Editing Ethane (section)

==Properties==
At standard temperature and pressure, ethane is a colorless, odorless gas. It has a boiling point of {{cvt|-88.5|°C|F}} and melting point of {{cvt|-182.8|°C|F}}. Solid ethane exists in several modifications.<ref name="Nes">{{cite journal |doi= 10.1107/S0567740878007037 |title= Single-crystal structures and electron density distributions of ethane, ethylene and acetylene. I. Single-crystal X-ray structure determinations of two modifications of ethane |journal= Acta Crystallographica Section B |volume=34 |issue=6 |page= 1947 |year= 1978 |last1= Van Nes |first1= G.J.H. |last2= Vos |first2= A. |bibcode= 1978AcCrB..34.1947V |s2cid= 55183235 |url= http://www.rug.nl/research/portal/files/3440910/c3.pdf}}</ref> On cooling under normal pressure, the first modification to appear is a [[plastic crystal]], crystallizing in the cubic system. In this form, the positions of the hydrogen atoms are not fixed; the molecules may rotate freely around the long axis. Cooling this ethane below ca. {{convert|89.9|K|C F}} changes it to monoclinic metastable ethane II ([[space group]] P 21/n).<ref>{{cite web |url= https://log-web.de/chemie/Start.htm?name=ethaneCryst&lang=en |title= Ethane as a solid |access-date= 2019-12-10}}</ref> Ethane is only very sparingly soluble in water.

The bond parameters of ethane have been measured to high precision by microwave spectroscopy and electron diffraction: ''r''<sub>C−C</sub> = 1.528(3) Å, ''r''<sub>C−H</sub> = 1.088(5) Å, and ∠CCH = 111.6(5)°  by microwave and ''r''<sub>C−C</sub> = 1.524(3) Å, ''r''<sub>C−H</sub> = 1.089(5) Å, and ∠CCH = 111.9(5)° by electron diffraction (the numbers in parentheses represents the uncertainties in the final digits).<ref>{{Cite journal|last=Harmony|first=Marlin D.|date=1990-11-15|title=The equilibrium carbon–carbon single-bond length in ethane|journal=The Journal of Chemical Physics|language=en|volume=93|issue=10|pages=7522–7523|doi=10.1063/1.459380|issn=0021-9606|bibcode=1990JChPh..93.7522H}}</ref>

[[File:Ethane conformations and relative energies.svg|left|thumb|300px|Ethane (shown in [[Newman projection]]) barrier to rotation about the carbon-carbon bond. The curve is potential energy as a function of rotational angle. [[Activation energy|Energy barrier]] is 12 [[kJ/mol]] or about 2.9 [[kcal/mol]].<ref>{{Cite book|title=Organic chemistry|last=J|first=McMurry|date=2012|publisher=Brooks|isbn=9780840054449|edition=8|location=Belmont, CA|pages=95}}</ref>]]
Rotating a molecular substructure about a twistable bond usually requires energy. The minimum energy to produce a 360° bond rotation is called the [[rotational barrier]].

