Acetylene

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
Melting point	-84 °C
Boiling point	-80.8 °C
SMILES	C#C
Chemical infobox

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.

As the molecule cannot twist around the triple bond, all four atoms lie in the same straight line, with 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 (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:

Ca O + 3 C \to Ca C_2 + C O

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.

Ca C_2 + 2 H_2 O \to Ca (O H)_2 + C_2 H_2

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

Toxic effects

Inhaling acetylene may cause dizziness, headache and nausea. Muir, GD (ed.) 1971, Hazards in the Chemical Laboratory, The Royal Institute of Chemistry, London. 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.

Fire hazard

Mixtures with air containing between 3% and 82% acetylene are explosive on ignition. The minimum ignition temperature is 335°C. The majority of acetylene's chemical energy is contained in the carbon-carbon triple bond.

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.

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. Combustion with oxygen produces a flame of over 3300°C, releasing 11,800 J/g.

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 used for carburization (that is, hardening) of steel. Research in the last ten years has concluded that acetylene is the best hydrocarbon available for this purpose.

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.

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

Acetylene at Chemistry Comes Alive!

Alkynes

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