Editing Alkane (section)

==Occurrence==
===Occurrence of alkanes in the Universe===
[[Image:Jupiter.jpg|thumb|right|[[Methane]] and [[ethane]] make up a tiny proportion<!-- 0.3% methane and 0.00006% ethane is tiny not large --> of [[Jupiter]]'s atmosphere]]
[[Image:Oil well.jpg|thumb|right|Extraction of oil, which contains many distinct [[hydrocarbon]]s including alkanes]]

Alkanes form a small portion<!-- 0.3% methane and 0.00006% ethane for Jupiter is small not significant: else find cite that claims this is significant, or specify in which context. Uranus and Neptune have more but still small --> of the [[Celestial body atmosphere|atmospheres]] of the outer gas planets such as [[Jupiter]] (0.1% methane, 2&nbsp;[[parts per million|ppm]] ethane), [[Saturn]] (0.2% methane, 5&nbsp;ppm ethane), [[Uranus]] (1.99% methane, 2.5&nbsp;ppm ethane) and [[Neptune]] (1.5% methane, 1.5&nbsp;ppm ethane). [[Titan (moon)|Titan]] (1.6% methane), a satellite of Saturn, was examined by the [[Huygens (spacecraft)|''Huygens'' probe]], which indicated that Titan's atmosphere periodically rains liquid methane onto the moon's surface.<ref>{{cite web|title=Titan: Arizona in an Icebox? |url=http://www.planetary.org/news/2005/huygens_science-results_0121.html |first=Emily |last=Lakdawalla |access-date=21 January 2004 |archive-url=https://web.archive.org/web/20080406104505/http://planetary.org/news/2005/0121_Titan_Arizona_in_an_Icebox.html |archive-date=6 April 2008 |url-status=dead  }}</ref> Also on Titan, the Cassini mission has imaged seasonal methane/ethane lakes near the polar regions of Titan. [[Methane]] and [[ethane]] have also been detected in the tail of the [[comet Hyakutake]]. Chemical analysis showed that the abundances of ethane and methane were roughly equal, which is thought to imply that its ices formed in interstellar space, away from the Sun, which would have evaporated these volatile molecules.<ref>{{cite journal | last1=Mumma |first1=M.J. | title=Detection of Abundant Ethane and Methane, Along with Carbon Monoxide and Water, in Comet C/1996 B2 Hyakutake: Evidence for Interstellar Origin | journal=Science | year=1996 | volume=272 | doi=10.1126/science.272.5266.1310 | pmid=8650540 | last2 = Disanti |first2=M.A. |last3=dello Russo |first3=N. |last4=Fomenkova |first4=M. |last5=Magee-Sauer |first5=K. |last6=Kaminski |first6=C.D. |last7=D.X. |first7=Xie | issue=5266 | bibcode=1996Sci...272.1310M | pages=1310–4| s2cid=27362518 }}</ref> Alkanes have also been detected in [[meteorite]]s such as [[carbonaceous chondrite]]s.

===Occurrence of alkanes on Earth===
Traces of methane gas (about 0.0002% or 1745&nbsp;ppb) occur in the Earth's atmosphere, produced primarily by [[methanogenesis|methanogenic]] microorganisms, such as [[Archaea]] in the gut of ruminants.<ref>{{cite journal | last1 = Janssen | first1 = P. H. | last2 = Kirs | first2 = M. | year = 2008 | title = Structure of the Archaeal Community of the Rumen | journal = Appl Environ Microbiol | volume = 74 | issue = 12| pages = 3619–25 | doi = 10.1128/AEM.02812-07 |pmc= 2446570 | pmid=18424540| bibcode = 2008ApEnM..74.3619J }}</ref>

The most important commercial sources for alkanes are natural gas and [[Petroleum|oil]].<ref name=m&b/> Natural gas contains primarily methane and ethane, with some [[propane]] and [[butane]]: oil is a mixture of liquid alkanes and other [[hydrocarbons]]. These hydrocarbons were formed when marine animals and plants (zooplankton and phytoplankton) died and sank to the bottom of ancient seas and were covered with sediments in an [[wikt:anoxic|anoxic]] environment and converted over many millions of years at high temperatures and high pressure to their current form. Natural gas resulted thereby for example from the following reaction:
:C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> → 3&nbsp;CH<sub>4</sub> + 3&nbsp;CO<sub>2</sub>

These hydrocarbon deposits, collected in porous rocks trapped beneath impermeable cap rocks, comprise commercial [[oil fields]]. They have formed over millions of years and once exhausted cannot be readily replaced. The depletion of these hydrocarbons reserves is the basis for what is known as the [[energy crisis]].

Alkanes have a low solubility in water, so the content in the oceans is negligible; however, at high pressures and low temperatures (such as at the bottom of the oceans), methane can co-crystallize with water to form a solid [[methane clathrate]] (methane hydrate). Although this cannot be commercially exploited at the present time, the amount of combustible energy of the known methane clathrate fields exceeds the energy content of all the natural gas and oil deposits put together. Methane extracted from methane clathrate is, therefore, a candidate for future fuels.