Ethane gives a classic, simple example of such a rotational barrier, sometimes called the "ethane barrier". Among the earliest experimental evidence of this barrier (see diagram at left) was obtained by modelling the entropy of ethane.<ref>{{cite journal |doi= 10.1021/ja01281a014 |title= The Entropy of Ethane and the Third Law of Thermodynamics. Hindered Rotation of Methyl Groups |journal= Journal of the American Chemical Society |volume=59 |issue=2 |pages=276 |year=1937 |last1=Kemp |first1=J. D. |last2=Pitzer |first2= Kenneth S.}}
</ref> The three hydrogens at each end are free to pinwheel about the central carbon–carbon bond when provided with sufficient energy to overcome the barrier. The physical origin of the barrier is still not completely settled,<ref>{{cite journal |doi= 10.1021/ed082p1703 |title= Determination of the Rotational Barrier in Ethane by Vibrational Spectroscopy and Statistical Thermodynamics |year=2005 |last1= Ercolani |first1=G. |journal= J. Chem. Educ. |volume=82 |issue=11 |pages= 1703–1708 |bibcode = 2005JChEd..82.1703E }}</ref> although the overlap (exchange) repulsion<ref>{{cite journal |doi= 10.1021/ar00090a004 |title= The Barrier to Internal Rotation in Ethane |year=1983 |last1= Pitzer |first1= R.M. |journal= Acc. Chem. Res. |volume=16 |issue=6 |pages= 207–210}}</ref> between the hydrogen atoms on opposing ends of the molecule is perhaps the strongest candidate, with the stabilizing effect of [[hyperconjugation]] on the staggered conformation contributing to the phenomenon.<ref>{{cite journal|doi=10.1002/anie.200352931|title=The Magnitude of Hyperconjugation in Ethane: A Perspective from Ab Initio Valence Bond Theory|year=2004|last1=Mo|first1=Y.|last2=Wu|first2=W.|last3=Song|first3=L.|last4=Lin|first4=M.|last5=Zhang|first5=Q.|last6=Gao|first6=J.|journal=Angew. Chem. Int. Ed.|volume=43|issue=15|pages=1986–1990|pmid=15065281}}</ref> Theoretical methods that use an appropriate starting point (orthogonal orbitals) find that hyperconjugation is the most important factor in the origin of the ethane rotation barrier.<ref>{{cite journal |author1= Pophristic, V. |author2=Goodman, L.  |title= Hyperconjugation not steric repulsion leads to the staggered structure of ethane |journal= Nature |volume= 411 |issue= 6837 |pages= 565–8 |doi= 10.1038/35079036 |pmid= 11385566 |year=2001|bibcode=2001Natur.411..565P |s2cid=205017635 }}</ref><ref>{{cite journal |author= Schreiner, P. R. |title= Teaching the right reasons: Lessons from the mistaken origin of the rotational barrier in ethane |journal= Angewandte Chemie International Edition |volume=41 |issue=19 |pages=3579–81, 3513 |pmid= 12370897 |year= 2002 |doi= 10.1002/1521-3773(20021004)41:19<3579::AID-ANIE3579>3.0.CO;2-S}}
</ref>

As far back as 1890–1891, chemists suggested that ethane molecules preferred the staggered conformation with the two ends of the molecule askew from each other.<ref>{{cite journal |author= Bischoff, CA |title= Ueber die Aufhebung der freien Drehbarkeit von einfach verbundenen Kohlenstoffatomen |year=1890 |journal= Chem. Ber. |volume=23 |page= 623 |doi= 10.1002/cber.18900230197|url= https://zenodo.org/record/1425584 }}</ref><ref>{{cite journal |author= Bischoff, CA |title= Theoretische Ergebnisse der Studien in der Bernsteinsäuregruppe |year= 1891 |journal= Chem. Ber. |volume=24 |pages= 1074–1085 |doi= 10.1002/cber.189102401195|url= https://zenodo.org/record/1425620 }}</ref><ref>{{cite journal |author= Bischoff, CA |title= Die dynamische Hypothese in ihrer Anwendung auf die Bernsteinsäuregruppe |year= 1891 |journal= Chem. Ber. |volume=24 |pages=1085–1095 |doi= 10.1002/cber.189102401196 |url= https://zenodo.org/record/1425622 }}</ref><ref>{{cite journal |year=1893 |volume=26 |issue=2 |page= 1452 |doi= 10.1002/cber.18930260254 |title= Die Anwendung der dynamischen Hypothese auf Ketonsäurederivate |journal= Berichte der Deutschen Chemischen Gesellschaft |last1= Bischoff |first1=C.A. |last2= Walden |first2= P.|url=https://zenodo.org/record/1425708 }}</ref>

===Atmospheric and extraterrestrial===
[[File:Titan North Pole Lakes PIA08630.jpg|right|thumb|250px|A photograph of [[Titan (moon)|Titan]]'s northern latitudes. The dark features are hydrocarbon lakes containing ethane]]

