organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...

, a hydrocarbon is an

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon- hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...

consisting entirely of

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

and

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...

. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or exemplified by the odors of

gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic ...

and lighter fluid. They occur in a diverse range of molecular structures and phases: they can be gases (such as methane and

propane Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used a ...

liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...

s (such as hexane and

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen ato ...

), low melting solids (such as paraffin wax and

naphthalene Naphthalene is an organic compound with formula . It is the simplest polycyclic aromatic hydrocarbon, and is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromati ...

) or polymers (such as polyethylene and

polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is a ...

). In the

fossil fuel A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels ...

industries, ''hydrocarbon'' refers to the naturally occurring petroleum,

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon ...

and

coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen ...

, and to 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 gas A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...

es are

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

from the burning of fossil fuels, and methane released from natural gas handling and from agriculture.

Types

As defined by the International Union of Pure and Applied Chemistry's nomenclature of organic chemistry, the classifications for hydrocarbons are: # Saturated hydrocarbons are the simplest of the hydrocarbon types. They are composed entirely of

single bond In chemistry, a single bond is a chemical bond between two atoms involving two valence electrons. That is, the atoms share one pair of electrons where the bond forms. Therefore, a single bond is a type of covalent bond. When shared, each of th ...

s and are saturated with hydrogen. The formula for acyclic saturated hydrocarbons (i.e., alkanes) is CH. The most general form (true of both linear and branched species and those with and those without one or more rings) of saturated hydrocarbons is CH, where ''r'' is the number of rings. Those with exactly one ring are the cycloalkanes. Saturated hydrocarbons are the basis of petroleum fuels and are found as either linear or branched species. One or more of the hydrogen atoms can be replaced with other atoms, for example chlorine or another halogen, which is called a substitution reaction. An example would be the conversion of methane to chloroform using a chlorination reaction. Note that halogenating a hydrocarbon produces something that is not a hydrocarbon. It is a very common and useful process. Hydrocarbons with the same molecular formula but different

structural formula The structural formula of a chemical compound is a graphic representation of the molecular structure (determined by structural chemistry methods), showing how the atoms are possibly arranged in the real three-dimensional space. The chemical bondi ...

e are called

structural isomer In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a chemical compound, compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct chemical bond, b ...

s. As given in the example of 3-methylhexane and its higher homologues, branched hydrocarbons can be chiral. Chiral saturated hydrocarbons constitute the side chains of biomolecules such as chlorophyll and tocopherol. # Unsaturated hydrocarbons 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 In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...

have the formula CH (assuming non-cyclic structures). Those containing triple bonds are called alkyne. Those with one triple bond have the formula CH. # Aromatic hydrocarbons, also known as arenes, are hydrocarbons that have at least one aromatic ring. 10% of total nonmethane organic carbon emission are aromatic hydrocarbons from the exhaust of gasoline-powered vehicles. The term 'aliphatic' refers to non-aromatic hydrocarbons. Saturated aliphatic hydrocarbons are sometimes referred to as 'paraffins'. Aliphatic hydrocarbons containing a double bond between carbon atoms are sometimes referred to as 'olefins'.

Usage

The predominant use of hydrocarbons is as a combustible

fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...

source. Methane is the predominant component of natural gas. The C⁶ through C¹⁰ alkanes, alkenes and isomeric cycloalkanes are the top components of

, naphtha,

jet fuel Jet fuel or aviation turbine fuel (ATF, also abbreviated avtur) is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is colorless to straw-colored in appearance. The most commonly used fuels for commercial a ...

and specialized industrial solvent mixtures. With the progressive addition of carbon units, the simple non-ring structured hydrocarbons have higher viscosities, lubricating indices, boiling points, solidification temperatures, and deeper color. At the opposite extreme from methane lie the heavy tars that remain as the ''lowest fraction'' in a crude oil

refining {{Unreferenced, date=December 2009 Refining (also perhaps called by the mathematical term affining) is the process of purification of a (1) substance or a (2) form. The term is usually used of a natural resource that is almost in a usable form, b ...

