Acetylene
Acetylene is the simplest
alkyne hydrocarbon, consisting of two
hydrogen atoms and two
carbon atoms connected by a triple
bond. Because it contains a triple bond, acetylene is an unsaturated chemical compound.
The carbon-carbon triple bond leaves the carbon atoms with two sp hybrid orbitals for sigma bonding, placing all four atoms in the same straight line, with CCH bond angles of 180.
Acetylene was discovered in 1836 by Edmund Davy, in
England, who identified it as a "new carburet of hydrogen." It was rediscovered in 1860 by French chemist
Marcellin Berthelot, who coined the name "acetylene."
Encyclopedia
| Acetylene |
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| Chemical name | Acetylene |
| Chemical formula | C2H2 |
| Other names | Ethyne
Ethine |
| Molecular mass | 26.0373 g/mol |
| CAS number | 74-86-2 |
| Density | 1.09670E-03 g/cm³ |
| Melting point | -84 °C |
| Boiling point | -80.8 °C |
| SMILES | C#C |
| NFPA 704 | |
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Acetylene is the simplest
alkyne hydrocarbon, consisting of two
hydrogen atoms and two
carbon atoms connected by a triple
bond. Because it contains a triple bond, acetylene is an unsaturated chemical compound.
The carbon-carbon triple bond leaves the carbon atoms with two sp hybrid orbitals for sigma bonding, placing all four atoms in the same straight line, with CCH bond angles of 180°.
Acetylene was discovered in 1836 by Edmund Davy, in
England, who identified it as a "new carburet of hydrogen." It was rediscovered in 1860 by French chemist
Marcellin Berthelot, who coined the name "acetylene."
Preparation
The principal raw materials for acetylene manufacture are
calcium carbonate and
coal. The calcium carbonate is first converted into calcium oxide and the coal into coke, then the two are reacted with each other to form
calcium carbide and
carbon monoxide:
- CaO + 3C ? CaC2 + CO
Calcium
carbide and water are then reacted by any of several methods to produce acetylene and
calcium hydroxide. This reaction was discovered by
Friedrich Wohler in 1862.
- CaC2 + 2H2O ? Ca2 + C2H2
Calcium carbide synthesis requires an extremely high temperature, ~2000 Celsius, so the reaction is performed in an
electric arc furnace. This reaction was an important part of the industrial revolution in chemistry that occurred as a product of massive amounts of cheap hydroelectric power liberated from Niagara Falls before the turn of the century.
Acetylene can also be manufactured by the partial combustion of
methane with
oxygen, or by the cracking of
hydrocarbons.
Berthelot was able to prepare acetylene from
methyl alcohol,
ethyl alcohol,
ethylene, or
ether, when he passed any one of these as a gas or vapour through a red-hot tube. Berthelot also found acetylene was formed by sparking electricity through mixed
cyanogen and
hydrogen gases. He was also able to form acetylene directly by combining pure hydrogen with
carbon using electrical discharge of a carbon arc.
Safety and handling
Compression
Acetylene can explode with extreme violence if the pressure of the gas exceeds about 100 kPa as a gas or when in liquid or solid form, so it is shipped and stored dissolved in
acetone or
Dimethylformamide . There are strict regulations on the shipment of dangerous
gas cylinders throughout the world.
Toxic effects
Inhaling acetylene may cause dizziness, headache and nausea. It may also contain toxic impurities: the has established a grading system for identifying and quantifying
phosphine,
arsine, and
hydrogen sulfide content in commercial grades of acetylene in order to limit exposure to these impurities. The sulfur, phosphorus and arsenic are carry overs from the synthesis ingredient coke, an impure form of carbon and different, organic impurities would be expected from the thermal cracking of hydrocarbons source.
