Acetylene

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Acetylene
Acetylene
Acetylene
Acetylene – space-filling model
space-filling model of solid acetylene
Names
Preferred IUPAC name
Acetylene[1]
Systematic IUPAC name
Ethyne[2]
Identifiers
3D model (
JSmol
)
906677
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard
100.000.743 Edit this at Wikidata
EC Number
  • 200-816-9
210
KEGG
RTECS number
  • AO9600000
UNII
UN number
3138 (in mixture with ethylene and propylene
)
  • InChI=1S/C2H2/c1-2/h1-2H checkY
    Key: HSFWRNGVRCDJHI-UHFFFAOYSA-N checkY
  • InChI=1/C2H2/c1-2/h1-2H
    Key: HSFWRNGVRCDJHI-UHFFFAOYAY
  • C#C
Properties
C2H2
Molar mass 26.038 g·mol−1
Appearance Colorless gas
Odor Odorless
Density 1.1772 g/L = 1.1772 kg/m3 (0 °C, 101.3 kPa)[3]
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
Solubility slightly soluble in alcohol
soluble in acetone, benzene
Vapor pressure 44.2 atm (20 °C)[4]
Acidity (pKa) 25[5]
Conjugate acid
Ethynium
−20.8×10−6 cm3/mol [6]
Thermal conductivity
21.4 mW·m−1·K−1 (300 K) [6]
Structure
Linear
Thermochemistry[6]
44.036 J·mol−1·K−1
200.927 J·mol−1·K−1
Std enthalpy of
formation
fH298)
227.400 kJ·mol−1
209.879 kJ·mol−1
Std enthalpy of
combustion
cH298)
1300 kJ·mol−1
Hazards
GHS labelling:
GHS02: FlammableGHS07: Exclamation mark
Danger
H220, H336
P202, P210, P233, P261, P271, P304, P312, P340, P377, P381, P403, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 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. propaneInstability 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. hydrogen peroxideSpecial hazards (white): no code
1
4
3
300 °C (572 °F; 573 K)
Explosive limits
2.5–100%
NIOSH (US health exposure limits):
PEL (Permissible)
none[4]
REL (Recommended)
C 2500 ppm (2662 mg/m3)[4]
IDLH
(Immediate danger)
N.D.[4]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Acetylene (systematic name: ethyne) is the chemical compound with the formula C2H2 and structure H−C≡C−H. It is a hydrocarbon and the simplest alkyne.[7] 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.[8] Pure acetylene is odorless, but commercial grades usually have a marked odor due to impurities such as divinyl sulfide and phosphine.[8][9]

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°.[10]

Discovery

Acetylene was discovered in 1836 by

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.[13] 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).[14] 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.[15][16]

Preparation

Except in China acetylene production is dominated by partial combustion of natural gas.[17]

Partial combustion of hydrocarbons

Since the 1950s, acetylene has mainly been manufactured by the partial combustion of methane.[18][19][20] It is a recovered side product in production of ethylene by cracking of hydrocarbons. Approximately 400,000 tonnes were produced by this method in 1983.[18] 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 PdAg catalysts.[21]

3 CH4 + 3 O2 → C2H2 + CO + 5 H2O.

Partial combustion of methane also produces acetylene:

Dehydrogenation of alkanes

The heaviest alkanes in petroleum and natural gas are cracked into lighter molecules which are dehydrogenated at high temperature:

C2H6 → C2H2 + 2 H2
2 CH4→ C2H2+ 3 H2

This last reaction is implemented in the process of anaerobic decomposition of methane by microwave plasma. The advantage of this technology is the absence of CO2 emissions and the joint production of hydrogen as a secondary product.[22] It makes it a low-carbon technology production and also an electrified process. For 32 t of methane transformed, production of 26 t of acetylene and 6 t of hydrogen (according to stoichiometry).

Carbochemical method

Acetylene is traditionally produced by hydrolysis (reaction with water) of calcium carbide:

CaC2 + 2 H2O → Ca(OH)2 + C2H2

1 kg of calcium carbide combines with 562.5 g of water to release 350 liters of acetylene. This reaction was discovered by Friedrich Wöhler in 1862.[23]

The use of this technology has declined worldwide with the notable exception of China, with its emphasis on coal-based chemical industry. Otherwise oil has increasingly supplanted coal as the chief source of reduced carbon.[24]

Calcium carbide production requires 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.[25]

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.[26]

Since acetylene is a linear symmetrical molecule, it possesses the D∞h point group.[27]

Physical properties

Changes of state

At atmospheric pressure, acetylene cannot exist as a liquid and does not have a melting point. The

vapour (gas) by sublimation. The sublimation point at atmospheric pressure is −84.0 °C.[28]

Other

At room temperature, the solubility of acetylene in acetone is 27.9 g per kg. For the same amount of dimethylformamide (DMF), the solubility is 51 g. At 20.26 bar, the solubility increases to 689.0 and 628.0 g for acetone and DMF, respectively. These solvents are used in pressurized gas cylinders.[29]

Applications

Welding

Approximately 20% of acetylene is supplied by the

decomposes explosively into hydrogen and carbon.[31]

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

Chemicals

Acetylene, despite its simplicity, is not used for many industrial processes.

