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Carbon monoxide, chemical formula CO, is a colourless, odourless, flammable and highly toxic gas. It is a major
product of the incomplete combustion of carbon and carbon-containing compounds.
Toxicity
Carbon monoxide is emitted by cars, when the temperature is too low to
effect complete oxidation of the hydrocarbons in the fuel to water and CO2, because the time
(i.e., the residence time) available in the combustion chamber
is too short, or because there is insufficient oxygen present. Usually, it is more
difficult to design and operate a combustor for very low CO than for very low unburned hydrocarbons.
Similarly, carbon monoxide is present in cigarette smoke.
CO binds very strongly to the iron atom in
hemoglobin, (the principal oxygen-carrying compound in blood); this renders the hemoglobin incapable of
releasing oxygen. A sufficient exposure to carbon monoxide can reduce the amount of oxygen taken up by the brain to the point
that the victim becomes unconscious, and can suffer brain damage or even death from anoxia. Hemoglobin acquires a bright red colour when bound to carbon monoxide, so a casualty of CO poisoning can
actually look abnormally pink-cheeked and healthy.
First aid for carbon monoxide poisoning is to immediately
remove the patient from the exposure without endangering yourself, call for
help, apply CPR and if able to do so
apply oxygen first aid. Hyperbaric oxygen therapy is a treatment for carbon monoxide poisoning.
Town gas, used for illumination and heating from the 19th century, was
made by passing steam through red-hot coke; the resultant reaction between the water and
carbon generated a mixture of hydrogen and carbon monoxide. Town gas has now been
replaced by natural gas (methane). Wood
gas, the result of the incomplete combustion of wood, also contains carbon monoxide as
a major component. The exhaust of the internal
combustion engine, when burning a carbon based fuel (i.e. almost any fuel except pure hydrogen) contains carbon monoxide as well.
As other poisons such as cyanide or
arsenic had their availability placed under more and more stringent legal
restrictions, the use of carbon monoxide in town gas became the principal method of suicide by poisoning. Later, suicide was often committed by inhaling exhaust fumes of running engines. Air-quality
regulations have begun to reduce suicide by this route, as catalytic converters designed to clean up the exhausts remove all but
a trace of CO.
A major problem of accidental CO poisoning that still exists is the use of heaters, particularly gas water heaters and gas
fires which are improperly vented. There are a number of deaths every year from this cause. CO poisoning can occur in SCUBA diving due to faulty or badly sited diving air compressors.
Physiological Role
Carbon monoxide is produced naturally by the body. The breakdown-product of hemoglobin, heme, is a substrate for the enzyme
heme oxygenase which produces CO and biliverdin. The biliverdin can then be reduced to bilirubin which is excreted by the liver.
The CO produced in the brain might act as a neurotransmitter.
Chemistry
The structure of the CO molecule is best described using molecular
orbital theory. The length of the bond (0.111 nm) indicates that it has a partial triple bond character. The molecule has a small
dipole moment and is often represented by three resonance structures:
-
Note that the octet rule is violated for the carbon atom in the two
structures on the right.
The metal nickel forms a volatile compound
with carbon monoxide, known as nickel carbonyl. The carbonyl
decomposes readily back to the metal and gas, and this was used as the basis for the industrial purification of nickel.
Many other metals may form carbonyl complexes containing covalently attached carbon monoxide, these can be made by a range of
different methods for instance boiling ruthenium trichloride with triphenyl phosphine in methoxyethanol (or DMF) the complex
[RuHCl(CO)(PPh3)3] can be obtained. Nickel carbonyl is special as it can be formed by the dirrect combination of carbon monoxide
and nickel metal at room temperature.
As in nickel carbonyl and other carbonyls, the electron pair on the carbon bonded to the metal. In this case carbon monoxide
is regarded as a the carbonyl ligand.
Carbon monoxide and methanol are reacted together using a homogenous rhodium cataylist to form acetic acid in the Monsanto
process which is responsible for most of the industrial production of acetic acid.
The CAS registry number of carbon monoxide is
630-08-0.
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