Ethane ( , ) is a naturally occurring organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petrochemical by-product of petroleum refining. Its chief use is as feedstock for ethylene production.
Related compounds may be formed by replacing a hydrogen atom with another functional group; the ethane moiety is called an ethyl group. For example, an ethyl group linked to a hydroxyl group yields ethanol, the alcohol in beverages.
History
Ethane was first synthesised in 1834 by Michael Faraday, applying electrolysis of a potassium acetate solution. He mistook the hydrocarbon product of this reaction for methane and did not investigate it further.
During the period 1847–1849, in an effort to vindicate the radical theory of organic chemistry, Hermann Kolbe and Edward Frankland produced ethane by the reductions of propionitrile (ethyl cyanide) and ethyl iodide with potassium metal, and, as did Faraday, by the electrolysis of aqueous acetates. They mistook the product of these reactions for the methyl radical (), of which ethane () is a dimer.
This error was corrected in 1864 by Carl Schorlemmer, who showed that the product of all these reactions was in fact ethane. Ethane was discovered dissolved in Pennsylvanian light crude oil by Edmund Ronalds in 1864.
Properties
At standard temperature and pressure, ethane is a colorless, odorless gas. It has a boiling point of and melting point of . Solid ethane exists in several modifications. 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. changes it to monoclinic metastable ethane II (space group P 21/n). 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).
Atmospheric and extraterrestrial
Ethane occurs as a trace gas in the Earth's atmosphere, currently having a concentration at sea level of 0.5 ppb, though its preindustrial concentration is likely to have been only around 0.25 part per billion since a significant proportion of the ethane in today's atmosphere may have originated as fossil fuels. Global ethane quantities have varied over time, likely due to flaring at natural gas fields. Global ethane emission rates declined from 1984 to 2010, though increased shale gas production at the Bakken Formation in the U.S. has arrested the decline by half.
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, 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. 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.
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.
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 December 2007 the Cassini probe found at least one lake at Titan's south pole, now called Ontario Lacus because of the lake's similar area to Lake Ontario on Earth (approximately 20,000 km2). Further analysis of infrared spectroscopic data presented in July 2008 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.