retort. They are collected and widely utilized as roofing compounds, pavement composition ( bitumen), wood preservatives (the creosote series) and as extremely high viscosity shear-resisting liquids. Some large-scale non-fuel applications of hydrocarbons begins with ethane and propane, which are obtained from petroleum and natural gas. These two gases are converted either to syngas or to

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...

and propylene. These two alkenes are precursors to polymers, including polyethylene, polystyrene, acrylates, polypropylene, etc. Another class of special hydrocarbons is BTX, a mixture of

, toluene, and the three xylene isomers. Global consumption of benzene in 2021 is estimated at more than 58 million metric tons, which will increase to 60 million tons in 2022. Hydrocarbons are also prevalent in nature. Some eusocial arthropods, such as the Brazilian stingless bee, '' Schwarziana quadripunctata'', use unique

cuticular hydrocarbon Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pair ...

"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 An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease. Endophytes are ubiquitous and have been found in all species of plants studied to dat ...

bacteria from plants that naturally produce hydrocarbons have been used in hydrocarbon degradation in attempts to deplete hydrocarbon concentration in polluted soils.

Reactions

The noteworthy feature of saturated hydrocarbons is their inertness. Unsaturated hydrocarbons (alkanes, alkenes and aromatic compounds) react more readily, by means of substitution, addition, polymerization. At higher temperatures they undergo dehydrogenation, oxidation and combustion.

Substitution

Of the classes of hydrocarbons, aromatic compounds uniquely (or nearly so) undergo substitution reactions. The chemical process practiced on the largest scale is the reaction of benzene and ethene to give ethylbenzene: : The resulting ethylbenzene is dehydrogenated to styrene and then polymerized to manufacture

, a common thermoplastic material.

Free-radical substitution

Substitution reactions occur also in saturated hydrocarbons (all single carbon–carbon bonds). Such reactions require highly reactive reagents, such as

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...

and

fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...

. In the case of chlorination, one of the chlorine atoms replaces a hydrogen atom. The reactions proceed via

free-radical pathway A free-radical reaction is any chemical reaction involving free radicals. This reaction type is abundant in organic reactions. Two pioneering studies into free radical reactions have been the discovery of the triphenylmethyl radical by Moses Gomber ...

s, in which the halogen first dissociates into a two neutral radical atoms ( homolytic fission). :CH + Cl → CHCl + HCl :CHCl + Cl → CHCl + HCl all the way to CCl ( carbon tetrachloride) :CH + Cl → CHCl + HCl :CHCl + Cl → CHCl + HCl all the way to CCl ( hexachloroethane)

Addition

Addition reactions apply to alkenes and alkynes. In this reaction a variety of reagents add "across" the pi-bond(s). Chlorine, hydrogen chloride, water, and

are illustrative reagents.

Addition polymerization

Alkenes and some alkynes also undergo polymerization by opening of the multiple bonds to produce polyethylene, polybutylene, and

. The alkyne acetylene polymerizes to produce polyacetylene. Oligomers (chains of a few monomers) may be produced, for example in the Shell higher olefin process, where α-olefins are extended to make longer α-olefins by adding ethylene repeatedly.

Hydrogenation

Metathesis

Some hydrocarbons undergo ''metathesis'', in which substituents attached by C–C bonds are exchanged between molecules. For a single C–C bond it is

alkane metathesis Alkane metathesis is a class of chemical reaction in which an alkane is rearranged to give a longer or shorter alkane product. It is similar to olefin metathesis, except that olefin metathesis cleaves and recreates a carbon-carbon double bond, but ...

, for a double C–C bond it is alkene metathesis (olefin metathesis), and for a triple C–C bond it is alkyne metathesis.

