Acetylene (systematic name:ethyne) is achemical compound with the formulaC2H2 and structureHC≡CH. It is ahydrocarbon and the simplestalkyne.[8] 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 asdivinyl sulfide andphosphine.[9][10]
As an alkyne, acetylene isunsaturated because its two carbon atoms arebonded together in atriple bond. The carbon–carbon triple bond places all four atoms in the same straight line, with CCH bond angles of 180°.[11] The triple bond in acetylene results in a high energy content that is released when acetylene is burned.[12]
Acetylene was discovered in 1836 byEdmund Davy, who identified it as a "new carburet of hydrogen".[13][14] It was an accidental discovery while attempting to isolatepotassium metal. By heatingpotassium carbonate with carbon at very high temperatures, he produced a residue of what is now known aspotassium carbide, (K2C2), which reacted with water to release the new gas.[12] It was rediscovered in 1860 by French chemistMarcellin Berthelot, who coined the nameacétylène.[15] 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).[16] Berthelot was able to prepare this gas by passing vapours of organic compounds (methanol, ethanol, etc.) through a red hot tube and collecting theeffluent. He also found that acetylene was formed by sparking electricity through mixedcyanogen andhydrogen gases. Berthelot later obtained acetylene directly by passing hydrogen between the poles of acarbon arc.[17][18]
Since the 1950s, acetylene has mainly been manufactured by the partial combustion ofmethane in the US, much of the EU, and many other countries:[9][19][20]
3 CH4 + 3 O2 → C2H2 + CO + 5 H2O
It is a recovered side product in production ofethylene bycracking ofhydrocarbons. Approximately 400,000 tonnes were produced by this method in 1983.[9] Its presence in ethylene is usually undesirable because of its explosive character and its ability to poisonZiegler–Natta catalysts. It is selectively hydrogenated into ethylene, usually usingPd–Ag catalysts.[21]
The first sample of acetylene produced was by Edmund Davy in 1836, via hydrolysis (decomposition using water) of potassium carbide.[23] Subsequently, the first industrial method for synthesis was by hydrolysis ofcalcium carbide:[12]
CaC2 + 2 H2O → Ca(OH)2 + C2H2
This reaction was discovered byFriedrich Wöhler in 1862,[24] but a suitable commercial scale production method which allowed acetylene to be put into wider scale use was not found until 1892 by the Canadian inventorThomas Willson while searching for a viable commercial production method for aluminum.[25]
As late as the early 21st century, China, Japan, and Eastern Europe produced acetylene primarily by this method.[26]
The use of this technology has since declined worldwide with the notable exception of China, with its emphasis on coal-based chemical industry, as of 2013. Otherwiseoil has increasingly supplantedcoal as the chief source ofreduced carbon.[27]
Calcium carbide production requires high temperatures, ~2000 °C, necessitating the use of anelectric arc furnace. In the US, this process was an important part of the late-19th century revolution in chemistry enabled by the massivehydroelectric power project atNiagara Falls.[28]
In terms ofvalence bond theory, in each carbon atom the 2sorbitalhybridizes with one 2p orbital thus forming an sp hybrid. The other two 2p orbitals remain unhybridized. The two ends of the two sp hybridorbital overlap to form a strongσ 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 weakerπ bonds.[29]
At atmospheric pressure, acetylene cannot exist as a liquid and does not have a melting point. Thetriple point on thephase diagram corresponds to the melting point (−80.8 °C) at the minimal pressure at which liquid acetylene can exist (1.27 atm). At temperatures below the triple point, solid acetylene can change directly to thevapour (gas) bysublimation. The sublimation point at atmospheric pressure is −84.0 °C.[31]
At room temperature and atmospheric pressure, the solubility of acetylene inacetone is 27.9 g per kg. For the same amount ofdimethylformamide (DMF), the solubility is 51 g. At20.26 bar, the solubility increases to 689.0 and 628.0 g for acetone and DMF, respectively. These solvents are used in pressurized gas cylinders.[32]
Approximately 20% of acetylene is supplied by theindustrial gases industry foroxyacetylenegas welding andcutting due to the high temperature of the flame. Combustion of acetylene with oxygen produces a flame of over 3,600 K (3,330 °C; 6,020 °F), releasing 11.8 kJ/g. Oxygen with acetylene is the hottest burning common gas mixture.[33] Acetylene is the third-hottest natural chemical flame afterdicyanoacetylene's 5,260 K (4,990 °C; 9,010 °F) andcyanogen at 4,798 K (4,525 °C; 8,177 °F).Oxy-acetylene welding was a popular welding process in previous decades. The development and advantages ofarc-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 weldingequipment 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 asoldering tool for sealing lead sleeve splices inmanholes and in some aerial locations. Oxyacetylene welding may also be used in areas where electricity is not readily accessible. Oxyacetylene cutting is used in many metal fabrication shops. For use in welding and cutting, the working pressures must be controlled by a regulator, since above 15 psi (100 kPa), if subjected to a shockwave (caused, for example, by aflashback), acetylenedecomposes explosively intohydrogen andcarbon.[34]
