Editing Alkane (section)

==Physical properties==
{{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}}</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)}}</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/m<sup>3</sup>] (at 20&nbsp;°C)
!Isomers<ref group="note" name="isomer">Total number of [[constitutional isomer]]s for this molecular formula</ref>
|-
|[[Methane]]
|CH<sub>4</sub>
| −162
| −182
| 0.656 (gas)
| 1
|-
|[[Ethane]]
|C<sub>2</sub>H<sub>6</sub>
| −89
| −183
| 1.26 (gas)
| 1
|-
|[[Propane]]
|C<sub>3</sub>H<sub>8</sub>
| −42
| −188
| 2.01 (gas)
| 1
|-
|[[Butane]]
|C<sub>4</sub>H<sub>10</sub>
| 0
| −138
| 2.48 (gas)
| 2
|-
|[[Pentane]]
|C<sub>5</sub>H<sub>12</sub>
| 36
| −130
| 626 (liquid)
| 3
|-
|[[Hexane]]
|C<sub>6</sub>H<sub>14</sub>
| 69
| −95
| 659 (liquid)
| 5
|-
|[[Heptane]]
|C<sub>7</sub>H<sub>16</sub>
| 98
| −91
| 684 (liquid)
| 9
|-
|[[Octane]]
|C<sub>8</sub>H<sub>18</sub>
| 126
| −57 
| 703 (liquid)
| 18
|-
|[[Nonane]]
|C<sub>9</sub>H<sub>20</sub>
| 151
| −54
| 718 (liquid)
| 35
|-
|[[Decane]]
|C<sub>10</sub>H<sub>22</sub>
| 174
| −30
| 730 (liquid)
| 75
|-
|[[Undecane]]
|C<sub>11</sub>H<sub>24</sub>
| 196
| −26
| 740 (liquid)
| 159
|-
|[[Dodecane]]
|C<sub>12</sub>H<sub>26</sub>
| 216
| −10
| 749 (liquid)
| 355
|-
|[[Tridecane]]
|C<sub>13</sub>H<sub>28</sub>
| 235
| −5.4
| 756 (liquid)
| 802
|-
|[[Tetradecane]]
|C<sub>14</sub>H<sub>30</sub>
| 253
| 5.9
| 763 (liquid)
| 1858
|-
|[[Pentadecane]]
|C<sub>15</sub>H<sub>32</sub>
| 270
| 10
| 769 (liquid)
| 4347
|-
|[[Hexadecane]]
|C<sub>16</sub>H<sub>34</sub>
| 287
| 18
| 773 (liquid)
| 10,359
|-
|[[Heptadecane]]
|C<sub>17</sub>H<sub>36</sub>
| 303
| 22
| 777 (solid)
| 24,894
|-
|[[Octadecane]]
|C<sub>18</sub>H<sub>38</sub>
| 317
| 28
| 781 (solid)
| 60,523
|-
|[[Nonadecane]]
|C<sub>19</sub>H<sub>40</sub>
| 330
| 32
| 785 (solid)
| 148,284
|-
|[[Icosane]]
|C<sub>20</sub>H<sub>42</sub>
| 343
| 37
| 789 (solid)
| 366,319
|-
|[[Triacontane]]
|C<sub>30</sub>H<sub>62</sub>
| 450
| 66
| 810 (solid)
| 4,111,846,763
|-
|[[Tetracontane]]
|C<sub>40</sub>H<sub>82</sub>
| 525
| 82
| 817 (solid)
| 62,481,801,147,341
|-
|[[Pentacontane]]
|C<sub>50</sub>H<sub>102</sub>
| 575
| 91
| 824 (solid)
| 1,117,743,651,746,953,270
|-
|[[Hexacontane]]
|C<sub>60</sub>H<sub>122</sub>
| 625
| 100
| 829 (solid)
| 2.21587345357704×10<sup>22</sup>
|-
|Heptacontane
|C<sub>70</sub>H<sub>142</sub>
| 653
| 109
| 869 (solid)
| 4.71484798515330×10<sup>26</sup>	
|-
|colspan=6 | {{reflist|group=note}}
|}

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

Alkanes experience 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 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 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]] (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 | last=Boese | first=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}}</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}}</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|sp<sup>3</sup>-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 sp<sup>3</sup> 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<sup>−1</sup>(−{{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.png|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 sp<sup>3</sup> orbital of carbon with the 1s orbital of a hydrogen; the latter by the overlap of two sp<sup>3</sup> orbitals on adjacent carbon atoms. The [[bond length]]s amount to 1.09&nbsp;×&nbsp;10<sup>−10</sup>&nbsp;m for a C–H bond and 1.54&nbsp;×&nbsp;10<sup>−10</sup>&nbsp;m for a C–C bond.

The spatial arrangement of the bonds is similar to that of the four sp<sup>3</sup> 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 is 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 a strong absorptions between 2850 and 2960&nbsp;[[Wavenumber|cm<sup>−1</sup>]] and weaker bands for the C-C stretching mode absorbs between 800 and 1300&nbsp;cm<sup>−1</sup>. The carbon–hydrogen bending modes depend on the nature of the group: methyl groups show bands at 1450&nbsp;cm<sup>−1</sup> and 1375&nbsp;cm<sup>−1</sup>, while methylene groups show bands at 1465&nbsp;cm<sup>−1</sup> and 1450&nbsp;cm<sup>−1</sup>.<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<sup>−1</sup>.

====NMR spectroscopy====
The proton resonances of alkanes are usually found at [[chemical shift|''δ''<sub>H</sub>]] = 0.5–1.5. The carbon-13 resonances depend on the number of hydrogen atoms attached to the carbon: ''δ''<sub>C</sub> = 8–30 (primary, methyl, –CH<sub>3</sub>), 15–55 (secondary, methylene, –CH<sub>2</sub>–), 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 CH<sub>2</sub> 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>