While the impurities in acetylene can be toxic and even fatal, pure acetylene is of a very low toxicity . Up to 80% percent, acetylene has been administered to surgical patients as a general anaesthetic. The trade name for acetylene was "narcylene" it was used a fair amount experimentally in Germany in their impoverished 1920's; perhaps on several thousand patients. Medically, acetylene was considered to be nearly as safe as nitrous oxide and with a slightly higher potency, allowing for the use of higher percentages of oxygen in the blend; it is about 50% more potent. However the use of acetylene and oxygen mixtures was dropped after several gas explosions-inside patient's lungs. The energy of these explosions would be expected to exceed any of the flammable inhalation anesthetics due to the instability of the triple bond; It was suggested by the Germans that such an internal thorax explosion could not occur with air mixtures . Many details on toxcity, explosion hazards and a little on the historical use of narcylene at
http://72.14.253.104/search?q=cache:nI0qCD-JsswJ:www.cdc.gov/niosh/wpfs/76-195ss.wpf+narcylene&hl=en&gl=us&ct=clnk&cd=7
Acetylene has been infrequently abused in a manner akin to nitrous oxide abuse up through modern times; according to the literature. This can result in the death of the abuser due to toxicity of the above mentioned impurities
phosphine,
arsine, and
hydrogen sulfide. Also the gas is charged into tanks soaked with acetone over a solid matrix and some acetone comes out with the gas, further contributing to the poisonings. The driver for this behavior is better understood with the view of acetylene's anesthetic properties and addictive behaviors.
What's an easy way to detect inpurities in acetylene? Smell it. "Pure acetylene, C2H2 , is a colorless and odorless gas. The characteristic garliclike odor of technical grade acetylene is attributable to contamination by impurities. Impurities which may be present include: divinyl sulfide, ammonia, oxygen, nitrogen, phosphine, arsine, methane, carbon dioxide, carbon monoxide, hydrogen sulfide, vinyl acetylene, divinyl acetylene, diacetylene, propadiene, hexadiene, butadienyl acetylene, and methyl acetylene." From the feds at the above niosh web site.
Fire hazard
Mixtures with air containing between 3% and 82% acetylene are explosive on ignition. The minimum ignition temperature is 335°C.actions
Above 400 °C , the pyrolysis of acetylene will start. The main products are the dimer vinylacetylene and benzene. At temperatures above 900 °C, the main product will be soot.
Polymerization with Ziegler-Natta catalysts produces polyacetylene films.
Using acetylene, Berthelot was the first to show that an aliphatic compound could form an aromatic compound when he heated acetylene in a glass tube to produce benzene with some toluene. Berthelot oxidized acetylene to yield acetic acid and oxalic acid. He found acetylene could be reduced to form ethylene and ethane.
Uses
Approximately 80 percent of the acetylene produced annually in the United States is used in chemical synthesis. The remaining 20 percent is used primarily for oxyacetylene gas welding and cutting due to the high temperature of the flame; combustion of acetylene with oxygen produces a flame of over 3300°C , releasing 11.8 kJ/g. Oxyacetylene is the hottest burning of all fuel gases.
Acetylene is also used in the acetylene lamp, once used by miners , on vintage cars, and still sometimes used by cavers. In this context, the acetylene is generated by dripping water from the upper chamber of the lamp onto calcium carbide pellets in the base of the lamp.
In former times a few towns used acetylene for lighting, including Tata in Hungary where it was installed on 24 July 1897, and North Petherton, England in 1898.
In modern times acetylene is sometimes used for carburization of steel when the object is too large to fit into a furnace. ylene has been proposed as a carbon feedstock for Molecular Manufacturing using Nanotechnology. Since it does not occur naturally, using acetylene could limit out-of-control self-replication.
Acetylene is used to volatilize carbon in radiocarbon dating. The carbonaceous material in the archeological sample reacted in a small specialized research furnace with lithium metal to form lithium carbide . The carbide can then be reacted with water, as usual, to form acetylene gas to be fed into mass spectrometer to sort out the isotopic ratio of carbon 14 to carbon 12.
Other meanings
Sometimes the plural "acetylenes" is used to more generally mean organic chemical compounds that contain the -C=C- group: see -yne.
References
External links
