Alkane

{{Short description|Type of saturated hydrocarbon compound}} {{Distinguish|alkene|alkyne|alkali|Alkali{{!}}alkaline}} {{Use dmy dates|date=January 2020}}

[[Image:Methane-2D-stereo.svg|right|thumb|Chemical structure of [[methane]], the simplest alkane]]

In [[organic chemistry]], an '''alkane''', or '''paraffin''' (a historical [[trivial name]] that also has [[Paraffin (disambiguation)|other meanings]]<!--intentional link to DAB page-->), is an [[Open-chain compound|acyclic]] [[Saturated and unsaturated compounds|saturated]] [[hydrocarbon]]. In other words, an alkane consists of [[hydrogen]] and [[carbon]] atoms arranged in a [[Tree (graph theory)|tree]] structure in which all the [[carbon–carbon bond]]s are [[Single bond|single]].<ref name="GoldBook alkanes"/> Alkanes have the general chemical formula {{chem2|C_{''n''}H_{2''n''+2} }}. The alkanes range in complexity from the simplest case of [[methane]] ({{chem2|CH4}}), where ''n''&nbsp;=&nbsp;1 (sometimes called the parent molecule), to arbitrarily large and complex molecules, like [[Higher alkanes#Pentapentacontane to hexacontane|hexacontane]] ({{chem2|C60H122}}) or 4-methyl-5-(1-methylethyl) [[octane]], an [[isomer]] of [[dodecane]] ({{chem2|C12H26}}).<ref>{{cite web|url=http://webbook.nist.gov/cgi/cbook.cgi?ID=C62183919 |title=4-methyl-5-(1-methylethyl)octane |publisher=National Institute of Standards and Technology |accessdate=July 7, 2015}}</ref>

The [[International Union of Pure and Applied Chemistry]] (IUPAC) defines alkanes as "acyclic branched or unbranched hydrocarbons having the general formula {{chem2|C_{''n''}H_{2''n''+2} }}, and therefore consisting entirely of hydrogen atoms and saturated carbon atoms". However, some sources use the term to denote ''any'' saturated hydrocarbon, including those that are either monocyclic (i.e. the [[cycloalkane]]s) or [[polycyclic compound|polycyclic]], despite them having a distinct general formula (e.g. cycloalkanes are {{chem2|C_{''n''}H_{2''n''} }}).

In an alkane, each carbon atom is [[Orbital hybridisation|sp³-hybridized]] with 4 [[sigma bond]]s (either C–C or [[Carbon–hydrogen bond|C–H]]), and each hydrogen atom is joined to one of the carbon atoms (in a C–H bond). The longest series of linked carbon atoms in a molecule is known as its [[Skeletal formula|carbon skeleton]] or carbon backbone. The number of carbon atoms may be considered as the size of the alkane.

One group of the [[higher alkanes]] are [[wax]]es, solids at [[Standard temperature and pressure|standard ambient temperature and pressure]] (SATP), for which the number of carbon atoms in the carbon backbone is greater than about 17. With their repeated –{{chem2|CH2}} units, the alkanes constitute a [[homologous series]] of organic compounds in which the members differ in [[molecular mass]] by multiples of 14.03&nbsp;[[Dalton (unit)|u]] (the total mass of each such [[methylene bridge]] unit, which comprises a single carbon atom of mass 12.01&nbsp;u and two hydrogen atoms of mass ~1.01&nbsp;u each).

Methane is produced by [[Methanogen|methanogenic archaea]] and some long-chain alkanes function as pheromones in certain animal species or as protective waxes in plants and fungi. Nevertheless, most alkanes do not have much [[biological activity]]. They can be viewed as molecular trees upon which can be hung the more active/reactive [[functional group]]s of biological molecules.

The alkanes have two main commercial sources: [[petroleum]] (crude oil) and [[natural gas]].

An [[alkyl]] group is an alkane-based molecular fragment that bears one open valence for bonding. They are generally abbreviated with the symbol for any [[organyl group]], R, although Alk is sometimes used to specifically symbolize an alkyl group (as opposed to an alkenyl group or aryl group).

==Structure and classification== Ordinarily, the C–C single bond distance is {{convert|1.53|angstrom|m|lk=in}}.<ref>{{March6th|page=23|mode=cs1}}</ref> Saturated hydrocarbons can be linear, branched, or [[Cyclic compound|cyclic]]. The third group is sometimes called [[cycloalkane]]s.<ref name="GoldBook alkanes">{{GoldBookRef | title=alkanes | file = A00222}}</ref> Very complicated structures are possible by combining linear, branched, cyclic alkanes.

==Isomerism== [[Image:Saturated C4 hydrocarbons ball-and-stick (C4H10 C4H8).png|thumb|upright=1.2|right| C₄ alkanes and cycloalkanes (left to right): [[n-butane|''n''-butane]] and [[isobutane]] are the two C₄H₁₀ isomers; [[cyclobutane]] and [[methylcyclopropane]] are the two C₄H₈ isomers.]] <!--<br/>[[Bicyclobutane|Bicyclo[1.1.0]butane]] is the only C₄H₆ alkane and has no alkane isomer; [[tetrahedrane]] (below) is the only C₄H₄ alkane and so has no alkane isomer.]]--> {{multiple image |total_width=270 |image1=Saturated C4 hydrocarbons ball-and-stick (C4H6).png|caption1=[[Bicyclobutane|Bicyclo[1.1.0]butane]] is the only C₄H₆ alkane and has no alkane isomer. |image2=Tetrahedrane-3D-balls.png|caption2=[[Tetrahedrane]] is the only C₄H₄ alkane and also has no alkane isomer. }}

