Acetylene

From Encyclopedia Jr, free information reference for Kids

Acetylene
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
Autoignition temperature	305 °C
Maximum burning temperature	2,780 °C
Melting point	-84 °C
Boiling point	-80.8 °C
SMILES	C#C
NFPA 704	4 0 3
PubChem	6326
EINECS	200-816-9
Disclaimer and references

Acetylene (IUPAC name: ethyne) 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 who identified it as a "new carburet of hydrogen." It was rediscovered in 1860 by French chemist Marcellin Berthelot, who coined the name "acetylene."

Contents 1 Preparation 2 Safety and handling 2.1 Compression 2.2 Toxic effects 2.3 Fire hazard 3 Reactions 4 Uses 5 Other meanings 6 References 7 External links

[edit] Preparation

The principal raw materials for acetylene manufacture are calcium carbonate (limestone) 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 (or calcium acetylide) 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 → Ca(OH)₂ + 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 (19th)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.

[edit] Safety and handling

[edit] 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 (DMF). There are strict regulations on the shipment of dangerous gas cylinders throughout the world.

[edit] Toxic effects

Inhaling acetylene may cause dizziness, headache and nausea. ^[1] It may also contain toxic impurities: the Compressed Gas Association Commodity Specification for acetylene 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 carryovers 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 (not counting the "narcotic" effects). Up to 80% percent, (v/v) 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; (cyclopropane would be nearly as bad) It was suggested by the Germans that such an internal thorax explosion could not occur with air mixtures (that is without purified oxygen). 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 (absorbed) 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 (molecular weight 26.04), 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.

[edit] Fire hazard

Mixtures with air containing between 3% and 82% acetylene are explosive on ignition. The minimum ignition temperature is 335°C.^[1] The majority of acetylene's chemical energy is what is not contained in the carbon-carbon triple bond; that is, it is greater than that of three carbon-carbon bonds spread out, but is disallowed therefrom because of the spaces between its mate carbon and all other carbons likewise shielded in charge.

[edit] Reactions

Above 400 °C (which is quite low for a hydrocarbon), the pyrolysis of acetylene will start. The main products are the dimer vinylacetylene (C₄H₄) 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.

[edit] 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 (5972°F), releasing 11.8 kJ/g. Oxyacetylene is the hottest burning of all fuel gases (hottest chemical flame).^[2]

Acetylene is also used in the acetylene ('carbide') lamp, once used by miners (not to be confused with the Davy lamp), 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 (CaC₂) 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 (that is, hardening) of steel when the object is too large to fit into a furnace. ^[2]

Acetylene 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 (or lithium acetylide). 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.

[edit] Other meanings

Sometimes the plural "acetylenes" may refer to the class of organic chemical compounds known as alkynes which contain the -C≡C- group.

[edit] References

1. ^ ^{a b} Muir, GD (ed.) 1971, Hazards in the Chemical Laboratory, The Royal Institute of Chemistry, London.
2. ^ ^{a b} http://www.boc.com/products_and_services/by_product/acetylene/index.asp

[edit] External links

• Acetylene at Chemistry Comes Alive!
• Acetylene, the Principles of Its Generation and Use, available freely at Project Gutenberg

• Link page to external chemical sources.