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Acetylene

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Acetylene

Acetylene
Acetylene
Acetylene
Acetylene – space-filling model
space-filling model of solid acetylene
Names
IUPAC name
Ethyne
Systematic IUPAC name
Ethyne[1]
Identifiers
 YesY
ChEBI  YesY
ChEMBL  YesY
ChemSpider  YesY
Jmol-3D images Image
KEGG  YesY
UNII  YesY
UN number 1001 (dissolved)
3138 (in mixture with ethylene and propylene)
Properties
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; 192.4 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
Linear
Thermochemistry
201 J·mol−1·K−1
+226.88 kJ/mol
Hazards
NFPA 704
4
1
3
300 °C (572 °F; 573 K)
US health exposure limits (NIOSH):
PEL (Permissible)
none[2]
REL (Recommended)
C 2500 ppm (2662 mg/m3)[2]
N.D.[2]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
 YesY  (: YesY/N?)

Acetylene (systematic name: ethyne) is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne.[4] This colorless 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 odorless, but commercial grades usually have a marked odor 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°.

Discovery

Acetylene was discovered in 1836 by 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.[10][11] 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.[12]

Until the 1950s, when hydrolysis of calcium carbide, a reaction discovered by Friedrich Wöhler in 1862[13] 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.[14]

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.[15]

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

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]

Formation of lithium acetylide from acetylene + BuLi

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 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. ^ a b c d
  3. ^ [1], Gas Encyclopaedia, Air Liquide
  4. ^
  5. ^ a b c d e f g . Article Online Posting Date: 15 October 2008
  6. ^ Compressed Gas Association (1995) Material Safety and Data Sheet – Acetylene
  7. ^ 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.
  8. ^
  9. ^ 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).)
  10. ^ 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.
  11. ^ Acetylene
  12. ^ Acetylene: How Products are Made
  13. ^ Wohler (1862) "Bildung des Acetylens durch Kohlenstoffcalcium" (Formation of actylene by calcium carbide), Annalen der Chemie und Pharmacie, 124 : 220.
  14. ^
  15. ^ Organic Chemistry 7th ed. by J. McMurry, Thomson 2008
  16. ^
  17. ^ ;
  18. ^
  19. ^
  20. ^ a b
  21. ^
  22. ^
  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. ^
  26. ^
  27. ^ The 100 most important chemical compounds: a reference guide
  28. ^
  29. ^
  30. ^
  31. ^
  32. ^
  33. ^
  34. ^
  35. ^
  36. ^

External links

  • Acetylene Production Plant and Detailed Process
  • Acetylene at Chemistry Comes Alive!
  • Acetylene, the Principles of Its Generation and Use at Project Gutenberg
  • Movie explaining acetylene formation from calcium carbide and the explosive limits forming fire hazards
  • Calcium Carbide & Acetylene at The Periodic Table of Videos (University of Nottingham)
  • CDC - NIOSH Pocket Guide to Chemical Hazards - Acetylene
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