1 Sources

Carbon monoxide has many common sources. The exhaust of the internal combustion engine, when burning a carbon-based fuel (i.e. almost any fuel except pure hydrogen) contains carbon monoxide, especially 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. Carbon monoxide is also present in small but significant concentrations in cigarette smoke.

In the past a significant source of CO was Town gas, used for illumination and heating from the 19th century. Town gas 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 Town gas has now been replaced by natural gas ( methane).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.

2 Sociological notes

As other poisons such as cyanide and 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.

3 Toxicity

CO binds very strongly to the iron atoms in hemoglobin, the principal oxygen-carrying compound in blood. The affinity between CO and hemoglobin is 200 times stronger than the affinity between hemoglobin and oxygen. As CO binds to the hemoglobin, it cannot be released nearly as readily as oxygen would be. Thus, the body's hemoglobin becomes saturated with CO and is rendered incapable of carrying oxygen to the body. 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.

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. A number of deaths occur every year from this cause. CO poisoning can occur in SCUBA diving due to faulty or badly sited diving air compressors.

First aid for carbon monoxide poisoning is to immediately remove the victim from the exposure without endangering oneself, call for help, apply CPR and if possible apply oxygen first aid. Hyperbaric oxygen therapy is a treatment for carbon monoxide poisoning.

4 Physiological Role

Carbon monoxide is produced naturally by the body. The breakdown-product of haemoglobin, haeme, is a substrate for the enzyme haeme 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.

5 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 direct 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 catalyst 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.

Oxides Toxicology

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