Acetylene

"HCCH" redirects here. For other uses, see HCCH (disambiguation).
Acetylene
Names
IUPAC name
Ethyne
Systematic IUPAC name
Ethyne[1]
Identifiers
74-86-2 Yes
ChEBI CHEBI:27518 Yes
ChEMBL ChEMBL116336 Yes
ChemSpider 6086 Yes
Jmol-3D images Image
KEGG C01548 Yes
UNII OC7TV75O83 Yes
UN number 1001 (dissolved)
3138 (in mixture with ethylene and propylene)
Properties
Molecular formula
 C2H2
Molar mass 26.04 g·mol−1
Density 1.097 g/L = 1.097 kg/m3
Melting point −80.8 °C (−113.4 °F; 192.3 K) Triple point at 1.27 atm
−84 °C; −119 °F; 189 K (1 atm)
slightly soluble
Vapor pressure 44.2 atm (20°C)[2]
Acidity (pKa) 25[3]
Structure
Molecular shape Linear
Thermochemistry
201 J·mol−1·K−1
Std enthalpy of
formation (ΔfHo298)
 +226.88 kJ/mol
Hazards
NFPA 704
Flammability code 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g., propane Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 3: Capable of detonation or explosive decomposition but requires a strong initiating source, must be heated under confinement before initiation, reacts explosively with water, or will detonate if severely shocked. E.g., fluorine Special hazards (white): no codeNFPA 704 four-colored diamond
4
1
3
300 °C (572 °F; 573 K)
US health exposure limits (NIOSH):
none[2]
C 2500 ppm (2662 mg/m3)[2]
N.D.[2]
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 Yes verify (what is: Yes/?)
Infobox references

Acetylene (systematic name: ethyne) is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne.[4] This colourless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.[5] Pure acetylene is odourless, but commercial grades usually have a marked odour due to impurities.[6]

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°. Since acetylene is a linear symmetrical molecule, it possesses the D∞h point group.[7]

Discovery

Acetylene was discovered in 1836 by Edmund Davy, who identified it as a "new carburet of hydrogen".[8][9] It was rediscovered in 1860 by French chemist Marcellin Berthelot, who coined the name "acetylene".[10] 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 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.[11][12] Commercially available acetylene gas could smell foul due to the common impurities hydrogen sulphide and phosphine. However, acetylene gas with high purity would generate a light and sweet smell.

Preparation

Today acetylene is mainly manufactured by the partial combustion of methane or appears as a side product in the ethylene stream from cracking of hydrocarbons. Approximately 400,000 tonnes are produced by this method annually.[5] Its presence in ethylene is usually undesirable because of its explosive character and its ability to poison Ziegler-Natta catalysts. It is selectively hydrogenated into ethylene, usually using Pd–Ag catalysts.[13]

Until the 1950s, when oil supplanted coal as the chief source of reduced carbon, acetylene (and the aromatic fraction from coal tar) was the main source of organic chemicals in the chemical industry. It was prepared by the hydrolysis of calcium carbide, a reaction discovered by Friedrich Wöhler in 1862[14] and still familiar to students:

CaC2 + 2H2O → Ca(OH)2 + C2H2

Calcium carbide production requires extremely high temperatures, ~2000 °C, necessitating the use of an electric arc furnace. In the US, this process was an important part of the late-19th century revolution in chemistry enabled by the massive hydroelectric power project at Niagara Falls.[15]

Bonding

In terms of valence bond theory, in each carbon atom the 2s orbital hybridizes with one 2p orbital thus forming an sp hybrid. The other two 2p orbitals remain unhybridized. The two ends of the two sp hybrid orbital overlap to form a strong σ valence bond between the carbons, while on each of the other two ends hydrogen atoms attach also by σ bonds. The two unchanged 2p orbitals form a pair of weaker π bonds.[16]

Physical properties

Changes of state

At atmospheric pressure, acetylene cannot exist as a liquid and does not have a melting point. The triple point on the phase diagram corresponds to the melting point (−80.8 °C) at the minimum pressure at which liquid acetylene can exist (1.27 atm). At temperatures below the triple point, solid acetylene can change directly to the vapour (gas) by sublimation. The sublimation point at atmospheric pressure is −84 °C.

Other

The adiabatic flame temperature in air at atmospheric pressure is 2534 °C.

Acetylene gas can be dissolved in acetone or dimethylformamide in room temperature and 1 atm.

