thumb|upright|[[Ball-and-stick model of the methane molecule, CH. Methane is part of a homologous series known as the alkanes, which contain single bonds only.]]
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may be similar to that of gasoline or lighter fluid. They occur in a diverse range of molecular structures and phases: they can be gases (such as methane and propane), liquids (such as hexane and benzene), low melting solids (such as paraffin wax and naphthalene) or polymers (such as polyethylene and polystyrene).
In the fossil fuel industries, hydrocarbon refers to naturally occurring petroleum, natural gas and coal, or their hydrocarbon derivatives and purified forms. Combustion of hydrocarbons is the main source of the world's energy. Petroleum is the dominant raw-material source for organic commodity chemicals such as solvents and polymers. Most anthropogenic (human-generated) emissions of greenhouse gases are either carbon dioxide released by the burning of fossil fuels, or methane released from the handling of natural gas or from agriculture.
Types
As defined by the International Union of Pure and Applied Chemistry's nomenclature of organic chemistry, hydrocarbons are classified as follows:
- Saturated hydrocarbons, which are the simplest of the hydrocarbon types. They are composed entirely of single bonds and are saturated with hydrogen. The formula for acyclic saturated hydrocarbons (i.e., alkanes) is CH.
- Unsaturated hydrocarbons, which have one or more double or triple bonds between carbon atoms. Those with one or more double bonds are called alkenes. Those with one double bond have the formula CH (assuming non-cyclic structures).
Hydrocarbons are also prevalent in nature. Some eusocial arthropods, such as the Brazilian stingless bee, Schwarziana quadripunctata, use unique cuticular hydrocarbon "scents" in order to determine kin from non-kin. This hydrocarbon composition varies between age, sex, nest location, and hierarchal position.
There is also potential to harvest hydrocarbons from plants like Euphorbia lathyris and E. tirucalli as an alternative and renewable energy source for vehicles that use diesel. Furthermore, endophytic bacteria from plants that naturally produce hydrocarbons have been used in hydrocarbon degradation in attempts to deplete hydrocarbon concentration in polluted soils.
Reactions
Saturated hydrocarbons are notable for their inertness. Unsaturated hydrocarbons (alkenes, alkynes and aromatic compounds) react more readily, by means of substitution, addition, polymerization. At higher temperatures they undergo dehydrogenation, oxidation and combustion.
Combustion
Combustion of hydrocarbons is currently the main source of the world's energy for electric power generation, heating (such as home heating), and transportation. Often this energy is used directly as heat such as in home heaters, which use either petroleum or natural gas. The hydrocarbon is burnt and the heat is used to heat water, which is then circulated. A similar principle is used to create electrical energy in power plants. Both saturated and unsaturated hydrocarbons undergo this process.
Common properties of hydrocarbons are the facts that they produce steam, carbon dioxide and heat during combustion and that oxygen is required for combustion to take place. The simplest hydrocarbon, methane, burns as follows:
:<chem>\underset{methane}{CH4} + 2O2 -> CO2 + 2H2O</chem>
In inadequate supply of air, carbon black and water vapour are formed:
:<chem>\underset{methane}{CH4} + O2 -> C + 2H2O</chem>
And finally, for any linear alkane of n carbon atoms,
:<math chem="">\ce{C}_n \ce{H}_{2n+2} + \left(