At standard temperature and pressure, ethane is a colorless, odorless gas. It has a boiling point of −88.5 °C (−127.3 °F) and melting point of −182.8 °C (−297.0 °F). Solid ethane exists in several modifications.[12] 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. 89.9 K (−183.2 °C; −297.8 °F) changes it to monoclinic metastable ethane II (space group P 21/n).[13] 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: rC−C = 1.528(3) Å, rC−H = 1.088(5) Å, and ∠CCH = 111.6(5)° by microwave and rC−C = 1.524(3) Å, rC−H = 1.089(5) Å, and ∠CCH = 111.9(5)° by electron diffraction (the numbers in parentheses represents the uncertainties in the final digits).[14]
Ethane (shown in Newman projection) barrier to rotation about the carbon-carbon bond. The curve is potential energy as a function of rotational angle. Energy barrier is 12 kJ/mol or about 2.9 kcal/mol.[15]
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.[16] 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,[17] although the overlap (exchange) repulsion[18] 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.[19] 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.[20][21]
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.[22][23][24][25]
Atmospheric and extraterrestrial
A photograph of Titan's northern latitudes. The dark features are hydrocarbon lakes containing ethane
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,[30] 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.[31] It has been detected as a trace component in the atmospheres of all four giant planets, and in the atmosphere of Saturn's moon Titan.[32]
Atmospheric ethane results from the Sun's photochemical action on methane gas, also present in these atmospheres: ultraviolet photons of shorter wavelengths than 160 nm can photo-dissociate the methane molecule into a methyl radical and a hydrogen atom. When two methyl radicals recombine, the result is ethane:
CH4 → CH3• + •H
CH3• + •CH3 → C2H6
In Earth's atmosphere, hydroxyl radicals convert ethane to methanol vapor with a half-life of around three months.[30]
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 orbiter discovered Ontario Lacus in Titan's south polar regions. Further analysis of infrared spectroscopic data presented in July 2008[33] 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,[34] 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.[35]
Combustion may also occur without an excess of oxygen, yielding carbon monoxide, acetaldehyde, methane, methanol, and ethanol. At higher temperatures, especially in the range 600–900 °C (1,112–1,652 °F), ethylene is a significant product:
2 C2H6 + O2 → 2 C2H4 + 2 H2O
Such oxidative dehydrogenation reactions are relevant to the production of ethylene.[37]
Production
After methane, ethane is the second-largest component of natural gas. Natural gas from different gas fields varies in ethane content from less than 1% to more than 6% by volume. Prior to the 1960s, ethane and larger molecules were typically not separated from the methane component of natural gas, but simply burnt along with the methane as a fuel. Today, ethane is an important petrochemicalfeedstock and is separated from the other components of natural gas in most well-developed gas fields. Ethane can also be separated from petroleum gas, a mixture of gaseous hydrocarbons produced as a byproduct of petroleum refining.
Ethane is most efficiently separated from methane by liquefying it at cryogenic temperatures. Various refrigeration strategies exist: the most economical process presently in wide use employs a turboexpander, and can recover more than 90% of the ethane in natural gas. In this process, chilled gas is expanded through a turbine, reducing the temperature to approximately −100 °C (−148 °F). At this low temperature, gaseous methane can be separated from the liquefied ethane and heavier hydrocarbons by distillation. Further distillation then separates ethane from the propane and heavier hydrocarbons.
Ehane has been investigated as a feedstock for other commodity chemicals. Oxidative chlorination of ethane has long appeared to be a potentially more economical route to vinyl chloride than ethylene chlorination. Many patent exist on this theme, but poor selectivity for vinyl chloride and corrosive reaction conditions have discouraged the commercialization of most of them. Presently, INEOS operates a 1000 t/a (tonnes per annum) ethane-to-vinyl chloride pilot plant at Wilhelmshaven in Germany.
SABIC operates a 34,000 t/a plant at Yanbu to produce acetic acid by ethane oxidation.[38] The economic viability of this process may rely on the low cost of ethane near Saudi oil fields, and it may not be competitive with methanol carbonylation elsewhere in the world.[39]
Ethane can be used as a refrigerant in cryogenic refrigeration systems.
In the laboratory
On a much smaller scale, in scientific research, liquid ethane is used to vitrify water-rich samples for cryo-electron microscopy. A thin film of water quickly immersed in liquid ethane at −150 °C or colder freezes too quickly for water to crystallize. Slower freezing methods can generate cubic ice crystals, which can disrupt soft structures by damaging the samples and reduce image quality by scattering the electron beam before it can reach the detector.
Health and safety
At room temperature, ethane is an extremely flammable gas. When mixed with air at 3.0%–12.5% by volume, it forms an explosive mixture.
^IUPAC 2014, p. 4. "Similarly, the retained names 'ethane', 'propane', and 'butane' were never replaced by systematic names 'dicarbane', 'tricarbane', and 'tetracarbane' as recommended for analogues of silane, 'disilane'; phosphane, 'triphosphane'; and sulfane, 'tetrasulfane'."
^"Ethane – Compound Summary". PubChem Compound. US: National Center for Biotechnology Information. 16 September 2004. Retrieved 7 December 2011.
^Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton, Florida: CRC Press. p. 8.88. ISBN0-8493-0486-5.
^Kemp, J. D.; Pitzer, Kenneth S. (1937). "The Entropy of Ethane and the Third Law of Thermodynamics. Hindered Rotation of Methyl Groups". Journal of the American Chemical Society. 59 (2): 276. doi:10.1021/ja01281a014.
^Ercolani, G. (2005). "Determination of the Rotational Barrier in Ethane by Vibrational Spectroscopy and Statistical Thermodynamics". J. Chem. Educ. 82 (11): 1703–1708. Bibcode:2005JChEd..82.1703E. doi:10.1021/ed082p1703.
^Pitzer, R.M. (1983). "The Barrier to Internal Rotation in Ethane". Acc. Chem. Res. 16 (6): 207–210. doi:10.1021/ar00090a004.
^Mo, Y.; Wu, W.; Song, L.; Lin, M.; Zhang, Q.; Gao, J. (2004). "The Magnitude of Hyperconjugation in Ethane: A Perspective from Ab Initio Valence Bond Theory". Angew. Chem. Int. Ed. 43 (15): 1986–1990. doi:10.1002/anie.200352931. PMID15065281.
^Brown, R. H.; Soderblom, L. A.; Soderblom, J. M.; Clark, R. N.; Jaumann, R.; Barnes, J. W.; Sotin, C.; Buratti, B.; et al. (2008). "The identification of liquid ethane in Titan's Ontario Lacus". Nature. 454 (7204): 607–10. Bibcode:2008Natur.454..607B. doi:10.1038/nature07100. PMID18668101. S2CID4398324.
^Stern, A. (November 1, 2006). "Making Old Horizons New". The PI's Perspective. Johns Hopkins University Applied Physics Laboratory. Archived from the original on August 28, 2008. Retrieved 2007-02-12.
^Dreher, Eberhard-Ludwig; Torkelson, Theodore R.; Beutel, Klaus K. (2011). "Chlorethanes and Chloroethylenes". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.o06_o01. ISBN978-3-527-30385-4.
^Najari, Sara; Saeidi, Samrand; Concepcion, Patricia; Dionysiou, Dionysios D.; Bhargava, Suresh K.; Lee, Adam F.; Wilson, Karen (2021). "Oxidative dehydrogenation of ethane: Catalytic and mechanistic aspects and future trends". Chemical Society Reviews. 50 (7): 4564–4605. doi:10.1039/D0CS01518K. PMID33595011. S2CID231946397.
