Ethane

Ethane (pronounced ETH-ayn in the US, EE-thayn in the UK) is an organic compound with the formula C₂H₆. Under standard conditions, it is a colorless and odorless gas. Industrially, it is extracted from natural gas and also arises as a by-product of petroleum refining. Its primary application is as a raw material for making ethylene. The ethyl group is formally derived from ethane, though this derivation is rarely carried out in practice. **History** In 1834, Michael Faraday first produced ethane by electrolyzing a potassium acetate solution. He mistakenly identified the hydrocarbon as methane and did not pursue it further. This reaction is now known as Kolbe electrolysis. Between 1847 and 1849, Hermann Kolbe and Edward Frankland, aiming to support the radical theory of organic chemistry, generated ethane by reducing propionitrile and ethyl iodide with potassium metal, and also by electrolyzing aqueous acetates as Faraday had done. They believed the product was the methyl radical (CH₃), but ethane (C₂H₆) is actually a dimer of that radical. Carl Schorlemmer corrected this error in 1864, showing that all these reactions yielded ethane. That same year, Edmund Ronalds discovered ethane dissolved in Pennsylvania light crude oil. **Properties** At standard temperature and pressure, ethane is a colorless, odorless gas. It boils at −88.5 °C and melts at −182.8 °C. Solid ethane takes several forms. Under normal pressure, it first forms a plastic crystal with a cubic structure, where hydrogen atoms are not fixed and molecules can rotate freely along the long axis. Cooling below about 89.9 K (−183.2 °C) converts it to a metastable monoclinic form, ethane II. Ethane is only slightly soluble in water. Precise bond parameters have been measured: microwave spectroscopy gives a C–C bond length of 1.528(3) Å, C–H of 1.088(5) Å, and a CCH angle of 111.6(5)°; electron diffraction gives 1.524(3) Å, 1.089(5) Å, and 111.9(5)°, respectively. Rotating a molecular substructure around a bond requires energy, and the minimum needed for a full 360° rotation is the rotational barrier. Ethane provides a classic example of this barrier, sometimes called the "ethane barrier." Early evidence came from modeling ethane’s entropy. With enough energy to overcome the barrier, the three hydrogens at each end can spin like a pinwheel around the central carbon–carbon bond. The barrier’s physical origin remains debated, though overlap repulsion between opposing hydrogen atoms is a strong candidate, and hyperconjugation stabilizing the staggered conformation also contributes. Theoretical methods using orthogonal orbitals suggest hyperconjugation is the most important factor. As early as 1890–1891, chemists proposed that ethane molecules prefer a staggered conformation, with the two ends twisted relative to each other. **Atmospheric and Extraterrestrial** Ethane is a trace gas in Earth’s atmosphere, with a current sea-level concentration of 0.5 ppb. Its global levels have varied over time, likely due to flaring at natural gas fields. Emissions declined from 1984 to 2010, but increased shale gas production at the Bakken formation in the U.S. has slowed that decline by half. Though ethane is a greenhouse gas, it is far less abundant than methane, has an atmospheric lifetime of only a few months (compared to over a decade for methane), and is less efficient at absorbing radiation per unit mass. Its global warming potential mainly arises from its conversion to methane in the atmosphere. Ethane has been detected as a trace component in the atmospheres of all four giant planets and in that of Saturn’s moon Titan. On Titan and the giant planets, ethane forms when ultraviolet photons (wavelengths shorter than 160 nm) break methane into a methyl radical and a hydrogen atom; two methyl radicals then combine to produce ethane. In Earth’s atmosphere, hydroxyl radicals convert ethane to methanol vapor, with a half-life of about three months. It is thought that ethane produced on Titan rains onto the surface, accumulating over time into hydrocarbon seas covering much of the polar regions. In mid-2005, the Cassini orbiter discovered Ontario Lacus in Titan’s south polar region. Further infrared spectroscopic data from July 2008 provided additional evidence for liquid ethane there. Several larger lakes, including Ligeia Mare and Kraken Mare, were found near Titan’s north pole using radar data from Cassini; these are believed to be filled mainly with a mixture of liquid ethane and methane. In 1996, ethane was detected in Comet Hyakutake, and it has since been found in other comets. Its presence in these distant solar system bodies suggests ethane may be a primordial component of the solar nebula from which the Sun and planets formed.