Organic compounds are chemicals whose molecules contain carbon. There is no universally accepted, more precise definition of "organic," which makes defining the properties of organic compounds more difficult. Today, "organic compounds" most commonly refers to compounds that contain significant amounts of carbon, though they can be defined more loosely to include any molecules containing carbon or more strictly to include only molecules that contain carbon-carbon or carbon-hydrogen bonds.
The organic-inorganic distinction is somewhat arbitrary and is in large part derived from a theory called vitalism, which posited that the biochemical processes of living beings involve an additional energy or force that is not present in chemical processes occurring in nonliving matter. This suggested the idea that the substances involved in the biology of living creatures are therefore different from other substances in a way that goes beyond their physical and chemical properties and makes up a special category distinct from other matter. Vitalism is no longer a commonly accepted theory, and modern biochemistry explains biological processes and the properties of organic compounds according to the same physical laws that govern other chemical reactions, but the category “organic” remains in use, though today the term also encompasses a number of compounds that do not occur in living beings.
There is a vast number of different organic compounds with different properties, which are divided into smaller subclasses based on their composition or the composition of their functional groups and tend to have similar properties. Some organic categories have interesting properties that people often encounter in their daily lives. Hydrocarbons, a category that includes groups of molecules called alkanes, alkenes, and arenes according to their structure, are made up entirely of hydrogen and carbon and release a great deal of energy when burnt, making hydrocarbons such as propane, butane, and octane valuable fuel sources. Another type of organic molecules, called esters, make up vegetable and animal oils and fats.
One of the important properties of organic compounds is the presence of structural units called functional groups. These are groups of atoms within a molecule that is joined to the rest of the molecule by a covalent bond between an atom of the functional group and a carbon atom in the molecule's main body. An organic molecule can have many functional groups, and these are largely responsible for the chemical properties of the molecule as a whole, because a given functional group will behave in more or less the same way regardless of the composition of the larger molecule it is attached to. Organic molecules are often classified and named according to the nature of their functional groups.
Molecules that are organic vary greatly in size. One of the important properties of organic compounds that makes them so important in biology comes from the fact that carbon atoms can bond to each other to form chains of carbon atoms in a row or loop, with additional atoms bonded to some of the carbon atoms. Many identical copies of individual structural units called monomers, which consist of a row of carbon atoms linked together with other atoms called side groups attached, can be linked together to form long repeating chains called polymers.
Organic compounds are not unique in their ability to do this, but carbon is particularly well suited to it. A single organic polymer molecule can have hundreds of repeated structural units, and more complex polymers can have separate branches splitting off from the central chain and form chemical bonds with other polymer chains to create a single molecule. Some organic compounds are made of vast structures called macromolecules.