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Jai Ganesh

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Acetylene

Acetylene

Gist

Acetylene is the simplest alkyne, a colorless and flammable gas with the chemical formula C2H2.. Its structure is two carbon atoms joined by a triple bond, and it is used as a fuel for oxy-acetylene torches for welding and cutting, and as a chemical building block in the production of plastics and other compounds. It is unstable in pure form and is stored under pressure dissolved in a solvent like acetone for safety. 

Acetylene's primary uses are in oxy-acetylene welding and cutting due to its extremely hot flame, and as a chemical feedstock for producing plastics like PVC, synthetic rubber, and other chemicals such as acetaldehyde, acrylonitrile, and acetic acid. Other applications include brazing, chemical synthesis for vitamins and solvents, and historically, as a source for portable lighting. 

Summary

Acetylene (systematic name: ethyne) is a chemical compound with the formula C2H2 and structure HC≡CH. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure form and thus is usually handled as a solution.[9] Pure acetylene is odorless, but commercial grades usually have a marked odor due to impurities such as divinyl sulfide and phosphine.

As an alkyne, acetylene is unsaturated because its two carbon atoms are bonded together in a triple bond. The carbon–carbon triple bond places all four atoms in the same straight line, with CCH bond angles of 180°. The triple bond in acetylene results in a high energy content that is released when acetylene is burned.

Discovery

Acetylene was discovered in 1836 by Edmund Davy, who identified it as a "new carburet of hydrogen". It was an accidental discovery while attempting to isolate potassium metal. By heating potassium carbonate with carbon at very high temperatures, he produced a residue of what is now known as potassium carbide, (K2C2), which reacted with water to release the new gas. It was rediscovered in 1860 by French chemist Marcellin Berthelot, who coined the name acétylène. Berthelot's empirical formula for acetylene (C4H2), as well as the alternative name "quadricarbure d'hydrogène" (hydrogen quadricarbide), were incorrect because many chemists at that time used the wrong atomic mass for carbon (6 instead of 12). Berthelot was able to prepare this gas by passing vapours of organic compounds (methanol, ethanol, etc.) through a red hot tube and collecting the effluent. He also found that acetylene was formed by sparking electricity through mixed cyanogen and hydrogen gases. Berthelot later obtained acetylene directly by passing hydrogen between the poles of a carbon arc.

Details

Acetylene is the simplest and best-known member of the hydrocarbon series containing one or more pairs of carbon atoms linked by triple bonds, called the acetylenic series, or alkynes. It is a colourless flammable gas widely used as a fuel in oxyacetylene welding and the cutting of metals and as raw material in the synthesis of many organic chemicals and plastics; its chemical formula is C2H2.

Pure acetylene is a colourless gas with a pleasant odour; as prepared from calcium carbide, it usually contains traces of phosphine that cause an unpleasant garliclike odour. Acetylene can be decomposed to its elements with the liberation of heat. The decomposition may or may not give rise to explosion, depending on conditions. Pure acetylene under pressure in excess of about 1.05 kilograms per square centimetre (15 pounds per square inch) or in liquid or solid form explodes with extreme violence.

Mixtures of air and acetylene are explosive over a wide range, from about 2.5 percent air in acetylene to about 12.5 percent acetylene in air. When burned with the correct amount of air, acetylene gives a pure white light, and for this reason it was at one time used for illumination in locations where electric power was not available—e.g., buoys, miners’ lamps, and road signals. The combustion of acetylene produces a large amount of heat, and, in a properly designed torch, the oxyacetylene flame attains the highest flame temperature (about 3,300 °C, or 6,000 °F) of any known mixture of combustible gases.

The hydrogen atoms in acetylene can be replaced by metallic elements to form acetylides—e.g., acetylides of silver, copper, or sodium. The acetylides of silver, copper, mercury, and gold are detonated by heat, friction, or shock. In addition to its reactive hydrogen atom, the carbon–carbon triple bond can readily add halogens, halogen acids, hydrogen cyanide, alcohols, amines, and amides. Acetylene can also add to itself or to aldehydes and ketones. Many of the reactions mentioned here are used for the commercial manufacture of various industrial and consumer products, such as acetaldehyde, the synthetic rubber neoprene, water-base paints, vinyl fabric and floor coverings, dry-cleaning solvents, and aerosol insecticide sprays. Acetylene is produced by any of three methods: by reaction of water with calcium carbide, by passage of a hydrocarbon through an electric arc, or by partial combustion of methane with air or oxygen.

Additional Information

Acetylene is the simplest member of alkyne hydrocarbon derivatives. In the first half of the 20th century acetylene was the most important of all starting materials for organic synthesis. Acetylene is a colorless, combustible gas with a distinctive odor. When acetylene is liquefied, compressed, heated, or mixed with air, it becomes highly explosive. As a result special precautions are required during its production and handling. The most common use of acetylene is as a raw material for the production of various organic chemicals including 1,4-butanediol, which is widely used in the preparation of polyurethane and polyester plastics. The second most common use is as the fuel component in oxy-acetylene welding and metal cutting. Some commercially useful acetylene compounds include acetylene black, which is used in certain dry-cell batteries, and acetylenic alcohols, which are used in the synthesis of vitamins.