High-temperature reactions

Cracking

=Dehydrogenation

Pyrolysis

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

. 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 Electrical energy is energy related to forces on electrically charged particles and the movement of electrically charged particles (often electrons in wires, but not always). This energy is supplied by the combination of electric current and electr ...

in power plants. Common properties of hydrocarbons are the facts that they produce steam,

and heat during combustion and that oxygen is required for combustion to take place. The simplest hydrocarbon, methane, burns as follows: :CH + 2 O → 2 HO + CO + energy In inadequate supply of air,

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

gas and water vapour are formed: :2 CH + 3 O → 2 CO + 4 HO Another example is the combustion of

: :CH + 5 O → 4 HO + 3 CO + energy And finally, for any

linear alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in ...

of n carbon atoms, :CH + O → (''n'' + 1) HO + ''n'' CO + energy. Partial oxidation characterizes the reactions of alkenes and oxygen. This process is the basis of rancidification and paint drying.

Origin

The vast majority of hydrocarbons found on

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...

occur in crude oil, petroleum,

, and natural gas. Petroleum (literally "rock oil") and coal are generally thought to be products of decomposition of organic matter. Coal, in contrast to petroleum, is richer in carbon and poorer in hydrogen. Natural gas is the product of

methanogenesis Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group ...

. A seemingly limitless variety of compounds comprise petroleum, hence the necessity of refineries. These hydrocarbons consist of saturated hydrocarbons, aromatic hydrocarbons, or combinations of the two. Missing in petroleum are alkenes and alkynes. Their production requires refineries. Petroleum-derived hydrocarbons are mainly consumed for fuel, but they are also the source of virtually all synthetic organic compounds, including plastics and pharmaceuticals. Natural gas is consumed almost exclusively as fuel. Coal is used as a fuel and as a reducing agent in

metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the sc ...

. A small fraction of hydrocarbon found on earth, and all currently-known hydrocarbon found on other planets and moons, is thought to be abiological. Hydrocarbons such as ethylene, isoprene, and monoterpenes are emitted by living vegetation. Some hydrocarbons also are widespread and abundant in the Solar System. Lakes of liquid methane and ethane have been found on

Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

's largest moon, as confirmed by the '' Cassini–Huygens'' space probe. Hydrocarbons are also abundant in nebulae forming polycyclic aromatic hydrocarbon compounds.

Environmental impact

Burning hydrocarbons as fuel, which produces

and water, is a major contributor to anthropogenic global warming. Hydrocarbons are introduced into the environment through their extensive use as fuels and chemicals as well as through leaks or accidental spills during exploration, production, refining, or transport of fossil fuels. Anthropogenic hydrocarbon contamination of soil is a serious global issue due to contaminant persistence and the negative impact on human health. Mechanisms involved in phytoremediation

When soil is contaminated by hydrocarbons, it can have a significant impact on its microbiological, chemical, and physical properties. This can serve to prevent, slow down or even accelerate the growth of vegetation depending on the exact changes that occur. Crude oil and natural gas are the two largest sources of hydrocarbon contamination of soil.

Bioremediation

Bioremediation of hydrocarbon from soil or water contaminated is a formidable challenge because of the chemical inertness that characterize hydrocarbons (hence they survived millions of years in the source rock). Nonetheless, many strategies have been devised, bioremediation being prominent. The basic problem with bioremediation is the paucity of enzymes that act on them. Nonetheless the area has received regular attention. Bacteria in the gabbroic layer of the ocean's crust can degrade hydrocarbons; but the extreme environment makes research difficult. Other bacteria such as ''

Lutibacterium anuloederans ''Lutibacterium'' is a genus of Gram-negative staining bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microo ...

'' can also degrade hydrocarbons. Mycoremediation or breaking down of hydrocarbon by mycelium and mushrooms is possible.

Safety

Hydrocarbons are generally of low toxicity, hence the widespread use of gasoline and related volatile products. Aromatic compounds such as benzene and toluene are narcotic and chronic toxins, and benzene in particular is known to be carcinogenic. Certain rare polycyclic aromatic compounds are carcinogenic. Hydrocarbons are highly

flammable A combustible material is something that can burn (i.e., ''combust'') in air. A combustible material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort and a flammable mat ...

References

External links

The Methane Molecule

Encyclopedia of Hydrocarbons
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