–82) discovered that ammonium cyanate, an inorganic substance, could be converted into urea, an organic substance, in a laboratory. This proved that organic matter could be produced artificially, or synthesized, rather than solely by natural processes. From then on, the field of organic chemistry would expand to include artificial compounds as well as natural compounds.
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A crucial breakthrough for organic chemistry was made by the German chemist August Kekulé (1829–96) in 1857. In that year he announced his discovery that carbon atoms made a large number of covalent bonds (four) with other atoms. In a paper published in May 1858, he demonstrated the ability of carbon atoms to link to each other to form a lattice (the same discovery was made independently by Scottish chemist Archibald Scott Couper shortly afterwards). In 1865, Kekulé described the structure of benzene, a sweet-smelling liquid, as a ring of six carbon atoms with alternating single and double bonds.

Natural compounds

Combining carbon with other elements such as hydrogen, oxygen, nitrogen, phosphorus and sulphur produces a wide range of organic compounds. Those compounds produced by photosynthesis, the natural process by which carbon dioxide and water are converted to oxygen and carbohydrates (molecules consisting of carbon, hydrogen and oxygen atoms), form the raw material for the many organic compounds found in living organisms, including animals and other life-forms as well as plants. These are known as natural compounds.
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Synthetic compounds

Organic compounds that are produced artificially are called artificial or synthetic compounds. They have properties that make them especially useful. New medicines and agrochemicals (including pesticides and fertilizers) are being developed all the time. By connecting atoms in various different ways, new compounds that have specific properties—for example, a medicine to treat cancer, or a fuel additive that improves engine efficiency—can be created in the laboratory.
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Functional groups

One of the ways to classify organic compounds is by their chemical composition, in particular by the type of functional groups they contain. A functional group (also called a substituent) is a group of atoms that give the molecule they make up its distinctive chemical properties. All molecules that contain the same functional group will react chemically in the same way.
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The atoms in a functional group are linked to each other and to the rest of the molecule by covalent bonds. Hydrocarbons contain only carbon and hydrogen; some hydrocarbon compounds have double or triple carbon bonds, which make up a functional group. Alcohols are a type of organic compound that contain a hydroxyl functional group. Carboxylic acids are organic compounds that contain a carboxyl functional group. Amines are organic compounds formed from ammonia by replacing hydrogen atoms with one of a range of functional groups containing carbon. Esters are organic compounds formed from a combination of carboxylic acid and alcohol.

Organic compounds can also be classified by their structure—the way their atoms fit together. The hydrocarbons are split into two types. Aliphatic compounds are those in which the carbon atoms bond together in chains. Aliphatic compounds are themselves subdivided into three families, known as homologous series. These are the alkanes (paraffins), which have just single bonds, alkenes (olefins), which contain one or more double bonds, and alkynes (acetylenes), which have one or more triple bonds. ​​​​​​​

Aromatic compounds are those that form rings with alternating single and double bonds between their carbon atoms. Benzene is the best-known aromatic compound. Other aromatics include toluene and the xylenes.

Homologous series

Aliphatic hydrocarbons that have only single bonds are called alkanes. Compounds in the alkane homologous series are named according to the number of carbon atoms in the molecule (a combination of a stem, such as eth- or prop-, and the ending –ane). Methane is an alkane with one carbon atom. Ethane is one with two carbon atoms, propane one with three carbon atoms and butane one with four carbon atoms.
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Some aliphatic hydrocarbons have one or more double bonds between their carbon atoms. These are called alkenes. The names of alkenes in their homologous series have the same stem as the alkane with the same number of carbon atoms, but with the ending –ene. So ethene is an alkene with two carbon atoms and propene has three carbon atoms. (Ethene is commonly called ethylene, while propene is commonly called propylene.)
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Alcohols and other organic compounds also form homologous series. Alcohols are named using the name of their "parent" hydrocarbon name but with the final –e dropped and the ending –ol attached. Thus the two simplest alcohols are methanol (used in solvents, fuels and cleaning products) and ethanol (used in fuels and alcoholic drinks).

Properties of hydrocarbons

Hydrocarbons exist in different states: gases (e.g. methane), liquids (e.g. benzene) and solids (e.g. naphthalene, an aromatic compound).

A hydrocarbon has different properties according to the number of molecules it contains. The lower the number, the​ ​lower​ ​the​ ​boiling​ ​point and the more flammable it is. So the smallest hydrocarbons, methane and propane, are flammable gases. Hydrocarbons with slightly higher numbers of molecules, such as octane and benzene, are liquids; they are also flammable and are commonly used as fuels. Hydrocarbons that contain large numbers of molecules are more viscous (thick, like syrup) but less flammable. Bitumen (also known as asphalt) is highly viscous and contains both aliphatic and aromatic compounds. Mixed with concrete, it is used in road construction.

Saturated and unsaturated

Alkanes, alkenes and alkynes all form homologous series of hydrocarbons. Alkanes, which contain only single bonds, are described as saturated, whereas alkenes and alkynes, which contain at least one double or triple bond, are said to be unsaturated. Having double or triple bonds makes alkenes (and alkynes) more reactive than alkanes. For example, ethene molecules can react together to form polyethene (polyethylene, commonly known as polythene, a well-known type of plastic).

Polymers

One important property of carbon is that it forms chains linked by bonds between its carbon atoms. The linking process is called polymerization and the chains are called polymers. They are made up of monomers, repeating sub-units consisting of the same groups of atoms. Polymers can be either naturally-occurring (biopolymers) or manufactured (synthetic or industrial polymers).

DNA, cellulose and proteins are examples of biopolymers. DNA polymers are made up of four different monomers called​ ​nucleotides ​structured in​ ​the​ ​form​ ​of​ ​a​ ​double​ ​helix. Cellulose is made up of glucose monomers, while proteins consist of amino acid monomers, joined together in different combinations to make long strands, which then fold into complex shapes.
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Alkenes​ ​can​ ​be​ ​used​ ​to​ ​make​ ​synthetic polymers​, ​such​ ​as​ ​poly(ethene)​, poly(propene) and poly(styrene), by​ ​a process called addition​ ​polymerization.​ Condensation polymers form in a different way: instead of just one product forming, a polymer molecule and a small molecule, often water, are produced as well. Polyesters are an example of condensation polymers.

Uses of hydrocarbons

Hydrocarbons are a key source of energy because they release heat when burned. They are also extremely useful as building blocks for making other products. This is because they link together in so many different ways.

Chemical engineers use the many thousands of different types of hydrocarbon contained in petroleum (crude oil) to produce a vast range of materials, including fuels, plastics, textiles, cosmetics, pharmaceuticals, agrochemicals, detergents, paints and adhesives—and hundreds more. Organic industrial chemistry is still based almost entirely on petroleum and natural gas, from which petrochemicals are produced.

Because of the urgent need both to decarbonize (limit the greenhouse gases in the atmosphere that cause climate change) and to eliminate plastic pollution, chemical engineers may in future switch to the use of oleochemicals, industrial chemicals synthesized from natural organic compounds such as plant oils and animal fats.