Reactions

Main article: Alkyne

One new application is the conversion of acetylene to ethylene for use in making a variety of polyethylene plastics. An important reaction of acetylene is its combustion, the basis of the acetylene welding technologies. Otherwise, its major applications involve its conversion to acrylic acid derivatives.[5]

Compared to most hydrocarbons, acetylene is relatively acidic, though it is still much less acidic than water or ethanol. Thus it reacts with strong bases to form acetylide salts. For example, acetylene reacts with sodium amide in liquid ammonia to form sodium acetylide, and with butyllithium in cold THF to give lithium acetylide.[17]

Acetylides of heavy metals are easily formed by reaction of acetylene with the metal ions. Several, e.g., silver acetylide (Ag2C2) and copper acetylide (Cu2C2), are powerful and very dangerous explosives.[18]

Reppe chemistry

Walter Reppe discovered that in the presence of metal catalysts, acetylene can react to give a wide range of industrially significant chemicals.[19][20]

1,4-Butynediol is produced industrially in this way from formaldehyde and acetylene.
\mathrm{ Fe(CO)_5 + 4C_2H_2 + 2H_2O \xrightarrow[basic\ conditions]{ \begin{array}{c} 50-80\ ^{\circ} \mathrm{C} \\ 20-25\ \mathrm{atm} \end{array} } 2C_6H_4(OH)_2 + FeCO_3 }

Applications

Welding

Approximately 20 percent of acetylene is supplied by the industrial gases industry for oxyacetylene gas welding and cutting due to the high temperature of the flame; combustion of acetylene with oxygen produces a flame of over 3,600 K (3,300 °C, 6,000 °F), releasing 11.8 kJ/g. Oxyacetylene is the hottest burning common fuel gas.[22] Acetylene is the third hottest natural chemical flame after dicyanoacetylene's 5260 K (4990 °C, 9010 °F) and cyanogen at 4798 K (4525 °C, 8180 °F). Oxy-acetylene welding was a very popular welding process in previous decades; however, the development and advantages of arc-based welding processes have made oxy-fuel welding nearly extinct for many applications. Acetylene usage for welding has dropped significantly. On the other hand, oxy-acetylene welding equipment is quite versatile – not only because the torch is preferred for some sorts of iron or steel welding (as in certain artistic applications), but also because it lends itself easily to brazing, braze-welding, metal heating (for annealing or tempering, bending or forming), the loosening of corroded nuts and bolts, and other applications. Bell Canada cable repair technicians still use portable acetylene fuelled torch kits as a soldering tool for sealing lead sleeve splices in manholes and in some aerial locations. Oxyacetylene welding may also be used in areas where electricity is not readily accessible. As well, oxy-fuel cutting is still very popular and oxy-acetylene cutting is utilized in nearly every metal fabrication shop. For use in welding and cutting, the working pressures must be controlled by a regulator, since above 15 psi,[23] if subjected to a shockwave (caused for example by a flashback),[24] acetylene will decompose explosively into hydrogen and carbon.

Acetylene fuel container/burner as used in the island of Bali

Portable lighting

Calcium carbide was used to generate acetylene used in the lamps for portable or remote applications. It was used for miners and cavers before the widespread use of incandescent lighting; or many years later low-power/high-lumen LED lighting; and is still used by mining industries in some nations without workplace safety laws. It was also used as an early light source for lighthouses.

Niche applications

In 1881, the Russian chemist Mikhail Kucherov[25] described the hydration of acetylene to acetaldehyde using catalysts such as mercury(II) bromide. Before the advent of the Wacker process, this reaction was conducted on an industrial scale.[26]

The polymerization of acetylene with Ziegler-Natta catalysts produces polyacetylene films. Polyacetylene, a chain of CH centres with alternating single and double bonds, was the one of first discovered organic semiconductors. Its reaction with iodine produces a highly electrically conducting material. Although such materials are not useful, these discoveries led to the developments of organic semiconductors, as recognized by the Nobel Prize in Chemistry in 2000 to Alan J. Heeger, Alan G MacDiarmid, and Hideki Shirakawa.[5]

In the early 20th Century acetylene was widely used for illumination, including street lighting in some towns.[27] Most early automobiles used carbide lamps before the adoption of electric headlights.