===Biological occurrence===
[[Image:Rotbuntes Rind.jpg|thumb|right|[[Methanogen]]ic [[archaea]] in the gut of cows produce [[methane]].]]
Aside from petroleum and natural gas, alkanes occur significantly in nature only as methane, which is produced by some [[archaea]] by the process of [[methanogenesis]].  These organisms are found in the gut of termites<ref>{{Cite journal |last1=Buczkowski|first1=Grzegorz|last2=Bertelsmeier|first2=Cleo|date=15 January 2017|title=Invasive termites in a changing climate: A global perspective|journal=Ecology and Evolution |volume=7|issue=3 |pages=974–985|doi=10.1002/ece3.2674|pmc=5288252|pmid=28168033|bibcode=2017EcoEv...7..974B }}</ref> and cows.<ref>{{Cite news |url=https://gizmodo.com/do-cow-farts-actually-contribute-to-global-warming-1562144730|title=Do Cow Farts Actually Contribute to Global Warming?|work=TodayIFoundOut.com|first=Matt|last=Blitz|via=Gizmodo |access-date=11 April 2018|language=en-US}}</ref>  The [[methane]] is produced from [[carbon dioxide]] or other organic compounds. Energy is released by the oxidation of [[hydrogen]]:
:CO<sub>2</sub> + 4&nbsp;H<sub>2</sub> → CH<sub>4</sub> + 2&nbsp;H<sub>2</sub>O
It is probable that our current deposits of natural gas were formed in a similar way.<ref>{{Cite news|url=https://education.nationalgeographic.org/resource/natural-gas|title=Natural Gas|work=Resources Library|publisher=National Geographic Society|access-date=11 April 2018|language=en}}</ref>


Certain types of bacteria can metabolize alkanes: they prefer even-numbered carbon chains as they are easier to degrade than odd-numbered chains.<ref>{{Cite web|url=http://equilibrator.weizmann.ac.il/static/classic_rxns/classic_reactions/fatty_acid_met.html|title=Metabolism of Alkanes and Fatty Acids – eQuilibrator 0.2 beta documentation|website=equilibrator.weizmann.ac.il|language=en|access-date=11 April 2018}}</ref>

Alkanes play a negligible role in higher organisms, with rare exception.
Some yeasts, e.g., ''Candida tropicale'', ''[[Pichia]]'' sp., ''[[Rhodotorula]]'' sp., can use alkanes as a source of carbon or energy. The fungus ''[[Amorphotheca resinae]]'' prefers the longer-chain alkanes in [[aviation fuel]], and can cause serious problems for aircraft in tropical regions.<ref name=Hendey>{{cite journal | last1 = Hendey | first1 = N. I. | year = 1964 | title = Some observations on ''Cladosporium resinae'' as a fuel contaminant and its possible role in the corrosion of aluminium alloy fuel tanks | journal = Transactions of the British Mycological Society | volume = 47 | issue = 7| pages = 467–475 | doi=10.1016/s0007-1536(64)80024-3}}</ref>

In plants, the solid long-chain alkanes are found in the [[plant cuticle]] and [[epicuticular wax]] of many species, but are only rarely major constituents.<ref name=Baker1982>{{cite book |first=E.A.|last=Baker |date=1982 |chapter=Chemistry and morphology of plant epicuticular waxes |pages=139–165 |title=The Plant Cuticle |editor-first=D.F. |editor-last=Cutler |editor2-first=K.L. |editor2-last=Alvin  |editor3-first=C.E. |editor3-last=Price |publisher=Academic Press |isbn=0-12-199920-3}}</ref> They protect the plant against water loss, prevent the [[Leaching (agriculture)|leaching]] of important minerals by the rain, and protect against bacteria, fungi, and harmful insects. The carbon chains in plant alkanes are usually odd-numbered, between 27 and 33 carbon atoms in length,<ref name=Baker1982/> and are made by the plants by [[decarboxylation]] of even-numbered [[fatty acid]]s. The exact composition of the layer of wax is not only species-dependent but also changes with the season and such environmental factors as lighting conditions, temperature or humidity.<ref name=Baker1982/>