Alkanes with more than three [[carbon]] atoms can be arranged in various ways, forming [[structural isomer]]s. The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the ''n''-isomer (''n'' for "normal", although it is not necessarily the most common). However, the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms. For example, for acyclic alkanes:<ref>[[On-Line Encyclopedia of Integer Sequences]] {{OEIS|id=A000602}} Number of n-node unrooted quartic trees; number of n-carbon alkanes C(n)H(2n+2) ignoring stereoisomers</ref> * C₁: [[methane]] only * C₂: [[ethane]] only * C₃: [[propane]] only * C₄: 2 isomers: ''n''-[[butane]] and [[isobutane]] (''i''-butane) * C₅: 3 isomers: ''n''-[[pentane]], [[isopentane]], and [[neopentane]] * C₆: 5 isomers: ''n''-[[hexane]], [[2-Methylpentane|2-methylpentane]] (isohexane), [[3-Methylpentane|3-methylpentane]], [[2,2-Dimethylbutane|2,2-dimethylbutane]] (neohexane), and [[2,3-Dimethylbutane|2,3-dimethylbutane]] * C₇: 9 isomers: ''n''-[[heptane]], [[2-methylhexane]] (isoheptane), [[3-methylhexane]], [[2,2-dimethylpentane]] (neoheptane), [[2,3-dimethylpentane]], [[2,4-dimethylpentane]], [[3,3-dimethylpentane]], [[3-ethylpentane]], [[2,2,3-trimethylbutane]] *C₈: 18 isomers: [[octane]], [[2-methylheptane]], [[3-methylheptane]], [[4-methylheptane]], [[2,2-dimethylhexane]], [[2,3-dimethylhexane]], [[2,4-dimethylhexane]], [[2,5-dimethylhexane]], [[3,3-dimethylhexane]], [[3,4-dimethylhexane]], [[3-ethylhexane]], [[2,2,3-trimethylpentane]], [[2,2,4-trimethylpentane]], [[2,3,3-trimethylpentane]], [[2,3,4-trimethylpentane]], [[3-ethyl-2-methylpentane]], [[3-ethyl-3-methylpentane]], [[2,2,3,3-tetramethylbutane]] * C₉: [[list of isomers of nonane|35 isomers]] of nonane * C₁₀: [[list of isomers of decane|75 isomers]] of decane * C₁₁: [[list of isomers of undecane|159 isomers]] of undecane * C₁₂: [[list of isomers of dodecane|355 isomers]] of dodecane * C₂₀: 366,319 isomers of [[eicosane]] (icosane) * C₃₀: 4,111,846,763 isomers of [[triacontane]] * C₄₀: 62,481,801,147,341 isomers of [[tetracontane]] * C₅₀: 1,117,743,651,746,953,270 isomers of [[pentacontane]] * C₆₀: 22,158,734,535,770,411,074,184 isomers of [[hexacontane]]

Branched alkanes can be [[chirality (chemistry)|chiral]]. For example, [[3-methylhexane]] and its higher [[Homologous series|homologues]] are chiral due to their [[stereogenic center]] at carbon atom number 3. The above list only includes differences of connectivity, not stereochemistry. In addition to the alkane isomers, the chain of carbon atoms may form one or more rings. Such compounds are called [[cycloalkane]]s, and are also excluded from the above list because changing the number of rings changes the [[molecular formula]]. For example, [[cyclobutane]] and [[methylcyclopropane]] are isomers of each other (C₄H₈), but are not isomers of butane (C₄H₁₀).

Branched alkanes are more thermodynamically stable than their linear (or less branched) isomers. For example, the highly branched 2,2,3,3-tetramethylbutane is about 1.9 kcal/mol more stable than its linear isomer, ''n''-octane.<ref>{{Cite book|title=Stereoelectronic effects : a bridge between structure and reactivity|last=Alabugin |first=Igor V. |isbn=978-1-118-90637-8|publisher=Wiley |oclc=957525299|year=2016}}</ref>

==Nomenclature== {{Main|IUPAC nomenclature of organic chemistry}} The [[IUPAC nomenclature of organic chemistry#Alkanes|IUPAC nomenclature]] (systematic way of naming compounds) for alkanes is based on identifying hydrocarbon chains. Unbranched, saturated hydrocarbon chains are named systematically with a Greek numerical prefix denoting the number of carbons and the suffix "-ane".<ref>{{cite book | chapter-url = http://www.acdlabs.com/iupac/nomenclature/93/r93_184.htm | access-date = 12 February 2007 | chapter = R-2.2.1: Hydrocarbons | author = IUPAC, Commission on Nomenclature of Organic Chemistry | title = A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993) | year = 1993 | publisher = Blackwell Scientific | isbn = 978-0-632-03488-8}}</ref>

In 1866, [[August Wilhelm von Hofmann]] suggested systematizing nomenclature by using the whole sequence of vowels a, e, i, o and u to create suffixes -ane, -ene, -ine (or -yne), -one, -une, for the [[hydrocarbons]] C_''n''H_2''n''+2, C_''n''H_2''n'', C_''n''H_2''n''−2, C_''n''H_2''n''−4, C_''n''H_2''n''−6.<ref>{{Cite web|url=http://www.chem.yale.edu/~chem125/125/history99/5Valence/Nomenclature/alkanenames.html|archiveurl=https://web.archive.org/web/20120202091842/http://www.chem.yale.edu/~chem125/125/history99/5Valence/Nomenclature/alkanenames.html|url-status=dead|title=Alkane Nomenclature<!-- Bot generated title -->|archivedate=2 February 2012}}</ref> In modern nomenclature, the first three specifically name hydrocarbons with single, double and triple bonds;<ref>Thus, the ending "-diene" is applied in some cases where von Hofmann had "-ine"</ref> while "-one" now represents a [[ketone]].

===Linear alkanes=== {{further|List of straight-chain alkanes}} <!-- redirected from [[linear paraffins]] --> Straight-chain alkanes are sometimes indicated by the prefix ''n''- (for "normal") where a non-linear [[isomer]] exists. Although this is not strictly necessary and is not part of the IUPAC naming system, the usage is still common in cases where one wishes to emphasize or distinguish between the straight-chain and branched-chain isomers, e.g., "[[Butane|''n''-butane]]" rather than simply "butane" to differentiate it from [[isobutane]]. Alternative names for this group used in the petroleum industry are '''linear paraffins''' or '''''n''-paraffins'''.

The first eight members of the series (in terms of number of carbon atoms) are named as follows: ; [[methane]]: CH₄ – one carbon and 4 hydrogen ; [[ethane]] : C₂H₆ – two carbon and 6 hydrogen ; [[propane]]: C₃H₈ – three carbon and 8 hydrogen ; [[butane]] : C₄H₁₀ – four carbon and 10 hydrogen ; [[pentane]]: C₅H₁₂ – five carbon and 12 hydrogen ; [[hexane]] : C₆H₁₄ – six carbon and 14 hydrogen ; [[heptane]]: C₇H₁₆ – seven carbons and 16 hydrogen ; [[octane]]: C₈H₁₈ – eight carbons and 18 hydrogen

The first four names were [[Back-formation|derived]] from [[methanol]], [[Diethyl ether|ether]], [[propionic acid]] and [[butyric acid]]. Alkanes with five or more carbon atoms are named by adding the [[Affix|suffix]] '''-ane''' to the appropriate [[IUPAC numerical multiplier|numerical multiplier]] prefix<ref name=reusch-nom>{{cite web | first = William |last=Reusch | work = Virtual Textbook of Organic Chemistry | title = Nomenclature – Alkanes | url = http://www.cem.msu.edu/~reusch/VirtualText/nomen1.htm | access-date = 5 April 2007 | archive-url = http://arquivo.pt/wayback/20160521131930/http://www.cem.msu.edu/~reusch/VirtualText/nomen1.htm | archive-date = 21 May 2016 | url-status = dead}}</ref> with [[elision]] of any terminal vowel (''-a'' or ''-o'') from the basic numerical term. Hence, [[pentane]], C₅H₁₂; [[hexane]], C₆H₁₄; [[heptane]], C₇H₁₆; [[octane]], C₈H₁₈; etc. The [[numeral prefix]] is generally Greek; however, alkanes with a carbon atom count ending in nine, for example [[nonane]], use the [[Latin language|Latin]] prefix '''non-'''.

===Branched alkanes=== [[Image:Isopentane-numbered-3D-balls.png|thumb|right|[[Ball-and-stick model]] of [[isopentane]] (common name) or 2-methylbutane (IUPAC systematic name)]] Simple branched alkanes often have a common name using a prefix to distinguish them from linear alkanes, for example ''n''-butane, isobutane (or ''i''-butane) for the two isomers of butane and ''n''-pentane, isopentane, neopentane for the three isomers of pentane.

IUPAC naming conventions can be used to produce a systematic name.

The key steps in the naming of more complicated branched alkanes are as follows:<ref>{{cite web | first = William |last=Reusch | work = Virtual Textbook of Organic Chemistry | title = Examples of the IUPAC Rules in Practice | url = http://www.cem.msu.edu/~reusch/VirtualText/nomexmp1.htm | access-date = 5 April 2007 | archive-url = http://arquivo.pt/wayback/20160521132003/http://www.cem.msu.edu/~reusch/VirtualText/nomexmp1.htm | archive-date = 21 May 2016 | url-status = dead}}</ref> * Identify the longest continuous chain of carbon atoms. * Name this longest root chain using standard naming rules. * Name each side chain by changing the suffix of the name of the alkane from "-ane" to "-yl". * Number the longest continuous chain in order to give the lowest possible numbers for the side-chains.<ref>{{Cite web|url=http://www.chem.uiuc.edu/GenChemReferences/nomenclature_rules.html|title=IUPAC Rules|website=www.chem.uiuc.edu|access-date=13 August 2018}}</ref> * Number and name the side chains before the name of the root chain. * If there are multiple side chains of the same type, use prefixes such as "di-" and "tri-" to indicate it as such, and number each one. * Add side chain names in alphabetical (disregarding "di-" etc. prefixes) order in front of the name of the root chain.

{| class="wikitable" style="text-align:center" |+ Comparison of nomenclatures for three isomers of C₅H₁₂ |- ! Common name ! ''n''-pentane || isopentane || neopentane |- ! IUPAC name ! pentane || 2-methylbutane || 2,2-dimethylpropane |- ! Structure | [[Image:Pentane-2D-Skeletal.svg|120px]] || [[Image:Isopentane-2D-skeletal.svg|90px]] || [[Image:Neopentane-2D-skeletal.svg|70px]] |}

===Saturated cyclic hydrocarbons=== {{Main|Cycloalkane}}

Though technically distinct from the alkanes, this class of hydrocarbons is referred to by some as the "cyclic alkanes." As their description implies, they contain one or more rings.

Simple cycloalkanes have a prefix "cyclo-" to distinguish them from alkanes. Cycloalkanes are named as per their acyclic counterparts with respect to the number of carbon atoms in their backbones, e.g., [[cyclopentane]] (C₅H₁₀) is a cycloalkane with 5 carbon atoms just like [[pentane]] (C₅H₁₂), but they are joined up in a five-membered ring. In a similar manner, [[propane]] and [[cyclopropane]], [[butane]] and [[cyclobutane]], etc.

Substituted cycloalkanes are named similarly to substituted alkanes – the cycloalkane ring is stated, and the substituents are according to their position on the ring, with the numbering decided by the [[Cahn–Ingold–Prelog priority rules]].<ref name=reusch-nom/>

===Trivial/common names=== {{main|List of straight-chain alkanes}} The trivial (non-[[IUPAC nomenclature|systematic]]) name for alkanes is "paraffins". Together, alkanes are known as the "paraffin series". Trivial names for compounds are usually historical artifacts. They were coined before the development of systematic names, and have been retained due to familiar usage in industry. Cycloalkanes are also called naphthenes.<ref>{{Cite web|title=Definition of CYCLOALKANES|url=https://www.merriam-webster.com/dictionary/cycloalkanes|access-date=2021-06-26|website=www.merriam-webster.com|language=en}}</ref><ref>{{Cite web|title=Definition of NAPHTHENES|url=https://www.merriam-webster.com/dictionary/naphthenes|access-date=2021-06-26|website=www.merriam-webster.com|language=en}}</ref>

Branched-chain alkanes are called '''isoparaffins'''. "Paraffin" is a general term and often does not distinguish between pure compounds and mixtures of [[isomer]]s, i.e., compounds of the same [[chemical formula]], e.g., [[pentane]] and [[isopentane]].

;In IUPAC The following trivial names are retained in the IUPAC system: * [[isobutane]] for 2-methylpropane * [[isopentane]] for 2-methylbutane * [[neopentane]] for 2,2-dimethylpropane.

;Non-IUPAC Some non-IUPAC trivial names are occasionally used: * cetane, for [[hexadecane]] * cerane, for [[hexacosane]]<ref>{{cite book |first=Donald |last=Mackay |title=Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals |date=14 March 2006 |isbn=1-4200-4439-7 |page=206|publisher=CRC Press }}</ref>

==Physical properties== {{see also|Higher alkane|List of straight-chain alkanes}} All alkanes are colorless.<ref>{{cite web|url=http://nsdl.niscair.res.in/bitstream/123456789/777/1/Revised+organic+chemistry.pdf |title=Pharmaceutical Chemistry |access-date=17 February 2014 |url-status=dead |archive-url=https://web.archive.org/web/20131029192647/http://nsdl.niscair.res.in/bitstream/123456789/777/1/Revised%2Borganic%2Bchemistry.pdf |archive-date=29 October 2013 }}</ref><ref>{{cite web|url=http://textbook.s-anand.net/ncert/class-11/chemistry/13-hydrocarbons |archive-url=https://web.archive.org/web/20110508081631/http://textbook.s-anand.net/ncert/class-11/chemistry/13-hydrocarbons |url-status=dead |archive-date=8 May 2011 |title=13. Hydrocarbons {{pipe}} Textbooks |publisher=textbook.s-anand.net |access-date=3 October 2014 }}</ref> Alkanes with the lowest molecular weights are gases, those of intermediate molecular weight are liquids, and the heaviest are waxy solids.<ref>{{Cite web|title=Molecule Gallery - Alkanes|url=https://www.angelo.edu/faculty/kboudrea/molecule_gallery/01_alkanes/00_alkanes.htm|access-date=2021-12-06|website=www.angelo.edu|archive-date=19 March 2022|archive-url=https://web.archive.org/web/20220319042437/https://www.angelo.edu/faculty/kboudrea/molecule_gallery/01_alkanes/00_alkanes.htm|url-status=dead}}</ref><ref>{{Cite book|url=https://search.credoreference.com/content/entry/andidsci/alkanes_paraffins/|title=Illustrated Dictionary of Science, Andromeda|publisher=Windmill Books (Andromeda International)|year=1988|editor-last=Allaby|editor-first=Michael|chapter=Alkanes (paraffins)|archive-date=11 December 2023|access-date=13 July 2021|archive-url=https://web.archive.org/web/20231211132627/https://search.credoreference.com/content/entry/andidsci/alkanes_paraffins|url-status=dead}}</ref>

===Table of alkanes===

{| class="wikitable" |- !Alkane !Formula !Boiling point<ref group="note" name="prop">Physical properties of the straight-chain isomer</ref><br>[°C] !Melting point<ref group="note" name="prop"/><br>[°C] !Density<ref group="note" name="prop"/><br>[kg/m³] (at 20&nbsp;°C) !Isomers<ref group="note" name="isomer">Total number of [[constitutional isomer]]s for this molecular formula</ref> |- |[[Methane]] |CH₄ | −162 | −182 | 0.656 (gas) | 1 |- |[[Ethane]] |C₂H₆ | −89 | −183 | 1.26 (gas) | 1 |- |[[Propane]] |C₃H₈ | −42 | −188 | 2.01 (gas) | 1 |- |[[Butane]] |C₄H₁₀ | 0 | −138 | 2.48 (gas) | 2 |- |[[Pentane]] |C₅H₁₂ | 36 | −130 | 626 (liquid) | 3 |- |[[Hexane]] |C₆H₁₄ | 69 | −95 | 659 (liquid) | 5 |- |[[Heptane]] |C₇H₁₆ | 98 | −91 | 684 (liquid) | 9 |- |[[Octane]] |C₈H₁₈ | 126 | −57 | 703 (liquid) | 18 |- |[[Nonane]] |C₉H₂₀ | 151 | −54 | 718 (liquid) | 35 |- |[[Decane]] |C₁₀H₂₂ | 174 | −30 | 730 (liquid) | 75 |- |[[Undecane]] |C₁₁H₂₄ | 196 | −26 | 740 (liquid) | 159 |- |[[Dodecane]] |C₁₂H₂₆ | 216 | −10 | 749 (liquid) | 355 |- |[[Tridecane]] |C₁₃H₂₈ | 235 | −5.4 | 756 (liquid) | 802 |- |[[Tetradecane]] |C₁₄H₃₀ | 253 | 5.9 | 763 (liquid) | 1858 |- |[[Pentadecane]] |C₁₅H₃₂ | 270 | 10 | 769 (liquid) | 4347 |- |[[Hexadecane]] |C₁₆H₃₄ | 287 | 18 | 773 (liquid) | 10,359 |- |[[Heptadecane]] |C₁₇H₃₆ | 303 | 22 | 777 (solid) | 24,894 |- |[[Octadecane]] |C₁₈H₃₈ | 317 | 28 | 781 (solid) | 60,523 |- |[[Nonadecane]] |C₁₉H₄₀ | 330 | 32 | 785 (solid) | 148,284 |- |[[Eicosane]] |C₂₀H₄₂ | 343 | 37 | 789 (solid) | 366,319 |- |[[Triacontane]] |C₃₀H₆₂ | ≈450 | 66 | 810 (solid) | 4,111,846,763 |- |[[Tetracontane]] |C₄₀H₈₂ | ≈525 | 82 | 817 (solid) | 62,481,801,147,341 |- |[[Pentacontane]] |C₅₀H₁₀₂ | ≈575 | 91 | 824 (solid) | ~ 1.1×10¹⁸ |- |[[Hexacontane]] |C₆₀H₁₂₂ | ≈625 | 100 | 829 (solid) | ~ 2.2×10²² |- |Heptacontane |C₇₀H₁₄₂ | ? | ? | ? (solid) | ~ 4.7×10²⁶ |- |Octacontane |C₈₀H₁₆₂ | ? | ? | ? (solid) | ~ 1.1×10³¹ |- |Nonacontane |C₉₀H₁₈₂ | ? | ? | ? (solid) | ~ 2.5×10³⁵ |- |Hectane |C₁₀₀H₂₀₂ | ? | ? | ? (solid) | ~ 5.9×10³⁹ |- |colspan=6 | {{reflist|group=note}} |}

===Boiling points=== [[Image:AlkaneBoilingMeltingPoint.png|right|thumb|upright=1.9|Melting (blue) and boiling (orange) points of the first 16 ''n''-alkanes in °C.]]

Alkanes experiences intermolecular [[van der Waals force]]s. The cumulative effects of these intermolecular forces give rise to greater boiling points of alkanes.<ref name=m&b>{{cite book|title = Organic Chemistry |author1=R. T. Morrison |author2=R. N. Boyd | isbn = 978-0-13-643669-0 | publisher = Prentice Hall | edition = 6th|year = 1992}}</ref>

Two factors influence the strength of the van der Waals forces: * the number of electrons surrounding the [[molecule]], which increases with the alkane's molecular weight * the surface area of the molecule

Under [[standard conditions]], from CH₄ to C₄H₁₀ alkanes are gaseous; from C₅H₁₂ to C₁₇H₃₆ they are liquids; and after C₁₈H₃₈ they are solids. As the boiling point of alkanes is primarily determined by weight, it should not be a surprise that the boiling point has an almost linear relationship with the size ([[molecular weight]]) of the molecule. As a rule of thumb, the boiling point rises 20–30&nbsp;°C for each carbon added to the chain; this rule applies to other homologous series.<ref name = m&b/>

A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact, and thus greater van der Waals forces, between adjacent molecules. For example, compare isobutane (2-methylpropane) and ''n''-butane, which boil at −12 and 0&nbsp;°C, and 2,2-dimethylbutane and 2,3-dimethylbutane which boil at 50 and 58&nbsp;°C, respectively.<ref name = m&b/>

On the other hand, cycloalkanes tend to have higher boiling points than their linear counterparts due to the locked conformations of the molecules, which give a plane of intermolecular contact.

===Melting points=== The [[melting point]]s of the alkanes follow a similar trend to [[boiling points]] for the same reason as outlined above. That is, (all other things being equal) the larger the molecule the higher the melting point. However, alkanes' melting points follow a more complex pattern, due to variations in the properties of their solid crystals.{{efn|For visualizations of the low-temperature crystal structures of alkanes (methane to nonane), see {{cite web |title=Solid methane |work=<!--Visualization of Molecules and Crystal Structures--> |url=https://log-web.de/chemie/Start.htm?name=methaneCryst&lang=en}}}}

One difference in crystal structure that even-numbered alkanes (from hexane onwards) tend to form denser-packed crystals compared to their odd-numbered neighbors. This causes them to have a greater [[enthalpy of fusion]] (amount of energy required to melt them), raising their melting point.<ref>{{cite journal | last1=Boese | first1=Roland | last2=Weiss | first2=Hans-Christoph | last3=Bläser | first3=Dieter | title=The Melting Point Alternation in the Short-Chain ''n''-Alkanes: Single-Crystal X-Ray Analyses of Propane at 30&nbsp;K and of ''n''-Butane to ''n''-Nonane at 90&nbsp;K | journal=Angewandte Chemie International Edition | volume=38 | issue=7 | date=1999-04-01 | issn=1433-7851 | doi=10.1002/(SICI)1521-3773(19990401)38:7<988::AID-ANIE988>3.0.CO;2-0 | pages=988–992| pmid=29711877 }}</ref> A second difference in crystal structure is that even-numbered alkanes (from octane onwards) tend to form more rotationally ordered crystals compared to their odd-numbered neighbors. This causes them to have a greater [[entropy of fusion]] (increase in disorder from the solid to the liquid state), lowering their melting point.<ref name=Brown2000>{{cite journal |last1=Brown |first1=RJC |last2=Brown |first2=RFC |title=Melting Point and Molecular Symmetry |journal=Journal of Chemical Education |date=June 2000 |volume=77 |issue=6 |pages=724 |doi=10.1021/ed077p724|bibcode=2000JChEd..77..724B }}</ref>

While these effects operate in opposing directions, the first effect tends to be slightly stronger, leading even-numbered alkanes to have slightly higher melting points than the average of their odd-numbered neighbors.

This trend does not apply to methane, which has an unusually high melting point, higher than both ethane and propane. This is because it has a very low entropy of fusion, attributable to its high molecular symmetry and the rotational disorder in solid methane near its melting point ([[Methane#Solid methane|Methane I]]).<ref name=Brown2000/>

The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, again depending on these two factors. More symmetric alkanes tend towards higher melting points, due to enthalpic effects when they form ordered crystals, and entropic effects when they form disordered crystals (e.g. [[Neopentane#Boiling and melting points|neopentane]]).<ref name=Brown2000/>

===Conductivity and solubility=== Alkanes do not conduct electricity in any way, nor are they substantially [[Relative static permittivity|polarized]] by an [[electric field]]. For this reason, they do not form [[hydrogen bond]]s and are insoluble in polar solvents such as water. Since the hydrogen bonds between individual water molecules are aligned away from an alkane molecule, the coexistence of an alkane and water leads to an increase in molecular order (a reduction in [[entropy]]). As there is no significant bonding between water molecules and alkane molecules, the [[second law of thermodynamics]] suggests that this reduction in entropy should be minimized by minimizing the contact between alkane and water: Alkanes are said to be [[Hydrophobe|hydrophobic]] as they are insoluble in water.

Their solubility in nonpolar solvents is relatively high, a property that is called [[lipophilicity]]. Alkanes are, for example, miscible in all proportions among themselves.

The density of the alkanes usually increases with the number of carbon atoms but remains less than that of water. Hence, alkanes form the upper layer in an alkane–water mixture.<ref>{{Cite book|chapter-url=https://www.sciencedirect.com/science/article/pii/B9780128024447000033|chapter=Alkanes and Cycloalkanes|date=2015-01-01|publisher=Elsevier|isbn=978-0-12-802444-7 |doi=10.1016/b978-0-12-802444-7.00003-3|title=Principles of Organic Chemistry|last1=Ouellette|first1=Robert J.|last2=Rawn|first2=J. David|pages=65–94}}</ref>

===Molecular geometry===<!-- This section is linked from [[Nylon]] --> [[Image:Ch4 hybridization.svg|thumb|upright|right|sp³-hybridization in methane.]] The molecular structure of the alkanes directly affects their physical and chemical characteristics. It is derived from the [[electron configuration]] of [[carbon]], which has four [[valence electron]]s. The carbon atoms in alkanes are described as sp³ hybrids; that is to say that, to a good approximation, the valence electrons are in orbitals directed towards the corners of a tetrahedron which are derived from the combination of the 2s orbital and the three 2p orbitals. Geometrically, the angle between the bonds are cos⁻¹(−{{sfrac|3}})&nbsp;≈&nbsp;109.47°. This is exact for the case of methane, while larger alkanes containing a combination of C–H and C–C bonds generally have bonds that are within several degrees of this idealized value.

===Bond lengths and bond angles=== [[Image:CH4-structure.svg|thumb|upright|right|The tetrahedral structure of methane.]] An alkane has only C–H and C–C single bonds. The former result from the overlap of an sp³ orbital of carbon with the 1s orbital of a hydrogen; the latter by the overlap of two sp³ orbitals on adjacent carbon atoms. The [[bond length]]s amount to 1.09&nbsp;×&nbsp;10⁻¹⁰&nbsp;m for a C–H bond and 1.54&nbsp;×&nbsp;10⁻¹⁰&nbsp;m for a C–C bond.

The spatial arrangement of the bonds is similar to that of the four sp³ orbitals—they are tetrahedrally arranged, with an angle of 109.47° between them. Structural formulae that represent the bonds as being at right angles to one another, while both common and useful, do not accurately depict the geometry.

===Conformation=== {{Main|Alkane stereochemistry}} [[Image:Newman projection ethane.png|thumb|right|Newman projections of two of many conformations of ethane: eclipsed on the left, staggered on the right.]] [[Image:Ethane-rotamers-3D-balls.png|thumb|right|[[Ball-and-stick model]]s of the two rotamers of ethane]]

The spatial arrangement of the C-C and C-H bonds are described by the torsion angles of the molecule, known as its [[conformational isomerism|conformation]]. In [[ethane]], the simplest case for studying the conformation of alkanes, there is nearly free rotation about a carbon–carbon single bond. Two limiting conformations are important: [[eclipsed conformation]] and [[staggered conformation]]. The staggered conformation is 12.6&nbsp;kJ/mol (3.0 kcal/mol) lower in energy (more stable) than the eclipsed conformation (the least stable). In highly branched alkanes, the bond angle may differ from the optimal value (109.5°) to accommodate bulky groups. Such distortions introduce a tension in the molecule, known as [[steric hindrance]] or strain. Strain substantially increases reactivity.<ref>{{March6th|page=195}}</ref>

===Spectroscopic properties=== Spectroscopic signatures for alkanes are obtainable by the major characterization techniques.<ref>{{cite book|title=Spectrometric Identification of Organic Compounds |edition=8th |first1=Robert M.|last1=Silverstein|first2=Francis X.|last2=Webster|first3=David J. |last3=Kiemle|first4=David L.|last4=Bryce|publisher=Wiley|isbn= 978-0-470-61637-6|year=2016}}</ref>

====Infrared spectroscopy==== The C-H stretching mode gives strong absorptions between 2850 and 2960&nbsp;[[Wavenumber|cm⁻¹]] and weaker bands for the C-C stretching mode absorbs between 800 and 1300&nbsp;cm⁻¹. The carbon–hydrogen bending modes depend on the nature of the group: methyl groups show bands at 1450&nbsp;cm⁻¹ and 1375&nbsp;cm⁻¹, while methylene groups show bands at 1465&nbsp;cm⁻¹ and 1450&nbsp;cm⁻¹.<ref>{{cite web |title=Dodecane: IR Spectrum |url=https://webbook.nist.gov/cgi/cbook.cgi?ID=C112403&Mask=80#IR-Spec |work=NIST Chemistry WebBook |id=SRD 69}}</ref> Carbon chains with more than four carbon atoms show a weak absorption at around 725&nbsp;cm⁻¹.

====NMR spectroscopy==== The proton resonances of alkanes are usually found at [[chemical shift|''δ''_H]] = 0.5–1.5. The carbon-13 resonances depend on the number of hydrogen atoms attached to the carbon: ''δ''_C = 8–30 (primary, methyl, –CH₃), 15–55 (secondary, methylene, –CH₂–), 20–60 (tertiary, methyne, C–H) and quaternary. The carbon-13 resonance of quaternary carbon atoms is characteristically weak, due to the lack of [[nuclear Overhauser effect]] and the long [[relaxation time]], and can be missed in weak samples, or samples that have not been run for a sufficiently long time.

====Mass spectrometry==== Since alkanes have high [[ionization energy|ionization energies]], their [[Electron ionization|electron impact mass spectra]] show weak currents for their molecular ions. The fragmentation pattern can be difficult to interpret, but in the case of branched chain alkanes, the carbon chain is preferentially cleaved at tertiary or quaternary carbons due to the relative stability of the resulting [[free radical]]s. The mass spectra for straight-chain alkanes is illustrated by that for [[dodecane]]: the fragment resulting from the loss of a single methyl group (''M''&nbsp;−&nbsp;15) is absent, fragments are more intense than the molecular ion and are spaced by intervals of 14 mass units, corresponding to loss of CH₂ groups.<ref>{{cite web |title=Dodecane |url=https://webbook.nist.gov/cgi/cbook.cgi?ID=C112403&Mask=200 |work=NIST Chemistry WebBook |id=SRD 69 }}</ref>

==Chemical properties== Alkanes are only weakly reactive with most chemical compounds. They only react with the strongest of electrophilic reagents by virtue of their strong C–H bonds (~100 kcal/mol) and C–C bonds (~90 kcal/mol). They are also relatively unreactive toward free radicals. This inertness is the source of the term ''paraffins'' (with the meaning here of "lacking affinity"). In [[crude oil]] the alkane molecules have remained chemically unchanged for millions of years.

===Acid-base behavior=== The [[acid dissociation constant]] (p''K''_a) values of all alkanes are estimated to range from 50 to 70, depending on the extrapolation method, hence they are extremely weak acids that are practically inert to bases (see: [[carbon acid]]s). They are also extremely weak bases, undergoing no observable protonation in pure [[sulfuric acid]] (''H''₀ ~ −12), although [[superacid]]s that are at least millions of times stronger have been known to protonate them to give hypercoordinate alkanium ions (see: [[Methanium|methanium ion]]). Thus, a mixture of [[antimony pentafluoride]] (SbF₅) and [[fluorosulfonic acid]] (HSO₃F), called [[magic acid]], can protonate alkanes.<ref>{{cite journal | author-link = George A. Olah |first1=G.A. |last1=Olah |last2=Schlosberg |first2=R.H. | title = Chemistry in Super Acids. I. Hydrogen Exchange and Polycondensation of Methane and Alkanes in FSO₃H–SbF₅ ("Magic Acid") Solution. Protonation of Alkanes and the Intermediacy of CH₅⁺ and Related Hydrocarbon Ions. The High Chemical Reactivity of "Paraffins" in Ionic Solution Reactions | journal = Journal of the American Chemical Society | year = 1968 | volume = 90 | pages = 2726–7 | doi = 10.1021/ja01012a066 | issue = 10 }}</ref>

===Reactions with oxygen (combustion reaction)=== All alkanes react with [[oxygen]] in a [[combustion]] reaction, although they become increasingly difficult to ignite as the number of carbon atoms increases. The general equation for complete combustion is: :C_''n''H_2''n''+2 + ({{sfrac|3|2}}''n''&nbsp;+&nbsp;{{sfrac|2}})&nbsp;O₂ → (''n''&nbsp;+&nbsp;1)&nbsp;H₂O + ''n''&nbsp;CO₂ :or C_''n''H_2''n''+2 + ({{sfrac|3''n'' + 1|2}})&nbsp;O₂ → (''n''&nbsp;+&nbsp;1)&nbsp;H₂O + ''n''&nbsp;CO₂

In the absence of sufficient oxygen, [[carbon monoxide]] or even [[soot]] can be formed, as shown below:

:C_''n''H_2''n''+2 + (''n''&nbsp;+&nbsp;{{sfrac|2}})&nbsp;O₂ → (''n''&nbsp;+&nbsp;1)&nbsp;H₂O + ''n''&nbsp;CO

:C_''n''H_2''n''+2 + ({{sfrac|2}}''n''&nbsp;+&nbsp;{{sfrac|2}})&nbsp;O₂ → (''n''&nbsp;+&nbsp;1)&nbsp;H₂O + ''n''&nbsp;C

For example, [[methane]]: :2&nbsp;CH₄ + 3&nbsp;O₂ → 4&nbsp;H₂O + 2&nbsp;CO :CH₄ + O₂ → 2&nbsp;H₂O + C

See the [[Standard enthalpy change of formation (data table)#Alkanes|alkane heat of formation table]] for detailed data. The [[standard enthalpy change of combustion]], Δ_c''H''^⊖, for alkanes increases by about 650&nbsp;kJ/mol per CH₂ group. Branched-chain alkanes have lower values of Δ_c''H''^⊖ than straight-chain alkanes of the same number of carbon atoms, and so can be seen to be somewhat more stable.

===Biodegradation=== Some organisms are capable of metabolizing alkanes.<ref>{{Cite journal|doi=10.3389/fmicb.2013.00058 |doi-access=free |title=Structural insights into diversity and n-alkane biodegradation mechanisms of alkane hydroxylases |date=2013 |last1=Ji |first1=Yurui |last2=Mao |first2=Guannan |last3=Wang |first3=Yingying |last4=Bartlam |first4=Mark |journal=Frontiers in Microbiology |volume=4 |page=58 |pmid=23519435 |pmc=3604635 }}</ref><ref>{{Cite journal| doi=10.1264/jsme2.ME14090| issn=1342-6311| volume=30| issue=1| pages=70–75| last1=Dashti| first1=Narjes| last2=Ali| first2=Nedaa| last3=Eliyas| first3=Mohamed| last4=Khanafer| first4=Majida| last5=Sorkhoh| first5=Naser A.| last6=Radwan| first6=Samir S.| title=Most Hydrocarbonoclastic Bacteria in the Total Environment are Diazotrophic, which Highlights Their Value in the Bioremediation of Hydrocarbon Contaminants| journal=Microbes and Environments| date=March 2015| pmid=25740314| pmc=4356466}}</ref> The [[methane monooxygenase]]s convert methane to [[methanol]]. For higher alkanes, [[cytochrome P450]] convert alkanes to alcohols, which are then susceptible to degradation.

=== Free radical reactions === [[Free radical]]s, molecules with unpaired electrons, play a large role in most reactions of alkanes. [[Free radical halogenation]] reactions occur with halogens, leading to the production of [[haloalkanes]]. The hydrogen atoms of the alkane are progressively replaced by halogen atoms. The reaction of alkanes and fluorine is highly [[exothermic reaction|exothermic]] and can lead to an explosion.<ref>{{Ullmann |doi=10.1002/14356007.a11_349 |chapter=Fluorine Compounds, Organic |last1=Siegemund |first1=Günter |last2=Schwertfeger |first2=Werner |last3=Feiring |first3=Andrew |last4=Smart |first4=Bruce |last5=Behr |first5=Fred |last6=Vogel |first6=Herward |last7=McKusick |first7=Blaine }}</ref> These reactions are an important industrial route to halogenated hydrocarbons. There are three steps: * '''Initiation''' the halogen radicals form by [[homolysis (chemistry)|homolysis]]. Usually, energy in the form of heat or light is required. * '''Chain reaction''' or '''Propagation''' then takes place—the halogen radical abstracts a hydrogen from the alkane to give an alkyl radical. This reacts further. * '''Chain termination''' where the radicals recombine. Experiments have shown that all halogenation produces a mixture of all possible isomers, indicating that all hydrogen atoms are susceptible to reaction. The mixture produced, however, is not statistical: Secondary and tertiary hydrogen atoms are preferentially replaced due to the greater stability of secondary and tertiary free-radicals. An example can be seen in the monobromination of propane:<ref name = m&b/> [[Image:Monobromination of propane.png|500px|center|Monobromination of [[propane]]]]

In the [[Reed reaction]], [[sulfur dioxide]] and [[chlorine]] convert hydrocarbons to [[sulfonyl chloride]]s under the influence of [[photochemistry|light]].

Under some conditions, alkanes will undergo [[nitration]].

===C–H activation=== Certain transition metal complexes promote non-radical reactions with alkanes, resulting in [[C–H bond activation]] reactions.<ref>{{cite journal |doi=10.1021/acs.chemrev.3c00207 |title=Transition-Metal-Catalyzed Silylation and Borylation of C–H Bonds for the Synthesis and Functionalization of Complex Molecules |date=2023 |last1=Yu |first1=Isaac F. |last2=Wilson |first2=Jake W. |last3=Hartwig |first3=John F. |journal=Chemical Reviews |volume=123 |issue=19 |pages=11619–63 |pmid=37751601 |s2cid=263150991 }}</ref>

=== Cracking === {{Main|Cracking (chemistry)}} Cracking breaks larger molecules into smaller ones. This reaction requires heat and catalysts. The thermal cracking process follows a [[homolysis (chemistry)|homolytic]] mechanism with formation of [[Radical (chemistry)|free radicals]]. The catalytic cracking process involves the presence of [[acid]] [[catalyst]]s (usually solid acids such as [[silica-alumina]] and [[zeolite]]s), which promote a [[heterolytic cleavage|heterolytic]] (asymmetric) breakage of bonds yielding pairs of ions of opposite charges, usually a [[carbocation]]. Carbon-localized free radicals and cations are both highly unstable and undergo processes of chain rearrangement, C–C scission in position [[beta scission|beta]] (i.e., cracking) and [[Intramolecular reaction|intra-]] and [[intermolecular]] hydrogen transfer or [[hydride]] transfer. In both types of processes, the corresponding [[reactive intermediate]]s (radicals, ions) are permanently regenerated, and thus they proceed by a self-propagating chain mechanism. The chain of reactions is eventually terminated by radical or ion recombination.{{Citation needed|date=January 2021}}

=== Isomerization and reformation === Dragan and his colleague were the first to report about isomerization in alkanes.<ref name="Asinger, Friedrich 1967">{{cite book |last= Asinger |first=Friedrich | title = Paraffins; Chemistry and Technology | url = https://archive.org/details/paraffinschemist0000asin | url-access = registration | publisher = Pergamon Press | date = 1967 |oclc=556032}}</ref> Isomerization and reformation are processes in which straight-chain alkanes are heated in the presence of a [[platinum]] catalyst. In isomerization, the alkanes become branched-chain isomers. In other words, it does not lose any carbons or hydrogens, keeping the same molecular weight.<ref name="Asinger, Friedrich 1967"/> In reformation, the alkanes become [[cycloalkane]]s or [[aromatic hydrocarbon]]s, giving off hydrogen as a by-product. Both of these processes raise the [[octane number]] of the substance. Butane is the most common alkane that is put under the process of isomerization, as it makes many branched alkanes with high octane numbers.<ref name="Asinger, Friedrich 1967"/>

===Other reactions=== In [[steam reforming]], alkanes react with [[steam]] in the presence of a [[nickel]] [[catalyst]] to give [[hydrogen]] and carbon monoxide.

==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 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 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: :C₆H₁₂O₆ → 3&nbsp;CH₄ + 3&nbsp;CO₂

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]]: :CO₂ + 4&nbsp;H₂ → CH₄ + 2&nbsp;H₂O 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 important as [[pheromone]]s, chemical messenger materials, on which insects depend for communication. 7-methyltricosane and 9-methyltricosane are active for [[ladybird]] beetles (''Adalia bipunctata'').<ref>{{cite journal |doi=10.1016/s0022-1910(98)00081-x |title=Mate recognition in the two-spot ladybird beetle, Adalia bipunctata: Role of chemical and behavioural cues |date=1998 |last1=Hemptinne† |first1=J.-L |last2=Lognay |first2=G. |last3=Dixon |first3=A.F.G |journal=Journal of Insect Physiology |volume=44 |issue=12 |pages=1163–1171 |pmid=12770316 |bibcode=1998JInsP..44.1163H }}</ref> The [[emerald ash borer]] (''Agrilus planipennis Fairmaire'') responds to 9-methylpentacosane.<ref>{{cite journal |doi=10.1007/s00114-009-0513-1 |title=A contact sex pheromone component of the emerald ash borer Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) |date=2009 |last1=Silk |first1=Peter J. |last2=Ryall |first2=Krista |last3=Barry Lyons |first3=D. |last4=Sweeney |first4=Jon |last5=Wu |first5=Junping |journal=Naturwissenschaften |volume=96 |issue=5 |pages=601–608 |pmid=19238346 |bibcode=2009NW.....96..601S }}</ref> Female [[Asian long-horned beetle]]s ''Anoplophora glabripennis'', which are very damaging, secrete 2-methyldocosane.<ref>{{cite journal |doi=10.1007/s10886-014-0385-5 |title=Sex-Specific Trail Pheromone Mediates Complex Mate Finding Behavior in Anoplophora glabripennis |date=2014 |last1=Hoover |first1=Kelli |last2=Keena |first2=Melody |last3=Nehme |first3=Maya |last4=Wang |first4=Shifa |last5=Meng |first5=Peter |last6=Zhang |first6=Aijun |journal=Journal of Chemical Ecology |volume=40 |issue=2 |pages=169–180 |pmid=24510414 |bibcode=2014JCEco..40..169H }}</ref> [[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]] (C₂₃H₄₈), [[pentacosane]] (C₂₅H₅₂) and [[heptacosane]] (C₂₇H₅₆) 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.