Acetylene is sometimes used for carburization (that is, hardening) of steel when the object is too large to fit into a furnace.[28]

Acetylene is used to volatilize carbon in radiocarbon dating. The carbonaceous material in an archeological sample is treated with lithium metal in a small specialized research furnace to form lithium carbide (also known as lithium acetylide). The carbide can then be reacted with water, as usual, to form acetylene gas to be fed into mass spectrometer to measure the isotopic ratio of carbon-14 to carbon-12.[29]

Natural occurrence

The energy richness of the C≡C triple bond and the rather high solubility of acetylene in water make it a suitable substrate for bacteria, provided an adequate source is available. A number of bacteria living on acetylene have been identified. The enzyme acetylene hydratase catalyzes the hydration of acetylene to give acetaldehyde.[30]

C2H2 + H2O → CH3CHO

Acetylene is a moderately common chemical in the universe, often associated with the atmospheres of gas giants.[31] One curious discovery of acetylene is on Enceladus, a moon of Saturn. Natural acetylene is believed to form from catalytic decomposition of long chain hydrocarbons at temperatures of 1,770 K and above. Since such temperatures are highly unlikely on such a small distant body, this discovery is potentially suggestive of catalytic reactions within that moon, making it a promising site to search for prebiotic chemistry.[32][33]

Safety and handling

Acetylene is not especially toxic but, when generated from calcium carbide, it can contain toxic impurities such as traces of phosphine and arsine, which give it a distinct garlic-like smell. It is also highly flammable, as most light hydrocarbons, hence its use in welding. Its most singular hazard is associated with its intrinsic instability, especially when it is pressurized: under certain conditions acetylene can react in an exothermic addition-type reaction to form a number of products, typically benzene and/or vinylacetylene, possibly in addition to carbon and hydrogen. Consequently, acetylene, if initiated by intense heat or a shockwave, can decompose explosively if the absolute pressure of the gas exceeds about 200 kPa (29 psi). Most regulators and pressure gauges on equipment report gauge pressure and the safe limit for acetylene therefore is 101 kPagage or 15 psig.[34][35] It is therefore supplied and stored dissolved in acetone or dimethylformamide (DMF),[36] contained in a gas cylinder with a porous filling (Agamassan), which renders it safe to transport and use, given proper handling. Copper catalyses the decomposition of acetylene and as a result acetylene should not be transported in copper pipes. Brass pipe fittings should also be avoided.