The [[Jeffrey pine]] is noted for producing exceptionally high levels of [[Heptane|''n''-heptane]] in its resin, for which reason its distillate was designated as the zero point for one [[octane rating]]. Floral scents have also long been known to contain volatile alkane components, and [[Nonane|''n''-nonane]] is a significant component in the scent of some [[rose]]s.<ref>{{cite journal | last1 = Kim | first1 =HyunJung | last2=Kim | first2=NamSun | last3=Lee | first3=DongSun | year =  2000 | title = Determination of floral fragrances of Rosa hybrida using solid-phase trapping-solvent extraction and gas chromatography–mass spectrometry. | journal = Journal of Chromatography A | volume = 902 | issue = 2| pages =  389–404 | doi = 10.1016/S0021-9673(00)00863-3 | pmid =11192171 }}</ref> Emission of gaseous and volatile alkanes such as [[ethane]], [[pentane]], and [[hexane]] by plants has also been documented at low levels, though they are not generally considered to be a major component of biogenic air pollution.<ref>{{cite journal | last1 = Kesselmeier | first1 = J. | last2 = Staudt | first2 = N. | year = 1999 | title = Biogenic Volatile Organic Compounds (VOC): An Overview on Emission, Physiology and Ecology | url = http://www.geo.uni-frankfurt.de/iau/epos/Gruppenintern/Kesselmeier___Staudt_JAC_1999.pdf | journal = Journal of Atmospheric Chemistry | volume = 33 | issue = 1 | pages = 22–38 | url-status = dead | archive-url = https://www.webcitation.org/6F5FQG2OP?url=http://www.geo.uni-frankfurt.de/iau/epos/Gruppenintern/Kesselmeier___Staudt_JAC_1999.pdf | archive-date = 13 March 2013| doi = 10.1023/A:1006127516791 | bibcode = 1999JAtC...33...23K | s2cid = 94021819 }}</ref>

Edible vegetable oils also typically contain small fractions of biogenic alkanes with a wide spectrum of carbon numbers, mainly 8 to 35, usually peaking in the low to upper 20s, with concentrations up to dozens of milligrams per kilogram (parts per million by weight) and sometimes over a hundred for the total alkane fraction.<ref>{{cite journal | last1 = Moreda
 | first1 =W. | last2=Perez-Camino | first2=M. C. |last3=Cert| first3=A.| year = 2001 | title = Gas and liquid chromatography of hydrocarbons in edible vegetable oils | journal = Journal of Chromatography A | volume = 936 | issue =1–2 | pages = 159–171 | doi=10.1016/s0021-9673(01)01222-5| pmid =11760997 | url=https://www.researchgate.net/publication/11596797}}</ref>

Alkanes are found in animal products, although they are less important than unsaturated hydrocarbons. One example is the shark liver oil, which is approximately 14% [[pristane]] (2,6,10,14-tetramethylpentadecane, C<sub>19</sub>H<sub>40</sub>). They are important as [[pheromone]]s, chemical messenger materials, on which insects depend for communication. In some species, e.g. the support beetle ''[[Xylotrechus colonus]]'', [[pentacosane]] (C<sub>25</sub>H<sub>52</sub>), 3-methylpentaicosane (C<sub>26</sub>H<sub>54</sub>) and 9-methylpentaicosane (C<sub>26</sub>H<sub>54</sub>) are transferred by body contact. With others like the [[tsetse fly]] ''Glossina morsitans morsitans'', the pheromone contains the four alkanes 2-methylheptadecane (C<sub>18</sub>H<sub>38</sub>), 17,21-dimethylheptatriacontane (C<sub>39</sub>H<sub>80</sub>), 15,19-dimethylheptatriacontane (C<sub>39</sub>H<sub>80</sub>) and 15,19,23-trimethylheptatriacontane (C<sub>40</sub>H<sub>82</sub>), and acts by smell over longer distances. [[waggle dance|Waggle-dancing]] [[honey bee]]s produce and release two alkanes, tricosane and pentacosane.<ref>{{cite journal |vauthors=Thom C, Gilley DC, Hooper J, Esch HE |date=21 August 2007 |title=The Scent of the Waggle Dance |journal=PLOS Biology |volume=5 |issue=9| page=e228 |doi=10.1371/journal.pbio.0050228 |pmid=17713987 |pmc=1994260 |doi-access=free }}</ref>

===Ecological relations===
[[Image:Ophrys sphegodes flower.jpg|thumb|upright|right|Early spider orchid (''[[Ophrys sphegodes]]'')]]
One example, in which both plant and animal alkanes play a role, is the ecological relationship between the [[sand bee]] (''[[Andrena nigroaenea]]'') and the [[early spider orchid]] (''[[Ophrys sphegodes]]''); the latter is dependent for [[pollination]] on the former. Sand bees use pheromones in order to identify a mate; in the case of ''A. nigroaenea'', the females emit a mixture of [[tricosane]] (C<sub>23</sub>H<sub>48</sub>), [[pentacosane]] (C<sub>25</sub>H<sub>52</sub>) and [[heptacosane]] (C<sub>27</sub>H<sub>56</sub>) in the ratio 3:3:1, and males are attracted by specifically this odor. The orchid takes advantage of this mating arrangement to get the male bee to collect and disseminate its pollen; parts of its flower not only resemble the appearance of sand bees but also produce large quantities of the three alkanes in the same ratio as female sand bees. As a result, numerous males are lured to the blooms and attempt to copulate with their imaginary partner: although this endeavor is not crowned with success for the bee, it allows the orchid to transfer its pollen,
which will be dispersed after the departure of the frustrated male to other blooms.