Editing Alkane (section)

==Isomerism==
[[Image:Saturated C4 hydrocarbons ball-and-stick (C4H10 C4H8).png|thumb|upright=1.2|right| 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.]]
<!--<br/>[[Bicyclobutane|Bicyclo[1.1.0]butane]] is the only C<sub>4</sub>H<sub>6</sub> alkane and has no alkane isomer; [[tetrahedrane]] (below) is the only C<sub>4</sub>H<sub>4</sub> 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 C<sub>4</sub>H<sub>6</sub> alkane and has no alkane isomer.
|image2=Tetrahedrane-3D-balls.png|caption2=[[Tetrahedrane]] is the only C<sub>4</sub>H<sub>4</sub> 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>
* C<sub>1</sub>: [[methane]] only
* C<sub>2</sub>: [[ethane]] only
* C<sub>3</sub>: [[propane]] only
* C<sub>4</sub>: 2 isomers: [[butane]] and [[isobutane]]
* C<sub>5</sub>: 3 isomers: [[pentane]], [[isopentane]], and [[neopentane]]
* C<sub>6</sub>: 5 isomers: [[hexane]], [[2-Methylpentane|2-methylpentane]], [[3-Methylpentane|3-methylpentane]], [[2,2-Dimethylbutane|2,2-dimethylbutane]], and [[2,3-Dimethylbutane|2,3-dimethylbutane]]
* C<sub>7</sub>: 9 isomers: [[heptane]], [[2-methylhexane]], [[3-methylhexane]], [[2,2-dimethylpentane]], [[2,3-dimethylpentane]], [[2,4-dimethylpentane]], [[3,3-dimethylpentane]], [[3-ethylpentane]], [[2,2,3-trimethylbutane]]
*C<sub>8</sub>: 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]]
* C<sub>9</sub>: [[list of isomers of nonane|35 isomers]]
* C<sub>10</sub>: [[list of isomers of decane|75 isomers]]
* C<sub>12</sub>: [[list of isomers of dodecane|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

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 (C<sub>4</sub>H<sub>8</sub>), but are not isomers of butane (C<sub>4</sub>H<sub>10</sub>).

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>