Acetylene
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."
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 â†' CaC
2 + 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.
CaC
2 + 2H
2O â†' Ca(OH)
2 + C
2H
2Acetylene 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.
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.
Above 400 °C (which is quite low for a hydrocarbon), the
pyrolysis of acetylene will start. The main products are the
dimer vinylacetylene (C
4H
4) 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.
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.
[http://www.boc.com/products_and_services/by_product/acetylene/index.asp]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 (CaC
2) 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.
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.
Sometimes the plural "acetylenes" is used to more generally mean organic chemical compounds that contain the -C≡C- group: see
-yne.
*
Acetylene at Chemistry Comes Alive!*
Free ebook of no=8144 at
Project Gutenberg
