In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon.[1]: 620 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:[2]
- 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 CnH2n+2.[1]: 623 The most general form of saturated hydrocarbons, (whether linear or branched species, and whether with or without one or more rings) is CnH2n+2(1-r), where r is the number of rings. Those with exactly one ring are the cycloalkanes. Saturated hydrocarbons are the basis of petroleum fuels and may be either linear or branched species. One or more of the hydrogen atoms can be replaced with other atoms, for example chlorine or another halogen: this is called a substitution reaction. An example is the conversion of methane to chloroform using a chlorination reaction. 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 formulae are called structural isomers.[1]: 625 As given in the example of 3-methylhexane and its higher homologues, branched hydrocarbons can be chiral.[1]: 627 Chiral saturated hydrocarbons constitute the side chains of biomolecules such as chlorophyll and tocopherol.[3]
- 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 CnH2n (assuming non-cyclic structures).[1]: 628 Those containing triple bonds are called alkyne. Those with one triple bond have the formula CnH2n−2.[1]: 631
- Aromatic hydrocarbons, also known as arenes, which 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.[citation needed]
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'.
| Number of carbon atoms |
Alkane (single bond) | Alkene (double bond) | Alkyne (triple bond) | Cycloalkane | Alkadiene |
|---|---|---|---|---|---|
| 1 | Methane | — | — | — | — |
| 2 | Ethane | Ethene (ethylene) | Ethyne (acetylene) | — | — |
| 3 | Propane | Propene (propylene) | Propyne (methylacetylene) | Cyclopropane | Propadiene (allene) |
| 4 | Butane | Butene (butylene) | Butyne | Cyclobutane | Butadiene |
| 5 | Pentane | Pentene | Pentyne | Cyclopentane | Pentadiene (piperylene) |
| 6 | Hexane | Hexene | Hexyne | Cyclohexane | Hexadiene |
| 7 | Heptane | Heptene | Heptyne | Cycloheptane | Heptadiene |
| 8 | Octane | Octene | Octyne | Cyclooctane | Octadiene |
| 9 | Nonane | Nonene | Nonyne | Cyclononane | Nonadiene |
| 10 | Decane | Decene | Decyne | Cyclodecane | Decadiene |
| 11 | Undecane | Undecene | Undecyne | Cycloundecane | Undecadiene |
| 12 | Dodecane | Dodecene | Dodecyne | Cyclododecane | Dodecadiene |
Usage
The predominant use of hydrocarbons is as a combustible fuel source. Methane is the predominant component of natural gas. C6 through C10 alkanes, alkenes, cycloalkanes, and aromatic hydrocarbons are the main components of gasoline, naphtha, jet fuel, 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, and solidification temperatures. At the opposite extreme from methane lie the heavy tars that remain as the lowest fraction in a crude oil refining retort. They are collected and widely utilized as roofing compounds, pavement material (bitumen), wood preservatives (the creosote series) and as extremely high viscosity shear-resisting liquids.
Some large-scale non-fuel applications of hydrocarbons begin with ethane and propane, which are obtained from petroleum and natural gas. These two gases are converted either to syngas or to ethylene and propylene respectively. Global consumption of benzene in 2021 is estimated at more than 58 million metric tons, which will increase to 60 million tons in 2022.[4]
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.[5]
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.[6] Furthermore, endophytic bacteria from plants that naturally produce hydrocarbons have been used in hydrocarbon degradation in attempts to deplete hydrocarbon concentration in polluted soils.[7]
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.[2]
Saturated hydrocarbons
Cracking
The cracking of saturated hydrocarbons is the main industrial route to alkenes and alkyne. These reactions require heterogeneous catalysts and temperatures >500 °C.
