544.0 kg/m3 (liquid at -88,5 °C)
206 kg/m3 (at critical point 305.322 K)
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Melting point −182.8 °C; −296.9 °F; 90.4 K Boiling point −88.5 °C; −127.4 °F; 184.6 K Critical point (T, P) 305.32 K (32.17 °C; 89.91 °F) 48.714 bars (4,871.4 kPa) 56.8 mg/L[4] Vapor pressure 3. MPa (at 21.1 °C) Henry's lawEthane ( ETH-ayn, EE-thayn) is a naturally occurring organic chemical compound with chemical formula C
2H
6. 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. The ethyl group is formally, although rarely practically, derived from ethane.
Ethane was first synthesised in 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.[6] The process is now called Kolbe electrolysis:
During the period –, 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)[7] and ethyl iodide[8] with potassium metal, and, as did Faraday, by the electrolysis of aqueous acetates. They mistook the product of these reactions for the methyl radical ( CH3), of which ethane ( C2H6) is a dimer.
This error was corrected in by Carl Schorlemmer, who showed that the product of all these reactions was in fact ethane.[9] Ethane was discovered dissolved in Pennsylvanian light crude oil by Edmund Ronalds in .[10][11]
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]
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 –, chemists suggested that ethane molecules preferred the staggered conformation with the two ends of the molecule askew from each other.[22][23][24][25]
Ethane occurs as a trace gas in the Earth's atmosphere, currently having a concentration at sea level of 0.5 ppb.[26] Global ethane quantities have varied over time, likely due to flaring at natural gas fields.[27] Global ethane emission rates declined from to ,[27] though increased shale gas production at the Bakken Formation in the U.S. has arrested the decline by half.[28][29]
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:
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-, the Cassini orbiter discovered Ontario Lacus in Titan's south polar regions. Further analysis of infrared spectroscopic data presented in July [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 , 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 , 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]
The reactions of ethane involve chiefly free radical reactions. Ethane can react with the halogens, especially chlorine and bromine, by free-radical halogenation. This reaction proceeds through the propagation of the ethyl radical:[36]
The combustion of ethane releases .7 kJ/mol, or 51.9 kJ/g, of heat, and produces carbon dioxide and water according to the chemical equation:
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:
Such oxidative dehydrogenation reactions are relevant to the production of ethylene.[37]
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 s, 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 petrochemical feedstock 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.
The chief use of ethane is the production of ethylene (ethene) by steam cracking. Steam cracking of ethane is fairly selective for ethylene, while the steam cracking of heavier hydrocarbons yields a product mixture poorer in ethylene and richer in heavier alkenes (olefins), such as propene (propylene) and butadiene, and in aromatic hydrocarbons.
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Ethane 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 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.
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.
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.
Ethane is not a carcinogen.[40]
The compounds of carbon with hydrogen are called hydrocarbons. In organic chemistry, depending on the chemical structure of such compounds, homologous series – or families – of chemical compounds are distinguised, characterised by a shared general formula. Ethane belongs to the homologous series of alkanes, or saturated hydrocarbons (also called paraffin hydrocarbons). They are characterised by the fact that all bonds between carbon and hydrogen atoms are saturated. The molecular formula of ethane is C2H6. Each of the carbon atoms in the molecule adopts sp3 hybridisation and is tetravalent. Individual atoms are connected to each other by covalent bonds. The two carbon atoms in an ethane molecule bond directly to each other with a single bond. Moreover, there are three bonds from each carbon atom to which hydrogen atoms are attached. Unlike unsaturated counterparts of ethane, ethene and ethyne (also known as ethylene and acetylene), ethane molecules do not show cis-trans isomerism that are common to these compounds and involve different spatial configurations of substituents around the carbon ring.
Small amounts of ethane are found in natural gas. However, it should be noted that compared to the main constituent of natural gas, namely methane, ethane constitutes only about 1% of it. Ethane also accompanies oil deposits. Other sources of ethane include biomass pyrolysis products and biogas generated in specifically designed anaerobic digesters. On an industrial scale, it is also extracted by steam cracking, refining and other crude oil processing.
On a larger scale, ethane is obtained as a product of thermal processing of crude oil and coal. On a laboratory scale, the compound can be obtained by electrolysis reaction of a concentrated sodium acetate solution. In addition to ethane, the reaction also produces carbon dioxide. Another method used frequently is catalytic hydrogenation of ethene (a representative of alkenes). In this process, a hydrogen molecule is attached to the unsaturated double bond shared by carbon atoms. Under normal conditions, the reaction is not very efficient, so ethene hydrogenation processes are carried out in the presence of a platinum catalyst.