Acetylene was discovered in 1836, when Edmund Davy was experimenting with potassium carbide. One of his chemical reactions produced a flammable gas, which is now known as acetylene. In 1859, Marcel Morren successfully generated acetylene when he used carbon electrodes to strike an electric arc in an atmosphere of hydrogen. The electric arc tore carbon atoms away from the electrodes and bonded them with hydrogen atoms to form acetylene molecules. He called this gas carbonized hydrogen.

By the late 1800s, a method had been developed for making acetylene by reacting calcium carbide with water. This generated a controlled flow of acetylene that could be combusted in air to produce a brilliant white light. Carbide lanterns were used by miners and carbide lamps were used for street illumination before the general availability of electric lights. In 1897, Georges Claude and A. Hess noted that acetylene gas could be safely stored by dissolving it in acetone. Nils Dalen used this new method in 1905 to develop long-burning, automated marine and railroad signal lights. In 1906, Dalen went on to develop an acetylene torch for welding and metal cutting.

Between 1960 and 1970, when worldwide acetylene production peaked, it served as the primary feedstock for a wide variety of commodity and specialty chemicals. Advances in olefin derivatives technology are related to the safety aspects of acetylene use, but mostly loss of cost-competitiveness, reduced and effectively limited the importance of acetylene. Now, with the current rise in crude oil prices, acetylene is finding a new place in the chemical industry.

Acetylene is the only petrochemical produced in significant quantity which contains a triple bond, and is a major intermediate species. The usefulness of acetylene is partly due to the variety of additional reactions which its triple bond undergoes, and partly due to the fact that its weakly acidic hydrogen atoms are replaceable by reaction with strong bases to form acetylide salts. However, acetylene is not easily shipped, and as a consequence its consumption is close to the point of origin.

However, acetylene was largely replaced by olefin feedstocks, such as ethylene and propylene, because of its high cost of production and the safety issues of handling acetylene at high pressures. Its use has largely been eliminated, except for the continued, and in some instances, growing production of vinyl chloride monomer, 1,4-butanediol, and carbon black. Up until the 1970s, acetylene was a basic chemical raw material used for the production of a wide range of chemicals.

Currently, there are several routes to acetylene. Hydrocarbon derivatives are the major feedstocks in the United States and Western Europe, either in the form of natural gas in partial oxidation processes or as byproducts in ethylene production. However, coal is becoming an ever increasing source of acetylene in countries with plentiful and cheap coal supplies, such as China, for the production of vinyl chloride and this source of lower cost acetylene may prove to be the impetus for returning acetylene to its place as a major chemical feedstock, especially in respect of the current and projected high oil prices and improvements in the safety, cost, and environmental protection of the calcium carbide process for the production of acetylene.

The resurgence of the use of acetylene for chemicals production will depend upon the relative cost of acetylene versus the more commonly used feedstocks. The technologies for the chemicals production are well known and have been improved since the heyday of acetylene. More importantly, the process technology to produce acetylene has been greatly improved and optimized, and now can offer attractive competitiveness in the right situations.

The classic commercial route to acetylene, first developed in the late 1800s, is the calcium carbide route in which lime is reduced by carbon (in the form of coke) in an electric furnace to yield calcium carbide. During this process a considerable amount of heat is produced, which is removed to prevent the acetylene from exploding. This reaction can occur via wet or dry processes depending on how much water is added to the reaction process. The calcium carbonate is first converted into calcium oxide and the coal into coke. The two are then reacted with each other to form calcium carbide and carbon monoxide:

CaO + 3C → CaC2 + CO

The calcium carbide is then hydrolyzed to produce acetylene:

CaC2 + 2H2O → C2H2 + Ca(OH)2

Acetylene can also be manufactured by the partial oxidation (partial combustion) combustion of methane with oxygen. The process employs a homogeneous gas phase hydrogen halide catalyst other than hydrogen fluoride to promote the pyrolytic oxidation of methane. The homogeneous gas phase catalyst employed can also consist of a mixture of gaseous hydrogen halide and gaseous halogen, or a halogen gas.

The electric arc or plasma pyrolysis of coal can also be used to produce acetylene. The electric arc process involves a 1 megawatt arc plasma reactor which utilizes a DC electric arc to generate and maintain a hydrogen plasma. The coal is then fed into the reactor and is heated to a high temperature as it passes through the plasma. It is then partially gasified to yield acetylene, hydrogen, carbon monoxide, hydrogen cyanide, and several hydrocarbon derivatives.

Acetylene can also be produced as a byproduct of ethylene steam cracking. The use of acetylene as a commodity feedstock decreased due to the competition of cheaper, more readily accessible and workable olefin derivatives when these olefin derivatives were produced from low cost crude oil products. With the rising cost of crude oil, natural gas, and the associated olefin derivative feedstocks (such as naphtha, ethane, propane, etc.) the olefin derivatives prices are no longer low enough to preclude the possibility of using acetylene. Additionally, regional shortages of these olefin derivatives and their feedstocks have forced the search for alternate routes to the commodity chemicals.

Acetylene is used as a special fuel gas (oxyacetylene torches) and as a chemical raw material. Historically, acetylene has been used to produce many important chemicals, such as (listed alphabetically): acetaldehyde, acrylate esters, acrylonitrile, 1,4-butynediol, 1,2-dichloroethane, polyacetylene, and polydiacetylene vinyl acetate, vinyl chloride monomer, and vinyl ether. Based on its availability, its many uses and prospective uses, acetylene is definitely an interesting possibility going forward, if available at competitive cost.

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