In addition to ethynylation, acetylene reacts withcarbon monoxide to giveacrylic acid, or acrylic esters. Metal catalysts are required. These derivatives form products such asacrylic fibers,glasses,paints,resins, andpolymers. Except in China, use of acetylene as a chemical feedstock has declined by 70% from 1965 to 2007 owing to cost and environmental considerations.[38] In China, acetylene is a major precursor tovinyl chloride.[35]
Prior to the widespread use of petrochemicals, coal-derived acetylene was a building block for several industrial chemicals. Thus acetylene can be hydrated to giveacetaldehyde, 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.[39]
Acetylene is sometimes used forcarburization (that is, hardening) of steel when the object is too large to fit into a furnace.[43]
Acetylene is used to volatilize carbon inradiocarbon dating. The carbonaceous material in an archeological sample is treated withlithium metal in a small specialized research furnace to formlithium carbide (also known as lithium acetylide). The carbide can then be reacted with water, as usual, to form acetylene gas to feed into amass spectrometer to measure the isotopic ratio of carbon-14 to carbon-12.[44]
Acetylene combustion produces a strong, bright light and the ubiquity ofcarbide lamps drove much acetylene commercialization in the early 20th century. Common applications included coastallighthouses,[45]street lights,[12] andautomobile[46] andminingheadlamps.[47] In most of these applications, direct combustion is afire hazard, and so acetylene has been replaced, first byincandescent 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 centralelectric grid.[47]
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.[48] A number of bacteria living on acetylene have been identified. Theenzymeacetylene hydratase catalyzes the hydration of acetylene to giveacetaldehyde:[49]
C2H2 + H2O → CH3CHO
Acetylene is a moderately common chemical in the universe, often associated with the atmospheres ofgas giants.[50] One curious discovery of acetylene is onEnceladus, a moon ofSaturn. Natural acetylene is believed to form fromcatalytic 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.[51][52]
The hydration of acetylene is a vinylation reaction, but the resulting vinyl alcohol isomerizes toacetaldehyde. The reaction is catalyzed bymercury salts. This reaction once was the dominant technology for acetaldehyde production, but it has been displaced by theWacker process, which affords acetaldehyde by oxidation ofethylene, a cheaper feedstock. A similar situation applies to the conversion of acetylene to the valuablevinyl chloride byhydrochlorination versus theoxychlorination of ethylene.
Vinyl acetate is used instead of acetylene for some vinylations, which are more accurately described astransvinylations.[53] Higher esters of vinyl acetate have been used in the synthesis ofvinyl formate.
Acetylene and its derivatives (2-butyne, diphenylacetylene, etc.) formcomplexes 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 asalkyne trimerisation to benzene, tetramerization tocyclooctatetraene,[9] and carbonylation tohydroquinone:[54]
Fe(CO)5 + 4 C2H2 + 2 H2O → 2 C6H4(OH)2 + FeCO3 at basic conditions (50–80 °C, 20–25 atm).
Acetylene is not especially toxic, but when generated fromcalcium carbide, or CaC2, it can contain toxic impurities such as traces ofphosphine andarsine, which gives it a distinctgarlic-like smell. It is also highly flammable, as are 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 anexothermic addition-type reaction to form a number of products, typicallybenzene and/orvinylacetylene, possibly in addition tocarbon andhydrogen.[citation needed] Although it is stable at normal pressures and temperatures, if it is subjected to pressures as low as 15 psig it can explode.[12] The safe limit for acetylene therefore is 101 kPagage, or 15 psig.[58][59] Additionally, if acetylene is initiated by intense heat or a shockwave, it can decompose explosively if the absolute pressure of the gas exceeds about 200 kilopascals (29 psi). It is therefore supplied and stored dissolved inacetone ordimethylformamide (DMF),[59][60][61] contained in agas cylinder witha porous filling, 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.[62]
Information on safe storage of acetylene in upright cylinders is provided by the OSHA,[63][64] Compressed Gas Association,[59] United States Mine Safety and Health Administration (MSHA),[65] EIGA,[62] and other agencies.
Copper catalyses the decomposition of acetylene, and as a result acetylene should not be transported in copper pipes.[66]
Cylinders should be stored in an area segregated from oxidizers to avoid exacerbated reaction in case of fire/leakage.[59][64] Acetylene cylinders should not be stored in confined spaces, enclosed vehicles, garages, and buildings, to avoid unintended leakage leading to explosive atmosphere.[59][64] In the US, National Electric Code (NEC) requires consideration for hazardous areas including those where acetylene may be released during accidents or leaks.[67] Consideration may include electrical classification and use of listed Group A electrical components in US.[67] Further information on determining the areas requiring special consideration is in NFPA 497.[68] In Europe, ATEX also requires consideration for hazardous areas where flammable gases may be released during accidents or leaks.[62]
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