Ethane is a hardly reactive chemical compound. This is because all the atoms in the molecule share single bonds that are difficult to break. However, ethane derivatives, representatives of alkenes and alkynes, such as ethene and ethyne, also called acetylene, demonstrate different behaviour. Their molecules contain unsaturated double and triple bonds between carbon atoms, respectively. Since such bonds are easy to break, they make the said compounds highly reactive, as in, for example, substitution reaction.
The main chemical reaction of ethane is combustion. Depending on the conditions of combustion, i.e. the supply of air or oxygen, the products of combustion may differ:
Along with propane and butane, ethane is used as energy feedstock. Ethane is one of the main constituents of fuels for camp stoves because of its high flammability, lack of odour and high calorific value. It is less important in heating houses and apartments.
Ethane is an important feedstock in the chemical industry. It is used not only for generating energy but also in a number of industrial processes. Ethane is used, among others, for the production of ethyl alcohol, acetylene or ethylene glycol. It is also an important component in plastics manufacturing processes.
Ethane is a petrol additive. The purpose of adding ethane to petrol is to increase its octane number.
Ethane is also a component of antifreeze agents and detergents. Thanks to its properties, it may serve as a refrigerant. In such applications, it is marked as R-170. Designed for use particularly in low-temperature refrigeration. An interesting application of ethane is its use in the food industry. It accelerates ripening of certain products, especially fruits.
Ethene is a representative of the homologous series of alkenes. Two hydrogen atoms that make up the hydrocarbon chain share an unsaturated double bond. Ethene, also commonly called ethylene, is a product of catalytic dehydrogenation of ethane. Dehydrogenation is usually performed with a nickel catalyst. An unsaturated bond in the ethene molecule makes the compound highly reactive. It is easily added to the double bond of molecules such as chlorine, bromine, hydrogen chloride, hydrogen bromide, water and more. Ethene also undergoes polymerisation, which involves joining of short fragments (monomers) into long chains (polymers). The product of this catalytic process is a transparent substance called polyethylene.
Polyethylene is a basic product broadly used for making packaging, plastic bottles, wrap film, photographic films and much more. In the industry, ethene is essentially used for the production of plastics. Moreover, it serves as a substrate in the processes of obtaining such compounds as acetic acid, ethyl alcohol and ethylene glycol.
Another derivative of ethane is acetylene. It is the first representative of the homologous series of alkynes. Acetylene molecule is made up of two atoms of carbon and two atoms of hydrogen. Such a chemical structure suggests that there is a triple unsaturated bond between the carbon atoms. The bond largely determines the properties of the compound. Acetylene is an extremely reactive gas. It undergoes combustion and addition reactions (with bromine, chlorine, bromine hydrogen, among others). Like ethylene, it undergoes polymerisation reaction to form polyacetylene. In spite of its interesting functions (such as electrical conductivity), the compound is difficult to obtain. In the chemical industry, it may also be used to synthesise organic solvents.
Acetylene has a wide range of applications in various sectors of the industry. One of the most important applications is oxy-acetylene torches. They are used for metal working and cutting, as well as welding. There are also acetylene lamps in which the gas is used as an illuminant. Historically, it was produced and burned in so-called Carbide lamps. Today, this method is no longer used. High-purity acetylene is administered to patients in the form of anaesthesia.
Ethanol is a colourless liquid with a characteristic odour, astringent taste and high volatility. Ethyl alcohol is an ethane derivative containing an -OH hydroxyl group in its molecule. Ethanol is highly polar and mixes with water in any ratio. Ethanol is also a solvent for several organic compounds. Although it is a derivative of ethane, it is obtained on an industrial scale through alcoholic fermentation of raw materials that contain sugars. These may be potatoes, corn or cereals. The finished product is separated and purified by distillation. The maximum achievable concentration of ethanol is about 96% (azeotropic mixture of ethanol and water).
Ethanol is has the greatest importance in the food industry. It is used not only for the production of alcoholic beverages, but also vinegar and food flavours. In the pharmaceutical industry, it is used to make syrups and topical disinfectants. Because of its properties, ethyl alcohol is also used in the cosmetic industry to make perfumes.
Like ethanol, ethylene glycol is another alcoholic derivative of ethane. This compound is made of two atoms of carbon connected by a single bond, to which two -OH hydroxyl groups are attached. Glycol comes in the form of a thick and colourless liquid. It is easily dissolved in water. Ethylene glycol is an example of the simplest polyhydroxy alcohol. It is relatively cheap to make but its main disadvantage is that is crystallises at low temperatures.
Ethylene glycol is one of the key ingredients used in anti-freeze products for HVAC and automotive systems. The substance is also found in a number of household products, including detergents, cosmetics, paints and solvents. Other uses for ethylene glycol include the production of plastics, inks and heat carriers.
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