In thefossil fuel industries,hydrocarbon refers to naturally occurringpetroleum,natural gas andcoal, 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 organiccommodity chemicals such as solvents and polymers. Most anthropogenic (human-generated) emissions ofgreenhouse gases are eithercarbon dioxide released by the burning offossil fuels, or methane released from the handling of natural gas or from agriculture.
Saturated hydrocarbons, which are the simplest of the hydrocarbon types. They are composed entirely ofsingle bonds and are saturated with hydrogen. The formula foracyclic 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), wherer is the number of rings. Those with exactly one ring are thecycloalkanes. Saturated hydrocarbons are the basis ofpetroleum 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 tochloroform using achlorination reaction. Halogenating a hydrocarbon produces something that is not a hydrocarbon. It is a very common and useful process. Hydrocarbons with the samemolecular formula but differentstructural formulae are calledstructural isomers.[1]: 625 As given in the example of3-methylhexane and its higherhomologues, branched hydrocarbons can bechiral.[1]: 627 Chiral saturated hydrocarbons constitute the side chains ofbiomolecules such aschlorophyll andtocopherol.[3]
Unsaturated hydrocarbons, which have one or more double or triple bonds between carbon atoms. Those with one or more double bonds are calledalkenes. Those with onedouble bond have the formula CnH2n (assuming non-cyclic structures).[1]: 628 Those containingtriple bonds are calledalkyne. Those with one triple bond have the formula CnH2n−2.[1]: 631
Aromatic hydrocarbons, also known asarenes, which are hydrocarbons that have at least onearomatic ring. 10% of total nonmethane organic carbon emission are aromatic hydrocarbons from the exhaust of gasoline-powered vehicles.[4]
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'.
Variations on hydrocarbons based on the number of carbon atoms
Oil refineries are one way hydrocarbons are processed for use.Crude oil is processed in several stages to form desired hydrocarbons, used as fuel and in other products.Tank wagon 33 80 7920 362–0 with hydrocarbon gas at Bahnhof Enns (2018)
The predominant use of hydrocarbons is as a combustiblefuel source. Methane is the predominant component of natural gas. C6 through C10 alkanes, alkenes, cycloalkanes, and aromatic hydrocarbons are the main components ofgasoline,naphtha,jet fuel, and specialized industrial solvent mixtures. With the progressive addition of carbon units, the simple non-ring structured hydrocarbons have higherviscosities, lubricating indices, boiling points, andsolidification temperatures. At the opposite extreme from methane lie the heavytars that remain as thelowest fraction in a crude oilrefining retort. They are collected and widely utilized as roofing compounds, pavement material (bitumen), wood preservatives (thecreosote 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 tosyngas or toethylene andpropylene 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.[5]
Hydrocarbons are also prevalent in nature. Some eusocial arthropods, such as the Brazilian stingless bee,Schwarziana quadripunctata, use uniquecuticular hydrocarbon "scents" in order to determine kin from non-kin. This hydrocarbon composition varies between age, sex, nest location, and hierarchal position.[6]
There is also potential to harvest hydrocarbons from plants likeEuphorbia lathyris andE. tirucalli as an alternative and renewable energy source for vehicles that use diesel.[7] Furthermore,endophytic bacteria from plants that naturally produce hydrocarbons have been used in hydrocarbon degradation in attempts to deplete hydrocarbon concentration in polluted soils.[8]
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]
The cracking of saturated hydrocarbons is the main industrial route toalkenes andalkyne. These reactions requireheterogeneous 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 frombutane,terephthalic acid fromxylenes,acetone together withphenol fromcumene (isopropylbenzene), andcyclohexanone fromcyclohexane. The process, which is calledautoxidation, begins with the formation ofhydroperoxides (ROOH).[9]
Combustion of hydrocarbons is currently the main source of the world's energy forelectric power generation, heating (such as home heating), and transportation.[10][11] Often this energy is used directly as heat such as in home heaters, which use eitherpetroleum ornatural gas. The hydrocarbon is burnt and the heat is used to heat water, which is then circulated. A similar principle is used to createelectrical energy inpower plants. Both saturated and unsaturated hydrocarbons undergo this process.
Common properties of hydrocarbons are the facts that they produce steam,carbon dioxide and heat duringcombustion and thatoxygen is required for combustion to take place. The simplest hydrocarbon,methane, burns as follows:
Saturated hydrocarbons react withchlorine andfluorine. In the case of chlorination, one of the chlorine atoms replaces a hydrogen atom. The reactions proceed viafree-radical pathways, in which the halogen first dissociates into two neutral radical atoms (homolytic fission).
Aromatic compounds, almost uniquely for hydrocarbons, undergo substitution reactions. The chemical process practiced on the largest scale is the reaction of benzene andethene to giveethylbenzene:
C6H6 + C2H4 → C6H5CH2CH3
The resulting ethylbenzene is dehydrogenated tostyrene and then polymerized to manufacturepolystyrene, a commonthermoplastic material.
Addition reactions apply to alkenes and alkynes. It is because they add reagents that they are called unsaturated. In this reaction a variety of reagents add "across" the pi-bond(s). Chlorine, hydrogen chloride,water, andhydrogen are illustrative reagents.
Some hydrocarbons undergometathesis, in which substituents attached by C–C bonds are exchanged between molecules. For a single C–C bond it isalkane metathesis, for a double C–C bond it isalkene metathesis (olefin metathesis), and for a triple C–C bond it isalkyne metathesis.
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 inmetallurgy.
A small fraction of hydrocarbon found on earth, and all currently known hydrocarbon found on other planets and moons, is thought to beabiological.[15]
Hydrocarbons such as ethylene, isoprene, and monoterpenes are emitted by living vegetation.[16]
Burning hydrocarbons as fuel, which producescarbon dioxide andwater, is a major contributor to anthropogenicglobal 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.[19]
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.[21]
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.[22]Bacteria in thegabbroic layer of the ocean's crust can degrade hydrocarbons; but the extreme environment makes research difficult.[23] Other bacteria such asLutibacterium anuloederans can also degrade hydrocarbons.[24]Mycoremediation or breaking down of hydrocarbon bymycelium andmushrooms is possible.[25][26]
Hydrocarbons are generally of low toxicity, hence the widespread use of gasoline and related volatile products. Aromatic compounds such as benzene andtoluene are narcotic and chronic toxins, and benzene in particular is known to becarcinogenic. Certain rare polycyclic aromatic compounds are carcinogenic.Hydrocarbons are highlyflammable.
^van Dijk, J.P. (2022); Unravelling the Maze of Scientific Writing Through the Ages: On the Origins of the Terms Hydrocarbon, Petroleum, Natural Gas, and Methane. Amazon Publishers, 166 pp. PaperBack Edition B0BKRZRKHW.ISBN979-8353989172
^Clayden, J., Greeves, N., et al. (2001)Organic Chemistry OxfordISBN0-19-850346-6, p. 21.
^McMurry, J. (2000).Organic Chemistry 5th ed. Brooks/Cole: Thomson Learning.ISBN0-495-11837-0. pp. 75–81.
^"Additives Affecting the Microbial Degradation of Petroleum Hydrocarbons",Bioremediation of Contaminated Soils, CRC Press, pp. 353–360, 9 June 2000,doi:10.1201/9781482270235-27,ISBN978-0-429-07804-0
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