Oxidation
Oxidation of hydrocarbons involves their reaction with oxygen. In the presence of excess oxygen, hydrocarbons combust. With careful conditions, which have been optimized for many years, partial oxidation results. Useful compounds can be obtained in this way: maleic acid from butane, terephthalic acid from xylenes, acetone together with phenol from cumene (isopropylbenzene), and cyclohexanone from cyclohexane. The process, which is called autoxidation, begins with the formation of hydroperoxides (ROOH).[8]
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.[9][10] 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:
![{\displaystyle {{\underset {\mathrm {methane} }{\mathrm {CH} {\vphantom {A}}_{\smash[{t}]{4}}}}{}+{}2\,\mathrm {O} {\vphantom {A}}_{\smash[{t}]{2}}{}\mathrel {\longrightarrow } {}\mathrm {CO} {\vphantom {A}}_{\smash[{t}]{2}}{}+{}2\,\mathrm {H} {\vphantom {A}}_{\smash[{t}]{2}}\mathrm {O} }}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8a72f60068ff686d0be397b219c224f7e0668254)
In inadequate supply of air, carbon black and water vapour are formed:
![{\displaystyle {{\underset {\mathrm {methane} }{\mathrm {CH} {\vphantom {A}}_{\smash[{t}]{4}}}}{}+{}\mathrm {O} {\vphantom {A}}_{\smash[{t}]{2}}{}\mathrel {\longrightarrow } {}\mathrm {C} {}+{}2\,\mathrm {H} {\vphantom {A}}_{\smash[{t}]{2}}\mathrm {O} }}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c27fa979f3dd2f59e1771a8029752c4839f4383e)
And finally, for any linear alkane of n carbon atoms,
![{\displaystyle {\mathrm {C} }_{n}{\mathrm {H} }_{2n+2}+\left({{3n+1} \over 2}\right){\mathrm {O} {\vphantom {A}}_{\smash[{t}]{2}}{}\mathrel {\longrightarrow } {}}n{\mathrm {CO} {\vphantom {A}}_{\smash[{t}]{2}}}+(n+1){\mathrm {H} {\vphantom {A}}_{\smash[{t}]{2}}\mathrm {O} }}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c740b1a9a6a3627f98b9f405188b8f5ec94ffa5b)
Partial oxidation characterizes the reactions of alkenes and oxygen. This process is the basis of rancidification and paint drying.
Benzene burns with sooty flame when heated in air:
![{\displaystyle {{\underset {\mathrm {benzene} }{\mathrm {C} {\vphantom {A}}_{\smash[{t}]{6}}\mathrm {H} {\vphantom {A}}_{\smash[{t}]{6}}}}{}+{}{{\text{15}} \over {\text{2}}}\mathrm {O} {\vphantom {A}}_{\smash[{t}]{2}}{}\mathrel {\longrightarrow } {}6\,\mathrm {CO} {\vphantom {A}}_{\smash[{t}]{2}}~{\text{+}}~3\,\mathrm {H} {\vphantom {A}}_{\smash[{t}]{2}}\mathrm {O} }}](https://wikimedia.org/api/rest_v1/media/math/render/svg/0cd785f5ae505bd3b350f6674755292345f40ad0)
Halogenation
Saturated hydrocarbons react with chlorine and fluorine. In the case of chlorination, one of the chlorine atoms replaces a hydrogen atom. The reactions proceed via free-radical pathways, in which the halogen first dissociates into two neutral radical atoms (homolytic fission).
- CH4 + Cl2 → CH3Cl + HCl
- CH3Cl + Cl2 → CH2Cl2 + HCl
all the way to CCl4 (carbon tetrachloride)
- C2H6 + Cl2 → C2H5Cl + HCl
- C2H4Cl2 + Cl2 → C2H3Cl3 + HCl
all the way to C2Cl6 (hexachloroethane)
Unsaturated hydrocarbons
Substitution
Aromatic compounds, almost uniquely for hydrocarbons, undergo substitution reactions. The chemical process practiced on the largest scale is the reaction of benzene and ethene to give ethylbenzene: