Alkane 639 224595044 2008-07-09T16:01:33Z 122.161.57.88 /* Melting point */ [[Image:Methane-2D-stereo.svg|right|thumb|Chemical structure of [[methane]], the simplest alkane]] '''Alkanes''', also known as [[paraffin]]s, are [[chemical compound]]s that consist only of the elements [[carbon]] (C) and [[hydrogen]] (H) (i.e., [[hydrocarbon]]s), wherein these atoms are linked together exclusively by [[single bond]]s (i.e., they are [[Saturation (chemistry)|saturated compounds]]) without any cyclic structure (i.e. loops). Alkanes belong to a [[homologous series]] of organic compounds in which the members differ by a constant relative atomic mass of 14. Each carbon atom must have 4 bonds (either C-H or C-C bonds), and each hydrogen atom must be joined to a carbon atom (H-C bonds). A series of linked carbon atoms is known as the carbon skeleton or carbon backbone. In general, the number of carbon atoms is often used to define the size of the alkane (e.g., C<sub>2</sub>-alkane). An [[alkyl]] group is a [[functional group]] or side-chain that, like an alkane, consists solely of singly-bonded carbon and hydrogen atoms, for example a [[methyl]] or [[ethyl group]]. Saturated hydrocarbons can be linear (general formula '''C<sub>''n''</sub>H<sub>2''n''+2</sub>''') wherein the carbon atoms are joined in a snake-like structure, [[Branching (chemistry)|branched]] (general formula '''C<sub>''n''</sub>H<sub>2''n''+2</sub>, ''n''>3''') wherein the carbon backbone splits off in one or more directions, or [[Cyclic compound|cyclic]] (general formula '''C<sub>''n''</sub>H<sub>2''n''</sub>, ''n''>2''') wherein the carbon backbone is linked so as to form a loop. According to the definition by [[International Union of Pure and Applied Chemistry|IUPAC]], the former two are alkanes, whereas the third group is called [[cycloalkane]]s.<ref>{{GoldBookRef | title=alkanes | file = A00222 | date = 1995}}</ref> In other words, saturated hydrocarbons are divided into alkanes and cycloalkanes, depending on whether or not they have cyclic structures, and, in the technical sense, cycloalkanes are ''not'' alkanes. However, cycloalkanes are sometimes called ''cyclic alkanes'', which can be confusing when "real" alkanes are called ''acyclic alkanes''. Saturated hydrocarbons can also combine any of the linear, cyclic (e.g., polycyclic) and branching structures, and they are still alkanes (no general formula) as long as they are [[acyclic]] (i.e., having no loops). The simplest possible alkane (the parent molecule) is [[methane]], CH<sub>4</sub>. There is no limit to the number of carbon atoms that can be linked together, the only limitation being that the molecule is acyclic, is [[Saturation (chemistry)|saturated]], and is a [[hydrocarbon]]. Saturated [[oils]] and [[waxes]] are examples of larger alkanes where the number of carbons in the carbon backbone tends to be greater than 10. Alkanes are not very reactive and have little [[biological activity]]. Alkanes can be viewed as a molecular [[scaffold]] upon which can be hung the interesting biologically-active/reactive portions ([[functional groups]]) of the molecule. ==Isomerism== [[Image:Saturated C4 hydrocarbons ball-and-stick.png|thumb|right| Different C<sub>4</sub>-alkanes and -cycloalkanes (left to right): [[n-butane|''n''-butane]] and [[isobutane]] are the two C<sub>4</sub>H<sub>10</sub> isomers; [[cyclobutane]] and [[methylcyclopropane]] are the two C<sub>4</sub>H<sub>8</sub> isomers; bicyclo[1.1.0]butane is the only C<sub>4</sub>H<sub>6</sub> isomer; [[tetrahedrane]] (not shown) is the only C<sub>4</sub>H<sub>4</sub> isomer.]] Alkanes with more than three carbon atoms can be arranged in a multiple number of ways, forming different [[structural isomer]]s. An isomer is like a chemical [[anagram]], in which the atoms of a [[chemical compound]] are arranged or joined together in a different order. 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 {{OEIS|id=A000602}}. For example: * C<sub>1</sub>: 1 isomer&mdash;[[methane]] * C<sub>2</sub>: 1 isomer&mdash;[[ethane]] * C<sub>3</sub>: 1 isomer&mdash;[[propane]] * C<sub>4</sub>: 2 isomers&mdash;[[n-butane|''n''-butane]], [[isobutane]] * C<sub>12</sub>: 355 isomers * C<sub>32</sub>: 27,711,253,769 isomers * C<sub>60</sub>: 22,158,734,535,770,411,074,184 isomers, many of which are not stable. In addition to these isomers, the chain of carbon atoms may form one or more loops. Such compounds are called [[cycloalkane]]s. ==Nomenclature== {{main|Organic nomenclature}} 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 | url = http://www.acdlabs.com/iupac/nomenclature/93/r93_184.htm | accessdate = 2007-02-12 | 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}}</ref> [[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.{{Fact|date=April 2007}} The first three name hydrocarbons with single, double and triple bonds; "-one" represents a [[ketone]]; "-ol" represents an alcohol or OH group; "-oxy-" means an [[ether]] and refers to oxygen between two carbons, so that methoxy-methane is the IUPAC name for dimethyl ether. It is difficult or impossible to find compounds with more than one IUPAC name. This is because shorter chains attached to longer chains are prefixes and the convention includes brackets. Numbers in the name, referring to which carbon a group is attached to, should be as low as possible, so that 1- is implied and usually omitted from names of organic compounds with only one side-group; "1-" is implied in Nitro-octane. Symmetric compounds will have two ways of arriving at the same name. ===Linear alkanes=== Straight-chain alkanes are sometimes indicated by the prefix ''n-'' (for ''normal'') where a non-linear [[isomer]] exists. Although this is not strictly necessary, the usage is still common in cases where there is an important difference in properties between the straight-chain and branched-chain isomers, e.g., [[hexane|''n''-hexane]] or 2- or 3-methylpentane. The first four members of the series (in terms of number of carbon atoms) are named as follows: :[[methane]], CH<sub>4</sub> :[[ethane]], C<sub>2</sub>H<sub>6</sub> :[[propane]], C<sub>3</sub>H<sub>8</sub> :[[butane]], C<sub>4</sub>H<sub>10</sub> Alkanes with five or more carbon atoms are named by adding the [[Affix|suffix]] '''-ane''' to the appropriate Greek-language prefix [[IUPAC numerical multiplier|numerical multiplier]]<ref name = reusch-nom>{{cite web | author = William Reusch | work = Virtual Textbook of Organic Chemistry | title = Nomenclature - Alkanes | url = http://www.cem.msu.edu/~reusch/VirtualText/nomen1.htm}}</ref> with elision of any terminal vowel (''-a'' or ''-o'') from the basic numerical term. Hence, [[pentane]], C<sub>5</sub>H<sub>12</sub>; [[hexane]], C<sub>6</sub>H<sub>14</sub>; [[heptane]], C<sub>7</sub>H<sub>16</sub>; [[octane]], C<sub>8</sub>H<sub>18</sub>; etc. For a more complete list, see [[List of alkanes]]. ===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 [[pentane|''n''-pentane]], [[isopentane]], and [[neopentane]]. 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 | author = William 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}}</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 root chain so that sum of the numbers assigned to each side group will be as low as possible * 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. {| class="wikitable" style="text-align:center" |+ Comparison of nomenclatures for three isomers of C<sub>5</sub>H<sub>12</sub> |- ! 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.png|90px]] || [[Image:Neopentane-2D-skeletal.png|70px]] |} ===Cyclic alkanes=== {{main|Cycloalkane}} So-called cyclic alkanes are, in the technical sense, ''not'' alkanes, but cycloalkanes. They are hydrocarbons just like alkanes, but are containing 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, e.g., [[cyclopentane]] (C<sub>5</sub>H<sub>10</sub>) is a cycloalkane with 5 carbon atoms just like [[pentane]] (C<sub>5</sub>H<sub>12</sub>), 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 similar to substituted alkanes — the cycloalkane ring is stated, and the substituents are according to their position on the ring, with the numbering decided by [[Cahn-Ingold-Prelog rules]].<ref name=reusch-nom/> ===Trivial names=== The trivial (non-[[IUPAC nomenclature|systematic]]) name for alkanes is "[[paraffin]]s." 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. It is almost certain that the term [[paraffin]] stems from the petrochemical industry. Branched-chain alkanes are called ''[[isoparaffins]]''. The use of the term "paraffin" is a general term and often does not distinguish between a pure compounds and mixtures of [[isomers]] with the same [[chemical formula]] (i.e., like a chemical [[anagram]]), e.g., [[pentane]] and [[isopentane]]. ;Examples The following trivial names are retained in the IUPAC system: * [[isobutane]] for 2-methylpropane * [[isopentane]] for 2-methylbutane * [[neopentane]] for 2,2-dimethylpropane ==Occurrence== ===Occurrence of alkanes in the Universe=== [[Image:Jupiter.jpg|thumb|right|Methane and ethane make up a large proportion of Jupiter's atmosphere]] Alkanes form a significant portion of the [[Celestial body atmosphere|atmospheres]] of the outer gas planets such as [[Jupiter]] (0.1% methane, 0.0002% ethane), [[Saturn]] (0.2% methane, 0.0005% ethane), [[Uranus]] (1.99% methane, 0.00025% ethane) and [[Neptune]] (1.5% methane, 1.5 ppm ethane). [[Titan (moon)|Titan]] (1.6% methane), a satellite of Saturn, was examined by the ''Huygens'' probe, which indicate that Titan's atmosphere periodically rains liquid methane onto the moon's surface.<ref>[http://www.planetary.org/news/2005/huygens_science-results_0121.html Titan: Arizona in an Icebox?], Emily Lakdawalla, [[2004-01-21]], verified [[2005-03-28]]</ref> Also on Titan, a methane-spewing volcano was spotted and this volcanism is believed to be a significant source of the methane in the atmosphere. There also appear to be Methane/Ethane lakes near the north polar regions of Titan, as discovered by Cassini's radar imaging. [[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 name="science">{{cite journal | author=Mumma, M.J. | co-authors = Disanti, M.A., dello Russo, N., Fomenkova, M., Magee-Sauer, K., Kaminski, C.D., and D.X. Xie | 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 | pages=1310 | doi=10.1126/science.272.5266.1310 | pmid=8650540 }}</ref> Alkanes have also been detected in [[meteorite]]s such as [[carbonaceous chondrite]]s. ===Occurrence of alkanes on Earth=== Traces of methane gas (about 0.0001% or 1 ppm) occur in the Earth's atmosphere, produced primarily by organisms such as [[Archaea]], found for example in the gut of cows.{{Fact|date=April 2007}} [[Image:Oil well.jpg|thumb|right|Extraction of oil, which contains many different hydrocarbons including alkanes]] 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 dead 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> → 3CH<sub>4</sub> + 3CO<sub>2</sub> These hydrocarbons collected in porous rocks, located beneath an impermeable cap rock and so are trapped. Unlike methane, which is constantly reformed in large quantities, higher alkanes (alkanes with 9 or more carbon atoms) rarely develop to a considerable extent in nature.{{Fact|date=April 2007}} These deposits, e.g., [[oil fields]], have formed over millions of years and once exhausted cannot be readily replaced. The depletion of these hydrocarbons is the basis for what is known as the [[energy crisis]]. Solid alkanes are known as [[tar]]s and are formed when more volatile alkanes such as gases and oil [[evaporation|evaporate]] from hydrocarbon deposits. One of the largest natural deposits of solid alkanes is in the [[asphalt]] lake known as the [[Pitch Lake]] in [[Trinidad and Tobago]]. Methane is also present in what is called [[biogas]], produced by animals and decaying matter, which is a possible [[renewable energy source]]. 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 hydrate]].{{Fact|date=April 2007}} Although this cannot be commercially exploited at the present time, the amount of combustible energy of the known methane hydrate fields exceeds the energy content of all the natural gas and oil deposits put together{{Fact|date=February 2007}};methane extracted from methane hydrate is considered therefore a candidate for future fuels. ===Biological occurrence=== Although alkanes occur in nature in various way, they do not rank biologically among the essential materials. Cycloalkanes with 14 to 18 carbon atoms occur in [[musk]], extracted from [[deer]] of the family [[Moschidae]].{{Fact|date=April 2007}} All further information refers to (acyclic) alkanes. ;Bacteria and archaea [[Image:Rotbuntes Rind.jpg|thumb|right|[[Methanogen]]ic [[archaea]] in the gut of this cow are responsible for some of the [[methane]] in Earth's atmosphere.]] Certain types of [[bacteria]] can metabolise alkanes: they prefer even-numbered carbon chains as they are easier to degrade than odd-numbered chains.{{Fact|date=April 2007}} On the other hand, certain [[archaea]], the [[methanogen]]s, produce large quantities of [[methane]] by the metabolism of [[carbon dioxide]] or other [[oxidation|oxidised]] organic compounds. The energy is released by the oxidation of [[hydrogen]]: :CO<sub>2</sub> + 4H<sub>2</sub> → CH<sub>4</sub> + 2H<sub>2</sub>O Methanogens are also the producers of [[marsh gas]] in [[wetlands]], and release about two billion tonnes of methane per year{{Fact|date=April 2007}}&mdash;the atmospheric content of this gas is produced nearly exclusively by them. The methane output of [[cattle]] and other [[herbivore]]s, which can release up to 150&nbsp;litres per day,{{Fact|date=April 2007}} and of [[termite]]s,{{Fact|date=April 2007}} is also due to methanogens. They also produce this simplest of all alkanes in the [[intestine]]s of humans. Methanogenic archaea are, hence, at the end of the [[carbon cycle]], with carbon being released back into the atmosphere after having been fixed by [[photosynthesis]]. It is probable that our current deposits of [[natural gas]] were formed in a similar way.{{Fact|date=April 2007}} ;Fungi and plants [[Image:Fuji apple.jpg|thumb|right|Water forms droplets on a thin film of alkane wax on the skin of the apple.]] Alkanes also play a role, if a minor role, in the biology of the three [[eukaryote|eukaryotic]] groups of organisms: [[Fungus|fungi]], plants and animals. Some specialised yeasts, e.g., ''Candida tropicale'', ''[[Pichia]]'' sp., ''[[Rhodotorula]]'' sp., can use alkanes as a source of carbon and/or energy. The fungus ''[[Amorphotheca resinae]]'' prefers the longer-chain alkanes in [[aviation fuel]], and can cause serious problems for aircraft in tropical regions.{{Fact|date=April 2007}} In plants, it is the solid long-chain alkanes that are found; they form a firm layer of wax, the [[Plant cuticle|cuticle]], over areas of the plant exposed to the air. This protects the plant against water loss, while preventing the [[leaching]] of important minerals by the rain. It is also a protection against bacteria, fungi, and harmful insects&mdash;the latter sink with their legs into the soft waxlike substance and have difficulty moving. The shining layer on fruits such as apples consists of long-chain alkanes. The carbon chains are usually between twenty and thirty carbon atoms in length and are made by the plants from [[fatty acid]]s. The exact composition of the layer of wax is not only species-dependent, but changes also with the season and such environmental factors as lighting conditions, temperature or humidity. ;Animals 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>).{{Fact|date=April 2007}} Their occurrence is more important in [[pheromone]]s, chemical messenger materials, on which above all insects are dependent for communication. With some kinds, as the support beetle ''[[Xylotrechus colonus]]'', primarily [[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>), they 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, a useful characteristic for [[pest control]].{{Fact|date=April 2007}} ===Ecological relations=== [[Image:Ophrys sphegodes flower.jpg|thumb|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 odour. 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 endeavour 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 different blooms. ==Production== ===Petroleum refining=== [[Image:ShellMartinez-refi.jpg|thumb|right|An [[oil refinery]] at [[Martinez]], [[California]].]] As stated earlier, 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]] and processed into many different products ===Fischer-Tropsch=== The [[Fischer-Tropsch process]] is a method to synthesize liquid hydrocarbons, including alkanes, from [[carbon monoxide]] and hydrogen. This method is used to produce substitutes for [[petroleum distillates]]. ===Laboratory preparation=== There is usually little need for alkanes to be synthesized in the laboratory, since they are usually commercially available. Also, alkanes are generally non-reactive chemically or biologically, and do not undergo functional group interconversions cleanly. When alkanes are produced in the laboratory, it is often a side-product of a reaction. For example, the use of [[n-butyllithium]] as a strong [[base (chemistry)|base]] gives the conjugate acid, n-butane as a side-product: : C<sub>4</sub>H<sub>9</sub>Li + H<sub>2</sub>O → C<sub>4</sub>H<sub>10</sub> + [[lithium hydroxide|LiOH]] However, at times it may be desirable to make a portion of a molecule into an alkane like functionality ([[alkyl]] group) using the above or similar methods. For example, an [[ethyl group]] is an alkyl group; when this is attached to a [[hydroxy]] group, it gives [[ethanol]], which is not an alkane. To do so, the best-known methods are [[hydrogenation]] of [[alkene]]s: :RCH=CH<sub>2</sub> + H<sub>2</sub> → RCH<sub>2</sub>CH<sub>3</sub> (R = [[alkyl]]) Alkanes or alkyl groups can also be prepared directly from [[alkyl halide]]s in the [[Corey-House-Posner-Whitesides reaction]]. The [[Barton-McCombie deoxygenation]]<ref>[[Derek Harold Richard Barton|Barton, D. H. R.]]; McCombie, S. W. ''J. Chem. Soc., Perkin Trans. 1'' '''1975''', ''16'', 1574-1585</ref><ref>Crich, D.; Quintero, L. ''[[Chem. Rev.]]'' '''1989''', ''89'', 1413-1432.</ref> removes hydroxyl groups from alcohols e.g. :[[Image:Barton-McCombie Deoxygenation Scheme.png|400px]] and the [[Clemmensen reduction]]<ref>Martin, E. L. ''Org. React.'' '''1942''', ''1'', 155. (Review)</ref><ref>Buchanan, J. G. St. C.; Woodgate, P. D. ''Quart. Rev.'' '''1969''', ''23'', 522. (Review)</ref><ref>Vedejs, E. ''Org. React.'' '''1975''', ''22'', 401. (Review)</ref><ref>Yamamura, S.; Nishiyama, S. ''Comp. Org. Syn.'' '''1991''', ''8'', 309-313.(Review)</ref> removes carbonyl groups from aldehydes and ketones to form alkanes or alkyl-substituted compounds e.g.: :[[Image:Clemmensen Reduction Scheme.png|250px]] ==Applications== The applications of a certain alkane can be determined quite well according to the number of carbon atoms. The first four alkanes are used mainly for heating and cooking purposes, and in some countries for electricity generation. [[Methane]] and [[ethane]] are the main components of natural gas; they are normally stored as gases under pressure. It is, however, easier to transport them as liquids: This requires both compression and cooling of the gas. [[Propane]] and [[butane]] can be liquefied at fairly low pressures, and are well known as [[liquified petroleum gas]] (LPG). Propane, for example, is used in the propane gas burner, butane in disposable cigarette lighters. The two alkanes are used as propellants in [[aerosol spray]]s. From [[pentane]] to [[octane]] the alkanes are reasonably volatile liquids. They are used as fuels in [[internal combustion engine]]s, as they vaporise 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 homologue. This propensity to premature ignition is measured by the [[octane rating]] of the fuel, where [[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]] and [[aviation fuel]]. Diesel fuels are characterised 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. Alkanes from hexadecane upwards form the most important components of [[fuel oil]] and [[lubricating oil]]. In 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]]. Some synthetic [[polymers]] such as [[polyethylene]] and [[polypropylene]] are alkanes with chains containing hundreds of thousands of carbon atoms. These materials are used in innumerable applications, and billions of kilograms of these materials are made and used each year. ==Physical properties== ====Boiling point==== [[Image:Alkanschmelzundsiedepunkt.png|right|thumb|300px|Melting (blue) and boiling (pink) points of the first 14 ''n''-alkanes in °C.]] Alkanes experience inter-molecular [[van der Waals force]]s. Stronger inter-molecular van der Waals forces give rise to greater boiling points of alkanes.<ref name=m&b>{{cite book|title = Organic Chemistry | author = R. T. Morrison, R. N. Boyd | isbn = 0-13-643669-2 | publisher = Prentice Hall | location = New Jersey | edition = 6th}}</ref> There are two determinants for 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 CH<sub>4</sub> to C<sub>4</sub>H<sub>10</sub> alkanes are gaseous; from C<sub>5</sub>H<sub>12</sub> to C<sub>17</sub>H<sub>36</sub> they are liquids; and after C<sub>18</sub>H<sub>38</sub> 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 almost a linear relationship with the size ([[molecular weight]]) of the molecule. As a rule of thumb, the boiling point rises 20 - 30 °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, thus the greater van der Waals forces, between adjacent molecules. For example, compare [[isobutane]] and [[n-butane]], which boil at -12 and 0 °C, and 2,2-dimethylbutane and 2,3-dimethylbutane which boil at 50 and 58 °C, respectively.<ref name = m&b/> For the latter case, two molecules 2,3-dimethylbutane can "lock" into each other better than the cross-shaped 2,2-dimethylbutane, hence the greater van der Waals forces. 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.{{Fact|date=April 2007}} ====Melting point==== 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. There is one significant difference between boiling points and melting points. Solids have more ridged and fixed structure than liquids. This rigid structure requires energy to break down. Thus the stronger better put together solid structures will require more energy to break apart. For alkanes, this can be seen from the graph above (i.e., the blue line). The odd-numbered alkanes have a lower trend in melting points than even numbered alkanes. This is because even numbered alkanes pack well in the solid phase, forming a well-organised structure, which requires more energy to break apart. The odd-number alkanes pack less well and so the "looser" organised solid packing structure requires less energy to break apart.<ref>{{cite journal | author=Boese R, Weiss HC, Blaser D | title= The melting point alternation in the short-chain n-alkanes: Single-crystal X-ray analyses of propane at 30 K and of n-butane to n-nonane at 90 K | journal= Angew Chemie Int Ed | year=1999 | volume=38 | pages=988–992 |doi=10.1002/(SICI)1521-3773(19990401)38:7<988::AID-ANIE988>3.3.CO;2-S}}</ref> The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, again depending on the ability of the alkane in question to packing well in the solid phase: This is particularly true for isoalkanes (2-methyl isomers), which often have melting points higher than those of the linear analogues. ====Conductivity==== Alkanes do not conduct [[electricity]], nor are they substantially [[Polarization|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 minimised by minimising the contact between alkane and water: Alkanes are said to be [[hydrophobic]] in that they repel water. Their solubility in nonpolar solvents is relatively good, a property that is called [[lipophilicity]]. Different alkanes are, for example, miscible in all proportions among themselves. The density of the alkanes usually increases with increasing number of carbon atoms, but remains less than that of water. Hence, alkanes form the upper layer in an alkane-water mixture. ===Molecular geometry===<!-- This section is linked from [[Nylon]] --> [[Image:Ch4 hybridization.svg|thumb|right|sp<sup>3</sup>-hybridisation 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 always [[Orbital hybridisation|sp<sup>3</sup> hybridised]], that is to say that the valence electrons are said to be in four equivalent orbitals derived from the combination of the 2s orbital and the three 2p orbitals. These orbitals, which have identical energies, are arranged spatially in the form of a tetrahedron, the angle of cos<sup>−1</sup>(−⅓) ≈ 109.47° between them. ===Bond lengths and bond angles=== An alkane molecule has only C &ndash; H and C &ndash; C single bonds. The former result from the overlap of a sp³-orbital of carbon with the 1s-orbital of a hydrogen; the latter by the overlap of two sp³-orbitals on different carbon atoms. The [[bond length]]s amount to 1.09×10<sup>−10</sup>&nbsp;m for a C &ndash; H bond and 1.54×10<sup>−10</sup>&nbsp;m for a C &ndash; C bond. [[Image:Ch4-structure.png|thumb|right|The tetrahedral structure of methane.]] The spatial arrangement of the bonds is similar to that of the four sp³-orbitals&mdash;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 correspond with the reality. ===Conformation=== {{Main|Alkane stereochemistry}} The structural formula and the [[bond angle]]s are not usually sufficient to completely describe the geometry of a molecule. There is a further [[degrees of freedom (physics and chemistry)|degree of freedom]] for each carbon &ndash; carbon bond: the [[torsion angle]] between the atoms or groups bound to the atoms at each end of the bond. The spatial arrangement described by the torsion angles of the molecule is known as its [[conformation]]. [[Image:Newman projection ethane.png|thumb|right|200px|Newman projections of the two conformations of ethane: eclipsed on the left, staggered on the right.]] [[Image:Ethane-rotamers-3D-balls.png|thumb|right|200px|[[Ball-and-stick model]]s of the two rotamers of ethane]] [[Ethane]] forms the simplest case for studying the conformation of alkanes, as there is only one C &ndash; C bond. If one looks down the axis of the C &ndash; C bond, one will see the so-called [[Newman projection]]. The hydrogen atoms on both the front and rear carbon atoms have an angle of 120° between them, resulting from the projection of the base of the tetrahedron onto a flat plane. However, the torsion angle between a given hydrogen atom attached to the front carbon and a given hydrogen atom attached to the rear carbon can vary freely between 0° and 360°. This is a consequence of the free rotation about a carbon &ndash; carbon single bond. Despite this apparent freedom, only two limiting conformations are important: [[eclipsed]] conformation and [[staggered]] conformation. The two conformations, also known as [[rotamer]]s, differ in energy: The staggered conformation is 12.6 kJ/mol lower in energy (more stable) than the eclipsed conformation (the least stable). This difference in energy between the two conformations, known as the [[torsion energy]], is low compared to the thermal energy of an ethane molecule at ambient temperature. There is constant rotation about the C-C bond. The time taken for an ethane molecule to pass from one staggered conformation to the next, equivalent to the rotation of one CH<sub>3</sub>-group by 120° relative to the other, is of the order of 10<sup>−11</sup>&nbsp;seconds. The case of higher alkanes is more complex but based on similar principles, with the antiperiplanar conformation always being the most favoured around each carbon-carbon bond. For this reason, alkanes are usually shown in a zigzag arrangement in diagrams or in models. The actual structure will always differ somewhat from these idealised forms, as the differences in energy between the conformations are small compared to the thermal energy of the molecules: Alkane molecules have no fixed structural form, whatever the models may suggest. {| class="wikitable" |'''NAME''' |'''Formula''' |'''B.P./<sup>o</sup>C''' |'''M.P./<sup>o</sup>C''' |'''Density/g cm <sup>-3</sup>(20<sup>o</sup>C''') |- |Methane |CH<sub>4</sub> | -162 | -183 | gas |- |Ethane |C<sub>2</sub>H<sub>6</sub> | -89 | -172 | gas |- |Propane |C<sub>3</sub>H<sub>8</sub> | -42 | -188 | gas |- |Butane |C<sub>4</sub>H<sub>10</sub> | -0.5 | -135 | gas |- |Pentane |C<sub>5</sub>H<sub>12</sub> | 36 | -130 | 0.626 |- |Hexane |C<sub>6</sub>H<sub>14</sub> | 69 | -95 | 0.659 |- |Heptane |C<sub>7</sub>H<sub>16</sub> | 98 | -91 | 0.684 |- |Octane |C<sub>8</sub>H<sub>18</sub> | 126 | -57 | 0.703 |- |Nonane |C<sub>9</sub>H<sub>20</sub> | 151 | -54 | 0.718 |- |Decane |C<sub>10</sub>H<sub>22</sub> | 174 | -30 | 0.730 |- |Undecane |C<sub>11</sub>H<sub>24</sub> | 196 | -26 | 0.740 |- |Dodecane |C<sub>12</sub>H<sub>26</sub> | 216 | -10 | 0.749 |- |Triacontane |C<sub>30</sub>H<sub>62</sub> | 343 | 37 | solid |} ===Spectroscopic properties=== Virtually all organic compounds contain carbon &ndash; carbon and carbon &ndash; hydrogen bonds, and so show some of the features of alkanes in their spectra. Alkanes are notable for having no other groups, and therefore for the ''absence'' of other characteristic spectroscopic features. ====Infrared spectroscopy==== The carbon &ndash; hydrogen stretching mode gives a strong absorption between 2850 and 2960&nbsp;nanometres, while the carbon &ndash; carbon stretching mode absorbs between 800 and 1300&nbsp;nm. The carbon &ndash; hydrogen bending modes depend on the nature of the group: methyl groups show bands at 1450&nbsp;nm and 1375&nbsp;nm, while methylene groups show bands at 1465&nbsp;nm and 1450&nbsp;nm. Carbon chains with more than four carbon atoms show a weak absorption at around 725&nbsp;nm. <!-- cm^-1 are millimetres, but that was wrong, because 2.960 metres (2960mm) is way down into broadcast bands (radar and broadcasting are divided at about one metre, cordless phones and wireless networks straddling that line). Infrared is about a thousand nanometres or more. --> ====NMR spectroscopy==== The proton resonances of alkanes are usually found at [[chemical shift|δ<sub>H</sub>]] = 0.5 &ndash; 1.5. The carbon-13 resonances depend on the number of hydrogen atoms attached to the carbon: δ<sub>C</sub> = 8 &ndash; 30 (primary, methyl, -CH<sub>3</sub>), 15 &ndash; 55 (secondary, methylene, -CH<sub>2</sub>-), 20 &ndash; 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 sample that have not been run for a sufficiently long time. ====Mass spectrometry==== Alkanes have a high [[ionisation energy]], and the molecular ion is usually weak. 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 fragment resulting from the loss of a single methyl group (M−15) is often absent, and other fragment are often spaced by intervals of fourteen mass units, corresponding to sequential loss of CH<sub>2</sub>-groups. ==Chemical properties== In general, alkanes show a relatively low reactivity, because their C bonds are relatively stable and cannot be easily broken. Unlike most other organic compounds, they possess no [[functional group]]s. They react only very poorly with ionic or other polar substances. The [[acid dissociation constant]] (pK<sub>a</sub>) values of all alkanes are above 60, hence they are practically inert to acids and bases (see: [[carbon acid]]s). 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. However redox reactions of alkanes, in particular with oxygen and the halogens, are possible as the carbon atoms are in a strongly-reduced condition; in the case of methane, the lowest possible oxidation state for carbon (−4) is reached. Reaction with oxygen leads to combustion without any smoke; with halogens, [[substitution (chemistry)|substitution]]. In addition, alkanes have been shown to interact with, and bind to, certain transition metal complexes in (See: [[carbon-hydrogen bond activation]]). [[Free radical]]s, molecules with unpaired electrons, play a large role in most reactions of alkanes, such as cracking and reformation where long-chain alkanes are converted into shorter-chain alkanes and straight-chain alkanes into branched-chain isomers. In highly-branched alkanes, the bond angle may differ significantly from the optimal value (109.5°) in order to allow the different groups sufficient space. This causes a tension in the molecule, known as [[steric hindrance]], and can substantially increase the reactivity. ===Reactions with oxygen=== 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<sub>''n''</sub>H<sub>2''n''+2</sub> + (1.5''n''+0.5)O<sub>2</sub> → (''n''+1)H<sub>2</sub>O + ''n''CO<sub>2</sub> In the absence of sufficient oxygen, [[carbon monoxide]] or even [[soot]] can be formed, as shown below: :C<sub>n</sub>H<sub>(2n+2)</sub> + ½ n[[oxygen|O<sub>2</sub>]] → (n+1)[[hydrogen|H<sub>2</sub>]] + n[[carbon monoxide|CO]] for example [[methane]]: :2CH<sub>4</sub> + 3O<sub>2</sub> → 2CO + 4H<sub>2</sub>O :CH<sub>4</sub> + O<sub>2</sub> → C + 2H<sub>2</sub>O 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<sub>c</sub>''H''<sup><s>o</s></sup>, for alkanes increases by about 650&nbsp;kJ/mol per CH<sub>2</sub> group. Branched-chain alkanes have lower values of Δ<sub>c</sub>''H''<sup><s>o</s></sup> than straight-chain alkanes of the same number of carbon atoms, and so can be seen to be somewhat more stable. ===Reactions with halogens=== {{Main article|Free radical halogenation}} Alkanes react with [[halogen]]s in a so-called ''free radical halogenation'' reaction. The hydrogen atoms of the alkane are progressively replaced by halogen atoms. [[Free-radical]]s are the reactive species that participate in the reaction, which usually leads to a mixture of products. The reaction is highly [[exothermic reaction|exothermic]], and can lead to an explosion. These reactions are an important industrial route to halogenated hydrocarbons. There are three steps: * '''Initiation''' the halogen radicals form by [[homolysis]]. Usually, energy in the form of heat or light is required. * '''Chain reaction''' then takes place&mdash;the halogen radical abstracts a hydrogen from the alkane to give an alkyl radical. This reacts further. * '''Chain termination''' where step 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 a statistical mixture: 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]] ===Cracking=== {{main|Cracking (chemistry)}} Cracking breaks larger molecules into smaller ones. This can be done with a thermal or catalytic method. The thermal cracking process follows a [[homolytic cleavage|homolytic]] mechanism with formation of [[free-radical]]s. 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 [[ion]]s of opposite charges, usually a carbo[[cation]] and the very unstable [[hydride]] [[anion]]. 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|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. ===Isomerization and reformation=== 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 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. ===Other reactions=== Alkanes will react with [[steam]] in the presence of a [[nickel]] [[catalyst]] to give [[hydrogen]]. Alkanes can by [[Chlorosulfonation|chlorosulfonated]] and [[nitration|nitrated]], although both reactions require special conditions. The [[fermentation (biochemistry)|fermentation]] of alkanes to [[carboxylic acid]]s is of some technical importance. In the [[Reed reaction]], [[sulfur dioxide]], [[chlorine]] and [[photochemistry|light]] convert hydrocarbons to [[Sulfonic acid|sulfonyl chloride]]s. ==Hazards== Methane is explosive when mixed with air (1 &ndash; 8% CH<sub>4</sub>) and is a strong [[greenhouse gas]]: Other lower alkanes can also form explosive mixtures with air. The lighter liquid alkanes are highly flammable, although this risk decreases with the length of the carbon chain. Pentane, hexane, heptane, and octane are classed as ''dangerous for the environment'' and ''harmful''. The straight-chain isomer of hexane is a [[neurotoxin]]. ==See also== {{wikibooks|alkanes}} {{Wiktionary}} * [[Alkene]] * [[Alkyne]] * [[Cycloalkane]] * [[Cracking (chemistry)]] * [[Functional group]] * [[List of alkanes|List of n-alkanes]] ==References== {{reflist|2}} ==Further reading== * [http://www.cem.msu.edu/~reusch/VirtualText/intro1.htm Virtual Textbook of Organic Chemistry] {{Alkanes}} {{Functional Groups}} {{BranchesofChemistry}} [[Category:Hydrocarbons]] [[Category:Alkanes| ]] {{Link FA|de}} [[ar:ألكان]] [[bs:Alkani]] [[bg:Алкан]] [[ca:Alcà]] [[cs:Alkany]] [[da:Alkan]] [[de:Alkane]] [[et:Alkaanid]] [[el:Αλκάνια]] [[es:Alcano]] [[eo:Alkano]] [[fa:آلکان]] [[fo:Alkan]] [[fr:Alcane]] [[ko:알케인]] [[hr:Alkani]] [[id:Alkana]] [[is:Alkanar]] [[it:Alcani]] [[he:אלקאן]] [[ka:ალკანი]] [[ku:Alkan]] [[la:Alcanum]] [[lv:Alkāni]] [[lt:Alkanas]] [[hu:Alkánok]] [[mk:Алкан]] [[ms:Alkana]] [[nl:Alkaan]] [[ja:アルカン]] [[no:Alkaner]] [[nn:Alkan]] [[pms:Parafin-a]] [[pl:Alkany]] [[pt:Alcano]] [[ro:Alcan]] [[ru:Алканы]] [[simple:Alkane]] [[sk:Alkán]] [[sl:Alkan]] [[sr:Алкан]] [[sh:Alkani]] [[su:Alkana]] [[fi:Alkaani]] [[sv:Alkan]] [[ta:ஆல்க்கேன்]] [[th:อัลเคน]] [[vi:Ankan]] [[tr:Alkan]] [[uk:Алкани]] [[zh:烷烃]]