One of the major chemical applications is ethynylation of formaldehyde.[8] Acetylene adds to aldehydes and ketones to form α-ethynyl alcohols:

The reaction gives butynediol, with propargyl alcohol as the by-product. Copper acetylide is used as the catalyst.[32][33]

In addition to ethynylation, acetylene reacts with

glasses, paints, resins, and polymers. Except in China, use of acetylene as a chemical feedstock has declined by 70% from 1965 to 2007 owing to cost and environmental considerations.[34]

Historical uses

Prior to the widespread use of petrochemicals, coal-derived acetylene was a building block for several industrial chemicals. Thus acetylene can be hydrated to give acetaldehyde, which in turn can be oxidized to acetic acid. Processes leading to acrylates were also commercialized. Almost all of these processes became obsolete with the availability of petroleum-derived ethylene and propylene.[35]

Niche applications

In 1881, the Russian chemist Mikhail Kucherov[36] 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.[37]

The

Alan G MacDiarmid, and Hideki Shirakawa.[8]

In the 1920s, pure acetylene was experimentally used as an

Acetylene is sometimes used for

carburization (that is, hardening) of steel when the object is too large to fit into a furnace.[39]

Acetylene is used to volatilize carbon in

mass spectrometer to measure the isotopic ratio of carbon-14 to carbon-12.[40]

Acetylene combustion produces a strong, bright light and the ubiquity of

fire hazard, and so acetylene has been replaced, first by incandescent lighting and many years later by low-power/high-lumen LEDs. Nevertheless, acetylene lamps remain in limited use in remote or otherwise inaccessible areas and in countries with a weak or unreliable central electric grid.[44]

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.[45] A number of bacteria living on acetylene have been identified. The enzyme acetylene hydratase catalyzes the hydration of acetylene to give acetaldehyde:[46]

C2H2 + H2O → CH3CHO

Acetylene is a moderately common chemical in the universe, often associated with the atmospheres of gas giants.[47] 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 1,700 K (1,430 °C; 2,600 °F) 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.[48][49]

Reactions

Vinylation reactions

In

vinylcarbazole are produced industrially by vinylation of 2-pyrrolidone and carbazole.[29][8]

The hydration of acetylene is a vinylation reaction, but the resulting vinyl alcohol isomerizes to

hydrochlorination vs the oxychlorination
of ethylene.

transvinylations.[50] Higher esters of vinyl acetate have been used in the synthesis of vinyl formate
.

Organometallic chemistry

Acetylene and its derivatives (2-butyne, diphenylacetylene, etc.) form complexes with transition metals. Its bonding to the metal is somewhat similar to that of ethylene complexes. These complexes are intermediates in many catalytic reactions such as alkyne trimerisation to benzene, tetramerization to cyclooctatetraene,[8] and carbonylation to hydroquinone:[51]

Fe(CO)5 + 4 C2H2 + 2 H2O → 2 C6H4(OH)2 + FeCO3 at basic conditions (50–80 °C, 20–25 atm).

Metal

aqueous solutions with ease due to a favorable solubility equilibrium.[52]

Acid-base reactions

Acetylene has a pKa of 25, acetylene can be deprotonated by a superbase to form an acetylide:[52]

Various

organometallic[53] and inorganic[54]
reagents are effective.

The New acetylene plant of BASF, commissioned in 2020

Hydrogenation

Acetylene can be

semihydrogenated to ethylene, providing a feedstock for a variety of polyethylene
plastics. Halogens add to the triple bond.

Safety and handling

Acetylene is not especially toxic, but when generated from

gauge pressure, and the safe limit for acetylene therefore is 101 kPagage, or 15 psig.[55][56] It is therefore supplied and stored dissolved in acetone or dimethylformamide (DMF),[56][57][58] contained in a gas cylinder with a porous filling (Agamassan), which renders it safe to transport and use, given proper handling. Acetylene cylinders should be used in the upright position to avoid withdrawing acetone during use.[59]

Information on safe storage of acetylene in upright cylinders is provided by the OSHA,[60][61] Compressed Gas Association,[56] United States Mine Safety and Health Administration (MSHA),[62] EIGA,[59] and other agencies.