References

  1. ↑ Acyclic Hydrocarbons. Rule A-3. Unsaturated Compounds and Univalent Radicals, IUPAC Nomenclature of Organic Chemistry
  2. ↑ 2.0 2.1 2.2 2.3 "NIOSH Pocket Guide to Chemical Hazards #0008". National Institute for Occupational Safety and Health (NIOSH).
  3. ↑ , Gas Encyclopaedia, Air Liquide
  4. ↑ R.H.Petrucci, W.S.Harwood and F.G.Herring "General Chemistry", 8th edn.(Prentice-Hall 2002), p.1072
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Pässler, Peter; Hefner, Werner; Buckl, Klaus; Meinass, Helmut; Meiswinkel, Andreas; Wernicke, Hans-Jürgen; Ebersberg, Günter; Müller, Richard; Bässler, Jürgen; Behringer, Hartmut; Mayer, Dieter (2008). "Acetylene". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a01_097.pub3. ISBN 3527306730.. Article Online Posting Date: 15 October 2008
  6. ↑ Compressed Gas Association (1995) Material Safety and Data Sheet – Acetylene.
  7. ↑ Housecroft, C. E.; Sharpe, A. G. (2008). Inorganic Chemistry (3rd ed.). Prentice Hall. pp. 94–95. ISBN 978-0131755536.
  8. ↑ Edmund Davy (August 1836) "Notice of a new gaseous bicarburet of hydrogen,", Report of the Sixth Meeting of the British Association for the Advancement of Science … , 5 : 62-63.
  9. ↑ Miller, S.A. (1965). Acetylene: Its Properties, Manufacture and Uses 1. Academic Press Inc.
  10. ↑ Bertholet (1860) "Note sur une nouvelle série de composés organiques, le quadricarbure d'hydrogène et ses dérivés" (Note on a new series of organic compounds, tetra-carbon hydride and its derivatives), Comptes rendus, series 3, 50 : 805–808. (Note: Berthelot's empirical formula for acetylene (C4H2) was incorrect because chemists at that time used the wrong atomic mass for carbon (6 instead of 12).)
  11. ↑ Berthelot (1862) "Synthèse de l'acétylène par la combinaison directe du carbone avec l'hydrogène" (Synthesis of acetylene by the direct combination of carbon with hydrogen), Comptes rendus, series 3, 54 : 640-644.
  12. ↑ Acetylene
  13. ↑ Acetylene: How Products are Made
  14. ↑ Wohler (1862) "Bildung des Acetylens durch Kohlenstoffcalcium" (Formation of actylene by calcium carbide), Annalen der Chemie und Pharmacie, 124 : 220.
  15. ↑ Freeman, Horace (1919). "Manufacture of Cyanamide". The Chemical News and the Journal of Physical Science 117: 232. Retrieved 23 December 2013.
  16. ↑ Organic Chemistry 7th ed. by J. McMurry, Thomson 2008
  17. ↑ Midland, M. M.; McLoughlin, J. I.; Werley, Ralph T. (Jr.) (1990). "Preparation and Use of Lithium Acetylide: 1-Methyl-2-ethynyl-endo-3,3-dimethyl-2-norbornanol". Org. Synth. 68: 14.; Coll. Vol. 8, p. 391
  18. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419.
  19. ↑ Peter Pässler, Werner Hefner, Klaus Buckl, Helmut Meinass, Andreas Meiswinkel, Hans-Jürgen Wernicke, Günter Ebersberg, Richard Müller, Jürgen Bässler, Hartmut Behringer and Dieter Mayer (2007). "Acetylene". Ullmann's Encyclopedia of Industrial Chemistry: pg. 44. doi:10.1002/14356007.a01_097.pub2. Retrieved 26 December 2013.
  20. ↑ 20.0 20.1 Reppe, Walter; Kutepow, N; and Magin, A (1969). "Cyclization of Acetylenic Compounds". Angewandte Chemie International Edition in English 8 (10): 727–733. doi:10.1002/anie.196907271. Retrieved 26 December 2013.
  21. ↑ Takashi Ohara, Takahisa Sato, Noboru Shimizu, Günter Prescher, Helmut Schwind, Otto Weiberg, Klaus Marten and Helmut Greim (2003). "Acrylic Acid and Derivatives". Ullmann's Encyclopedia of Industrial Chemistry: pg. 7. doi:10.1002/14356007.a01_161.pub2. Retrieved 26 December 2013.
  22. ↑ "Acetylene". Products and Supply > Fuel Gases. Linde. Retrieved November 30, 2013.
  23. ↑ Acetylene - Properties, Purity and Packaging - Acetylene is simplest member of unsaturated hydrocarbons called alkynes or acetylenes. Most important of all starting materials ...
  24. ↑ ESAB Oxy-acetylene welding handbook - Acetylene properties
  25. ↑ Kutscheroff, M. (1881). "Ueber eine neue Methode direkter Addition von Wasser (Hydratation) an die Kohlenwasserstoffe der Acetylenreihe". Berichte der deutschen chemischen Gesellschaft 14: 1540–1542. doi:10.1002/cber.188101401320.
  26. ↑ Dmitry A. Ponomarev and Sergey M. Shevchenko (2007). "Hydration of Acetylene: A 125th Anniversary" (PDF). J. Chem. Ed. 84 (10): 1725. doi:10.1021/ed084p1725.
  27. ↑ The 100 most important chemical compounds: a reference guide
  28. ↑ "Acetylene". Products and Services. BOC. Archived from the original on May 17, 2006.
  29. ↑ Geyh, Mebus (1990). "Radiocarbon dating problems using acetylene as counting gas". Radiocarbon 32 (3): 321–324. doi:10.2458/azu_js_rc.32.1278. Retrieved 26 December 2013.
  30. ↑ ten Brink, Felix (2014). "Chapter 2. Living on acetylene. A Primordial Energy Source". In Peter M.H. Kroneck and Martha E. Sosa Torres. The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment. Metal Ions in Life Sciences 14. Springer. pp. 15–35. doi:10.1007/978-94-017-9269-1_2.
  31. ↑ "Precursor to Proteins and DNA found in Stellar Disk" (Press release). W. M. Keck Observatory. 20 December 2005.
  32. ↑ Emily Lakdawalla (17 March 2006). "LPSC: Wednesday afternoon: Cassini at Enceladus". The Planetary Society.
  33. ↑ John Spencer and David Grinspoon (25 January 2007). "Planetary science: Inside Enceladus". Nature 445 (7126): 376–377. doi:10.1038/445376b. PMID 17251967.
  34. ↑ Korzun, Mikołaj (1986). 1000 słów o materiałach wybuchowych i wybuchu. Warszawa: Wydawnictwo Ministerstwa Obrony Narodowej. ISBN 83-11-07044-X. OCLC 69535236.
  35. ↑ "Acetylene Specification". CFC StarTec LLC. Retrieved 2012-05-02.
  36. ↑ Downie, N. A. (1997). Industrial Gases. London; New York: Blackie Academic & Professional. ISBN 978-0-7514-0352-7.

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