==Production== ===Petroleum refining=== [[Image:ShellMartinez-refi.jpg|thumb|right|An [[oil refinery]] at [[Martinez, California]].]] The most important source of alkanes is natural gas and [[crude oil]].<ref name=m&b/> Alkanes are separated in an [[oil refinery]] by [[fractional distillation]]. Unsaturated hydrocarbons are converted to alkanes by [[hydrogenation]]:<ref name=Ull/> : {{chem2|RCH\dCH2 + H2 -> RCH2\sCH3}} (R = [[alkyl]])

Another route to alkanes is [[hydrogenolysis]], which entails cleavage of C-heteroatom bonds using hydrogen. In industry, the main substrates are organonitrogen and organosulfur impurities, i.e. the heteroatoms are N and S. The specific processes are called [[hydrodenitrification]] and [[hydrodesulfurization]]: :{{chem2|R3N + 3 H2 -> 3 RH + H3N}} :{{chem2|R2S + 2 H2 -> 2 RH + H2S}} Hydrogenolysis can be applied to the conversion of virtually any functional group into hydrocarbons. Substrates include haloalkanes, alcohols, aldehydes, ketones, carboxylic acids, etc. Both hydrogenolysis and hydrogenation are practiced in refineries. They can be effected by using [[lithium aluminium hydride]], [[Clemmenson reduction]] and other specialized routes.

===Coal=== Coal is a more traditional precursor to alkanes. A wide range of technologies have been intensively practiced for centuries.<ref name=Ull>{{Ullmann |doi=10.1002/14356007.a13_227.pub3 |chapter=Hydrocarbons |last1=Schmidt |first1=Roland |last2=Griesbaum |first2=Karl |last3=Behr |first3=Arno |last4=Biedenkapp |first4=Dieter |last5=Voges |first5=Heinz-Werner |last6=Garbe |first6=Dorothea |last7=Paetz |first7=Christian |last8=Collin |first8=Gerd |last9=Mayer |first9=Dieter |last10=Höke |first10=Hartmut }}</ref> Simply heating coal gives alkanes, leaving behind [[Coke (fuel)|coke]]. Relevant technologies include the [[Bergius process]] and [[coal liquefaction]]. Partial combustion of coal and related solid organic compounds generates [[carbon monoxide]], which can be hydrogenated using the [[Fischer–Tropsch process]]. This technology allows the synthesis of liquid hydrocarbons, including alkanes. This method is used to produce substitutes for [[petroleum distillate]]s.

===Laboratory preparation=== Rarely is there any interest in the synthesis of alkanes, since they are usually commercially available and less valued than virtually any precursor. The best-known method is [[hydrogenation]] of [[alkene]]s. Many C−X bonds can be converted to C−H bonds using [[lithium aluminium hydride]], [[Clemmenson reduction]], and other specialized routes.<ref>{{March6th|page=1790}}</ref> Hydrolysis of alkyl [[Grignard reagent]]s and alkyl [[organolithium reagent]]s gives alkanes.<ref>{{cite journal |doi=10.15227/orgsyn.011.0084 |first=C.R. |last=Noller|title=n-Pentane |journal=Organic Syntheses |date=1931 |volume=11 |page=84 }}</ref>

==Applications== ===Fuels=== The dominant use of alkanes is as fuels. [[Propane]] and [[butane]], easily liquified gases, are commonly known as [[liquified petroleum gas]] (LPG).<ref>{{Cite web |url=http://www.ferrellgas.com/Resource_/PageResource/LA_Transit_Case_Study.pdf |title=Using propane as a fuel |access-date=27 November 2012 |archive-url=https://web.archive.org/web/20131012000108/http://ferrellgas.com/Resource_/PageResource/LA_Transit_Case_Study.pdf |archive-date=12 October 2013 |url-status=dead }}</ref> From [[pentane]] to [[octane]] the alkanes are highly volatile liquids. They are used as fuels in [[internal combustion engine]]s, as they vaporize easily on entry into the combustion chamber without forming droplets, which would impair the uniformity of the combustion. Branched-chain alkanes are preferred as they are much less prone to premature ignition, which causes [[Engine knocking|knocking]], than their straight-chain homologues. This propensity to premature ignition is measured by the [[octane rating]] of the fuel, where [[2,2,4-Trimethylpentane|2,2,4-trimethylpentane]] (''isooctane'') has an arbitrary value of 100, and [[heptane]] has a value of zero. Apart from their use as fuels, the middle alkanes are also good [[solvent]]s for nonpolar substances. Alkanes from [[nonane]] to, for instance, [[hexadecane]] (an alkane with sixteen carbon atoms) are liquids of higher [[viscosity]], less and less suitable for use in gasoline. They form instead the major part of [[Diesel fuel|diesel]] and [[aviation fuel]]. Diesel fuels are characterized by their [[cetane number]], cetane being an old name for hexadecane. However, the higher melting points of these alkanes can cause problems at low temperatures and in polar regions, where the fuel becomes too thick to flow correctly.

===Precursors to chemicals=== By the process of [[Cracking (chemistry)|cracking]], alkanes can be converted to [[alkene]]s. Simple alkenes are precursors to polymers, such as [[polyethylene]] and [[polypropylene]]. When the cracking is taken to extremes, alkanes can be converted to [[carbon black]], which is a significant tire component.

Chlorination of methane gives chloromethanes, which are used as solvents and building blocks for complex compounds. Similarly treatment of methane with sulfur gives [[carbon disulfide]]. Still other chemicals are prepared by reaction with [[sulfur trioxide]] and [[nitric oxide]].

===Other=== Some light hydrocarbons are used as [[aerosol spray]]s.

Alkanes from hexadecane upwards form the most important components of [[fuel oil]] and [[lubricating oil]]. In the latter function, they work at the same time as anti-corrosive agents, as their hydrophobic nature means that water cannot reach the metal surface. Many solid alkanes find use as [[paraffin wax]], for example, in [[candle]]s. This should not be confused however with true [[wax]], which consists primarily of [[ester]]s.

Alkanes with a chain length of approximately 35 or more carbon atoms are found in [[bitumen]], used, for example, in road surfacing. However, the higher alkanes have little value and are usually split into lower alkanes by [[Cracking (chemistry)|cracking]].

==Hazards== {{See also|Hydrocarbon poisoning}} Alkanes are highly flammable, but they have low toxicities. Methane "is toxicologically virtually inert." Alkanes can be asphyxiants and narcotic.<ref name=Ull/>

==See also== {{Commons}} {{Wiktionary}} * [[Alkene]] * [[Alkyne]] * [[Cycloalkane]] * [[Higher alkane]] * [[Aliphatic compound]]

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==Notes== {{notelist}}

==References== {{Reflist}}

==Further reading== * [https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/intro1.htm Virtual Textbook of Organic Chemistry] * [https://log-web.de/chemie/Start.htm?name=ethaneCryst&lang=en Visualizations of the low-temperature crystal structures of alkanes (methane to nonane)] * {{cite EB1911 |wstitle=Paraffin |volume=20 |pages=752–756 |first=Boverton |last=Redwood |short=1}}

{{Hydrocarbons}} {{Alkanes}} {{Functional Groups}} {{BranchesofChemistry}} {{Hydrides by group}}

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[[Category:Alkanes| ]] [[Category:Hydrocarbons]]