Hydrocarbon

Ball-and-stick model of the methane molecule, CH4. 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.[1]: 620  Hydrocarbons are examples of group 14 hydrides.[2] Hydrocarbons are generally colourless and hydrophobic with only weak odours. Because of their diverse molecular structures, it is difficult to generalize further. In the oil & gas industry, hydrocarbon is a generalised term, which combines petroleum and natural gas as the two naturally occurring phases of hydrocarbon commoditised by the sector. Most anthropogenic emissions of greenhouse gases are from the burning of fossil fuels including fuel production and combustion. Natural sources of hydrocarbons such as methane, ethylene, isoprene, and monoterpenes come from the emissions of vegetation.[3]

Types

As defined by IUPAC nomenclature of organic chemistry, the classifications for hydrocarbons are:

  1. Saturated hydrocarbons 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 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 are found as either linear or branched species. Substitution reaction is their characteristics property (like chlorination reaction to form chloroform). 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.[4]
  2. Unsaturated hydrocarbons have one or more double or triple bonds between carbon atoms. Those with double bond 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 
  3. 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.[5]

Hydrocarbons can be gases (e.g. methane and propane), liquids (e.g. hexane and benzene), waxes or low melting solids (e.g. paraffin wax and naphthalene) or polymers (e.g. polyethylene, polypropylene and polystyrene).

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
Number of
carbon atoms
Alkane (single bond)Alkene (double bond)Alkyne (triple bond)CycloalkaneAlkadiene
1Methane
2EthaneEthene (ethylene)Ethyne (acetylene)
3PropanePropene (propylene)Propyne (methylacetylene)CyclopropanePropadiene (allene)
4ButaneButene (butylene)ButyneCyclobutaneButadiene
5PentanePentenePentyneCyclopentanePentadiene (piperylene)
6HexaneHexeneHexyneCyclohexaneHexadiene
7HeptaneHepteneHeptyneCycloheptaneHeptadiene
8OctaneOcteneOctyneCyclooctaneOctadiene
9NonaneNoneneNonyneCyclononaneNonadiene
10DecaneDeceneDecyneCyclodecaneDecadiene
11UndecaneUndeceneUndecyneCycloundecaneUndecadiene
12DodecaneDodeceneDodecyneCyclododecaneDodecadiene

Usage

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 combustible fuel source. Methane is the predominant component of natural gas. The C6 through C10 alkanes, alkenes and isomeric cycloalkanes are the top 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, 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 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[6] or to ethylene and propylene.[7][8] These two alkenes are precursors to polymers, including polyethylene, polystyrene, acrylates,[9][10][11] polypropylene, etc. Another class of special hydrocarbons is BTX, a mixture of benzene, toluene, and the three xylene isomers.[12] Global consumption of benzene in 2021 is estimated at more than 58 million tons, which will increase to 60 million tons in 2022.[13]

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.[14]

There is also potential to harvest hydrocarbons from plants like Euphorbia lathyris and Euphorbia tirucalli as an alternative and renewable energy source for vehicles that use diesel.[15] Furthermore, endophytic bacteria from plants that naturally produce hydrocarbons have been used in hydrocarbon degradation in attempts to deplete hydrocarbon concentration in polluted soils.[16]

Reactions

The noteworthy feature of hydrocarbons is their inertness, especially for saturated members. Otherwise, three main types of reactions can be identified:

Free-radical reactions

Substitution reactions only occur in saturated hydrocarbons (single carbon–carbon bonds). Such reactions require highly reactive reagents, such as chlorine and fluorine. In the case of chlorination, one of the chlorine atoms replaces a hydrogen atom. The reactions proceed via free-radical pathways.

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)

Substitution

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

Addition reactions

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 hydrogen are illustrative reagents. Alkenes and some alkynes also undergo polymerization, alkene metathesis, and alkyne metathesis.

Oxidation

Hydrocarbons are currently the main source of the world's electric energy and heat sources (such as home heating) because of the energy produced when they are combusted.[17][18] 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.

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:

CH4 + 2 O2 → 2 H2O + CO2 + energy

In inadequate supply of air, carbon monoxide gas and water vapour are formed:

2 CH4 + 3 O2 → 2 CO + 4 H2O

Another example is the combustion of propane:

C3H8 + 5 O2 → 4 H2O + 3 CO2 + energy

And finally, for any linear alkane of n carbon atoms,

CnH2n+2 +3n + 1/2 O2 → (n + 1) H2O + n CO2 + energy.

Partial oxidation characterizes the reactions of alkenes and oxygen. This process is the basis of rancidification and paint drying.

Origin

Natural oil spring in Korňa, Slovakia.

The vast majority of hydrocarbons found on Earth occur in crude oil, petroleum, coal, and natural gas. Petroleum (literally "rock oil" – petrol for short) 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.[19][20]

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.