Ethane occurs as a trace gas in the [[Earth's atmosphere]], currently having a concentration at [[sea level]] of 0.5 [[parts per billion|ppb]].<ref>{{cite web|url=http://www.atmosphere.mpg.de/enid/3tg.html|title=Trace gases (archived)|website=Atmosphere.mpg.de|archive-url=https://web.archive.org/web/20081222061502/http://www.atmosphere.mpg.de/enid/3tg.html |access-date=2011-12-08|archive-date=2008-12-22 }}</ref> Global ethane quantities have varied over time, likely due to [[Gas flare|flaring]] at [[natural gas field]]s.<ref name="SimpsonSulbaek Andersen2012">{{cite journal|last1=Simpson|first1=Isobel J.|last2=Sulbaek Andersen|first2=Mads P.|last3=Meinardi|first3=Simone|last4=Bruhwiler|first4=Lori|last5=Blake|first5=Nicola J.|last6=Helmig|first6=Detlev|last7=Rowland|first7=F. Sherwood|last8=Blake|first8=Donald R.|title=Long-term decline of global atmospheric ethane concentrations and implications for methane|journal=Nature|volume=488|issue=7412|year=2012|pages=490–494|doi=10.1038/nature11342|pmid=22914166|url=https://zenodo.org/record/898122|bibcode=2012Natur.488..490S|s2cid=4373714}}</ref> Global ethane emission rates declined from 1984 to 2010,<ref name="SimpsonSulbaek Andersen2012"/> though increased [[shale gas]] production at the [[Bakken Formation]] in the U.S. has arrested the decline by half.<ref name="KortSmith2016">{{cite journal|last1=Kort|first1=E. A.|last2=Smith|first2=M. L.|last3=Murray|first3=L. T.|last4=Gvakharia|first4=A.|last5=Brandt|first5=A. R.|last6=Peischl|first6=J.|last7=Ryerson|first7=T. B.|last8=Sweeney|first8=C.|last9=Travis|first9=K.|title=Fugitive emissions from the Bakken shale illustrate role of shale production in global ethane shift|journal=Geophysical Research Letters|year=2016|doi=10.1002/2016GL068703|volume=43|issue=9|pages=4617–4623|bibcode=2016GeoRL..43.4617K|doi-access=free|hdl=2027.42/142509|hdl-access=free}}</ref><ref>{{cite web|url=http://ns.umich.edu/new/multimedia/videos/23735-one-oil-field-a-key-culprit-in-global-ethane-gas-increase|title=One oil field a key culprit in global ethane gas increase|date=April 26, 2016|publisher=University of Michigan}}</ref>

Although ethane is a [[greenhouse gas]], it is much less abundant than methane, has a lifetime of only a few months compared to over a decade,<ref name="Feasibility">{{cite journal|last1=Aydin|first1=Kamil Murat|last2=Williams|first2=M.B.|last3=Saltzman|first3=E.S.|title=Feasibility of reconstructing paleoatmospheric records of selected alkanes, methyl halides, and sulfur gases from Greenland ice cores|journal=Journal of Geophysical Research|volume=112|date=April 2007|issue=D7 |doi=10.1029/2006JD008027 |bibcode=2007JGRD..112.7312A }}</ref> and is also less efficient at absorbing radiation relative to mass. In fact, ethane's [[global warming potential]] largely results from its conversion in the atmosphere to methane.<ref>{{cite journal|last1=Hodnebrog|first1=Øivind|last2=Dalsøren|first2=Stig B.|last3=Myrhe|first3=Gunnar|title=Lifetimes, direct and indirect radiative forcing, and global warming potentials of ethane (C<sub>2</sub>H<sub>6</sub>), propane (C<sub>3</sub>H<sub>8</sub>), and butane (C<sub>4</sub>H<sub>10</sub>)|journal=Atmospheric Science Letters|year=2018|volume=19 |issue=2 |doi=10.1002/asl.804|doi-access=free|bibcode=2018AtScL..19E.804H }}</ref> It has been detected as a trace component in the atmospheres of all four [[giant planet]]s, and in the atmosphere of [[Saturn]]'s moon [[Titan (moon)|Titan]].<ref>{{cite web|first = Bob|last = Brown|year = 2008|url = http://www.jpl.nasa.gov/news/news.cfm?release=2008-152|title = NASA Confirms Liquid Lake on Saturn Moon|display-authors = et al|publisher = NASA Jet Propulsion Laboratory|access-date = 2008-07-30|archive-date = 2011-06-05|archive-url = https://web.archive.org/web/20110605031218/http://www.jpl.nasa.gov/news/news.cfm?release=2008-152|url-status = dead}}</ref>

Atmospheric ethane results from the Sun's [[photochemistry|photochemical]] action on methane gas, also present in these atmospheres: [[ultraviolet]] photons of shorter [[wavelength]]s than 160 [[nanometer|nm]] can photo-dissociate the methane molecule into a [[methyl]] radical and a [[hydrogen]] atom. When two methyl radicals recombine, the result is ethane:

: CH<sub>4</sub> &nbsp;→&nbsp; CH<sub>3</sub>• + •H
: CH<sub>3</sub>• + •CH<sub>3</sub> &nbsp;→&nbsp; C<sub>2</sub>H<sub>6</sub>

In Earth's atmosphere, [[hydroxyl radical]]s convert ethane to [[methanol]] vapor with a half-life of around three months.<ref name="Feasibility"/>

It is suspected that ethane produced in this fashion on Titan rains back onto the moon's surface, and over time has accumulated into hydrocarbon seas covering much of the moon's polar regions. In mid-2005, the ''[[Cassini-Huygens|Cassini]]'' orbiter discovered [[Ontario Lacus]] in Titan's south polar regions. Further analysis of infrared spectroscopic data presented in July 2008<ref>{{cite journal|doi=10.1038/nature07100|title=The identification of liquid ethane in Titan's Ontario Lacus|year=2008|last1=Brown|first1=R. H.|last2=Soderblom|first2=L. A.|last3=Soderblom|first3=J. M.|last4=Clark|first4=R. N.|last5=Jaumann|first5=R.|last6=Barnes|first6=J. W.|last7=Sotin|first7=C.|last8=Buratti|first8=B.|last9=Baines|first9=K. H.|last10=Nicholson|first10=P. D.|journal=Nature|volume=454|issue=7204|pages=607–10|pmid=18668101|bibcode = 2008Natur.454..607B |s2cid=4398324|display-authors=8}}</ref> provided additional evidence for the presence of liquid ethane in Ontario Lacus. Several significantly larger hydrocarbon lakes, [[Ligeia Mare]] and [[Kraken Mare]] being the two largest, were discovered near Titan's north pole using radar data gathered by Cassini. These lakes are believed to be filled primarily by a mixture of liquid ethane and methane.

In 1996, ethane was detected in [[Comet Hyakutake]],<ref name= Mumma/> and it has since been detected in some other [[comets]]. The existence of ethane in these distant solar system bodies may implicate ethane as a primordial component of the [[solar nebula]] from which the sun and planets are believed to have formed.

In 2006, Dale Cruikshank of NASA/Ames Research Center (a ''[[New Horizons]]'' co-investigator) and his colleagues announced the spectroscopic discovery of ethane on [[Pluto]]'s surface.<ref>{{Cite web |last=Stern |first= A. |author-link=Alan Stern |date=November 1, 2006 |title=Making Old Horizons New |url=http://pluto.jhuapl.edu/overview/piPerspectives/piPerspective_11_1_2006.php |url-status=dead |archive-url=https://web.archive.org/web/20080828012339/http://pluto.jhuapl.edu/overview/piPerspectives/piPerspective_11_1_2006.php |archive-date=August 28, 2008 |access-date=2007-02-12 |website=The PI's Perspective |publisher=Johns Hopkins University Applied Physics Laboratory}}</ref>