Copper catalyses the decomposition of acetylene, and as a result acetylene should not be transported in copper pipes.[63]

Cylinders should be stored in an area segregated from oxidizers to avoid exacerbated reaction in case of fire/leakage.[56][61] Acetylene cylinders should not be stored in confined spaces, enclosed vehicles, garages, and buildings, to avoid unintended leakage leading to explosive atmosphere.[56][61] In the US, National Electric Code (NEC) requires consideration for hazardous areas including those where acetylene may be released during accidents or leaks.[64] Consideration may include electrical classification and use of listed Group A electrical components in US.[64] Further information on determining the areas requiring special consideration is in NFPA 497.[65] In Europe, ATEX also requires consideration for hazardous areas where flammable gases may be released during accidents or leaks.[59]

References

  1. . The name acetylene is retained for the compound HC≡CH. It is the preferred IUPAC name, but substitution of any kind is not allowed; however, in general nomenclature, substitution is allowed, for example fluoroacetylene [fluoroethyne (PIN)], but not by alkyl groups or any other group that extends the carbon chain, nor by characteristic groups expressed by suffixes.
  2. ^ Acyclic Hydrocarbons. Rule A-3. Unsaturated Compounds and Univalent Radicals Archived 10 October 2000 at the Wayback Machine, IUPAC Nomenclature of Organic Chemistry
  3. ^ Record of Acetylene in the GESTIS Substance Database of the Institute for Occupational Safety and Health
  4. ^ a b c d NIOSH Pocket Guide to Chemical Hazards. "#0008". National Institute for Occupational Safety and Health (NIOSH).
  5. ^ "Acetylene – Gas Encyclopedia Air Liquide". Air Liquide. Archived from the original on 4 May 2022. Retrieved 27 September 2018.
  6. ^
    OCLC 930681942. Archived from the original on 4 May 2022. Retrieved 4 May 2022.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link
    )
  7. ^ R. H. Petrucci; W. S. Harwood; F. G. Herring (2002). General Chemistry (8th ed.). Prentice-Hall. p. 1072.
  8. ^ .
  9. ^ Compressed Gas Association (1995) Material Safety and Data Sheet – Acetylene Archived 11 July 2012 at the Wayback Machine
  10. .
  11. ^ Edmund Davy (August 1836) "Notice of a new gaseous bicarburet of hydrogen" Archived 6 May 2016 at the Wayback Machine, Report of the Sixth Meeting of the British Association for the Advancement of Science …, 5: 62–63.
  12. ^ Miller, S. A. (1965). Acetylene: Its Properties, Manufacture and Uses. Vol. 1. Academic Press Inc. Archived from the original on 15 April 2021. Retrieved 16 July 2021.
  13. ^ Bertholet (1860) "Note sur une nouvelle série de composés organiques, le quadricarbure d'hydrogène et ses dérivés" Archived 13 July 2015 at the Wayback Machine (Note on a new series of organic compounds, tetra-carbon hydride and its derivatives), Comptes rendus, series 3, 50: 805–808.
  14. from the original on 30 December 2021. Retrieved 29 December 2021. Atomic weights of 6 and 12 were both in use for carbon.
  15. ^ Berthelot (1862) "Synthèse de l'acétylène par la combinaison directe du carbone avec l'hydrogène" Archived 14 August 2020 at the Wayback Machine (Synthesis of acetylene by the direct combination of carbon with hydrogen), Comptes rendus, series 3, 54: 640–644.
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  17. .
  18. ^ .
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  27. .
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  29. ^
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  31. ^ ESAB Oxy-acetylene welding handbook – Acetylene properties Archived 10 May 2020 at the Wayback Machine.
  32. from the original on 19 March 2022, retrieved 3 March 2022
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  40. from the original on 26 December 2013. Retrieved 26 December 2013.
  41. ^ "Lighthouse Lamps Through Time by Thomas Tag | US Lighthouse Society". uslhs.org. Archived from the original on 25 February 2017. Retrieved 24 February 2017.
  42. from the original on 17 June 2016. Retrieved 21 November 2015.
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  45. . Retrieved 28 July 2022.
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  49. S2CID 4427890
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  57. .
  58. .
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  60. ^ "OSHA 29 CFR 1910.102 Acetylene". Archived from the original on 1 December 2016. Retrieved 30 November 2016.
  61. ^ a b c "OSHA 29 CFR 1926.350 Gas Welding and cutting". Archived from the original on 1 December 2016. Retrieved 30 November 2016.
  62. ^ Special Hazards of Acetylene Archived 24 March 2016 at the Wayback Machine UNITED STATES DEPARTMENT OF LABOR Mine Safety and Health Administration – MSHA.
  63. ^ Daniel_Sarachick (16 October 2003). "ACETYLENE SAFETY ALERT" (PDF). Office of Environmental Health & Safety (EHS). Archived (PDF) from the original on 13 July 2018. Retrieved 27 September 2018.
  64. ^ a b "NFPA free access to 2017 edition of NFPA 70 (NEC)". Archived from the original on 1 December 2016. Retrieved 30 November 2016.
  65. ^ "NFPA Free Access to NFPA 497 – Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas". Archived from the original on 1 December 2016. Retrieved 30 November 2016.

External links