Abiological hydrocarbons A small fraction of hydrocarbon found on earth is thought to be abiological.[21]

Some hydrocarbons also are widespread and abundant in the solar system. Lakes of liquid methane and ethane have been found on Titan, Saturn's largest moon, confirmed by the Cassini-Huygens Mission.[22] Hydrocarbons are also abundant in nebulae forming polycyclic aromatic hydrocarbon (PAH) compounds.[23]

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.[24] Bacteria in the gabbroic layer of the ocean's crust can degrade hydrocarbons; but the extreme environment makes research difficult.[25] Other bacteria such as Lutibacterium anuloederans can also degrade hydrocarbons.[26] Mycoremediation or breaking down of hydrocarbon by mycelium and mushrooms is possible.[27][28]

Safety

Hydrocarbons are generally of low toxicity, hence the widespread use of gasoline and related volatile products. Aromatic compounds such as benzene are narcotic and chronic toxins and are carcinogenic. Certain rare polycyclic aromatic compounds are carcinogenic. Hydrocarbons are highly flammable.

Environmental impact

Burning hydrocarbons as fuel, which produces carbon dioxide 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.[29]

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.[30]

See also

References

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  2. ^ IUPAC Goldbook hydrocarbyl groups Archived 7 January 2010 at the Wayback Machine
  3. ^ Dewulf, Jo. "Hydrocarbons in the Atmosphere" (PDF). Retrieved 26 October 2020.
  4. ^ Meierhenrich, Uwe. Amino Acids and the Asymmetry of Life Archived 2 March 2017 at the Wayback Machine. Springer, 2008.ISBN 978-3-540-76885-2
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  7. ^ Ge, Meng; Chen, Xingye; Li, Yanyong; Wang, Jiameng; Xu, Yanhong; Zhang, Lihong (1 June 2020). "Perovskite-derived cobalt-based catalyst for catalytic propane dehydrogenation". Reaction Kinetics, Mechanisms and Catalysis. 130 (1): 241–256. doi:10.1007/s11144-020-01779-8. ISSN 1878-5204. S2CID 218496057.
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  15. ^ Calvin, Melvin (1980). "Hydrocarbons from plants: Analytical methods and observations". Naturwissenschaften. 67 (11): 525–533. Bibcode:1980NW.....67..525C. doi:10.1007/BF00450661. S2CID 40660980.
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This is a mosaic image, one of the largest ever taken by NASA's Hubble Space Telescope, of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion. Japanese and Chinese astronomers recorded this violent event in 1054 CE, as did, almost certainly, Native Americans.

The orange filaments are the tattered remains of the star and consist mostly of hydrogen. The rapidly spinning neutron star embedded in the center of the nebula is the dynamo powering the nebula's eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. The neutron star, like a lighthouse, ejects twin beams of radiation that appear to pulse 30 times a second due to the neutron star's rotation. A neutron star is the crushed ultra-dense core of the exploded star.

The Crab Nebula derived its name from its appearance in a drawing made by Irish astronomer Lord Rosse in 1844, using a 36-inch telescope. When viewed by Hubble, as well as by large ground-based telescopes such as the European Southern Observatory's Very Large Telescope, the Crab Nebula takes on a more detailed appearance that yields clues into the spectacular demise of a star, 6,500 light-years away.

The newly composed image was assembled from 24 individual Wide Field and Planetary Camera 2 exposures taken in October 1999, January 2000, and December 2000. The colors in the image indicate the different elements that were expelled during the explosion. Blue in the filaments in the outer part of the nebula represents neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen.
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This is a montage of planetary images taken by spacecraft managed by the Jet Propulsion Laboratory in Pasadena, CA. Included are (from top to bottom) images of Mercury, Venus, Earth (and Moon), Mars, Jupiter, Saturn, Uranus and Neptune. The spacecraft responsible for these images are as follows:
  • the Mercury image was taken by Mariner 10,
  • the Venus image by Magellan,
  • the Earth and Moon images by Galileo,
  • the Mars image by Mars Global Surveyor,
  • the Jupiter image by Cassini, and
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  • Sun (to photosphere) 1,392,684 km
  • Mercury 4,879.4 km
  • Venus 12,103.7 km
  • Earth 12,756.28 km
  • Moon 3,476.2 km
  • Mars 6,804.9 km
  • Jupiter 142,984 km
  • Saturn 120,536 km
  • Uranus 51,118 km
  • Neptune 49,528 km
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This view of the rising Earth greeted the Apollo 8 astronauts as they came from behind the Moon after the fourth nearside orbit. Earth is about five degrees above the horizon in the photo. The unnamed surface features in the foreground are near the eastern limb of the Moon as viewed from Earth. The lunar horizon is approximately 780 kilometers from the spacecraft. Width of the photographed area at the horizon is about 175 kilometers. On the Earth 240,000 miles away, the sunset terminator bisects Africa.
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Temperature changes to date have been most pronounced in northern latitudes and over land masses. The image uses longer term averages of at least a decade to smooth out climate variability due to factors such as El Niño. The map is improved from the highest quality rendering that NASA’s Scientific Visualization Studio generates, with horizontal and vertical lines removed and with a more legible projection of Kavraiskiy VII. Grey areas in the image have insufficient data for rendering. For a version that includes Fahrenheit, see File:Change in Average Temperature With Fahrenheit.svg
2018-05-04 (303) Tank wagon 33 80 7920 362-0 with hydrocarbon gas at Bahnhof Enns.jpg
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Tank wagon 33 80 7920 362-0 with hydrocarbon gas at Bahnhof Enns
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Natural oil seep in Korňa
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