Carbon monoxide (CO) is a colorless, odorless and tasteless gas, yet very toxic to humans and animals. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond. It is the simplest oxocarbon, and is an anhydride of formic acid.
Carbon monoxide is produced from the partial oxidation of carbon-containing compounds; it forms when there is not enough oxygen to produce carbon dioxide (CO 2 ), such as when operating a stove or an internal combustion engine in an enclosed space. Carbon monoxide burns with a blue flame, producing carbon dioxide. Despite its toxicity, coal gas, which was widely used before the 1960s for domestic lighting, cooking and heating, produced carbon monoxide as a byproduct. Some processes in modern technology, such as iron smelting, still produce carbon monoxide as a byproduct.
History
Carbon monoxide has been unknowingly used by humans since prehistoric times, for the smelting of iron and other metallic ores. The gas was used for executions by the Greek and Romans in Classical Antiquity, and was described by the Spanish doctor Arnaldus de Villa Nova in the 11th century. In 1776 the French chemist de Lassone produced CO by heating zinc oxide with coke, but mistakenly concluded that the gaseous product was hydrogen as it burned with a blue flame. The gas was identified as a compound containing carbon and oxygen by the English chemist William Cumberland Cruikshank in the year 1800. Its toxic properties on dogs were thoroughly investigated by Claude Bernard around 1846.
During World War II, carbon monoxide was used to keep motor vehicles running in parts of the world where gasoline was scarce. External charcoal or wood burners were fitted, and the carbon monoxide produced by gasification was piped to a carburetor. The carbon monoxide produced by this process is known as wood gas. Carbon monoxide was also reportedly used on a small scale during the Holocaust at some Nazi extermination camps(most notably by gas vans in Chelmno, and in the Action T4 "euthanasia" program.
Molecular properties
The bond length between the carbon atom and the oxygen atom is 112.8 pm. Atomic formal charge and electronegativity result in a small bond dipole moment with the negative end of the molecule on the carbon atom. This is due to the highest occupied molecular orbital having energy much closer than that of carbon's p orbitals, despite oxygen's greater electronegativity. This means that greater electron density is found near the carbon atom. In addition, carbon's lower electronegativity creates a much more diffuse electron cloud, enhancing the polarizability. This is also the reason why almost all chemistry involving carbon monoxide occurs through the carbon atom, and not the oxygen.
The bond length of CO is consistent with a partial triple bond, and the molecule can be represented by three resonance structures:
In this model, the leftmost structure contributes the most. Carbon monoxide resembles molecular nitrogen, and it has nearly the same molecular mass. Their physical properties (boiling point, melting point, etc.) are very similar.
Biological and physiological properties
Toxicity
Main article: Carbon monoxide poisoningCarbon monoxide poisoning is the most common type of fatal air poisoning in many countries. Carbon monoxide is colorless, odorless and tasteless, but extremely toxic. It combines with hemoglobin to produce carboxyhemoglobin, which is ineffective for delivering oxygen to bodily tissues. This condition is known as anoxemia). Concentrations as low as 667 ppm may cause up to 50% of the body's hemoglobin to convert to carboxyhemoglobin. In the United States, the OSHA limits long-term workplace exposure levels above 50 ppm.
The most common symptoms of carbon monoxide poisoning may resemble other types of poisonings and infections (such as the flu), including headache, nausea, vomiting, dizziness, lethargy and a feeling of weakness. Infants may be irritable and feed poorly. Neurological signs include confusion, disorientation, visual disturbance, syncope and seizures.
Some descriptions of carbon monoxide poisoning include retinal hemorrhages, and an abnormal cherry-red blood hue.. In most clinical diagnoses these signs are seldom seen.
Carbon monoxide binds to other molecules such as myoglobin and mitochondrial cytochrome oxidase. Exposures to carbon monoxide may cause significant damage to the heart and central nervous system, especially to the globus pallidus, often with long-term sequelae. Carbon monoxide may have severe adverse effects on the fetus of a pregnant woman.
Human physiology
Carbon monoxide is produced naturally by the human body. This carbon monoxide may have positive physiological roles in the body, such as a neurotransmitter or a blood vessel relaxant. Because of carbon monoxide's role in the body, abnormalities in its metabolism have been linked to a variety of diseases, including neurodegenerations, hypertension, heart failure, and inflammation.
Microbiology
Carbon monoxide is a nutrient for methanogenic bacteria, a building block for acetylcoenzyme A. This the theme for the emerging field of bioorganometallic chemistry. In bacteria, carbon monoxide is produced via the reduction of carbon dioxide by the enzyme carbon monoxide dehydrogenase, an Fe-Ni-S-containing protein.
CooA is a carbon monoxide sensor protein. The scope of its biological role is still unknown; it may be part of a signaling pathway in bacteria and archaea. Its occurrence in mammals is not established.
Occurrence
Carbon monoxide occurs in various natural and artificial environments. Typical concentrations in parts per million are as follows:
Atmospheric presence
Carbon monoxide is present in small amounts in the atmosphere, chiefly as a product of volcanic activity but also from natural and man-made fires (such as forest and bushfires, burning of crop residues, and sugarcane fire-cleaning). The burning of fossil fuels also contributes to carbon monoxide production. Carbon monoxide occurs dissolved in molten volcanic rock at high pressures in the Earth's mantle. Because natural sources of carbon monoxide are so variable from year to year, it is extremely difficult to accurately measure natural emissions of the gas.
Carbon monoxide has an indirect radiative forcing effect by elevating concentrations of methane and tropospheric ozone through chemical reactions with other atmospheric constituents (e.g., the hydroxyl radical, OH . ) that would otherwise destroy them. Through natural processes in the atmosphere, it is eventually oxidized to carbon dioxide. Carbon monoxide concentrations are both short-lived in the atmosphere and spatially variable.
Urban pollution
Carbon monoxide is a major atmospheric pollutant in some urban areas, chiefly from the exhaust of internal combustion engines (including vehicles, portable and back-up generators, lawn mowers, power washers, etc.), but also from improper burning of various other fuels (including wood, coal, charcoal, oil, paraffin, propane, natural gas, and trash). Along with aldehydes, it reacts photochemically to produce peroxy radicals. Peroxy radicals react with nitrogen oxide to increase the ratio of NO 2 to NO, which reduces the quantity of NO that is available to react with ozone.
Indoor pollution
In closed environments the concentration of carbon monoxide can easily rise to lethal levels. On average, 170 people in the United States die every year from carbon monoxide produced by non-automotive consumer products. These products include malfunctioning fuel-burning appliances such as furnaces, ranges, water heaters and room heaters; engine-powered equipment such as portable generators; fireplaces; and charcoal that is burned in homes and other enclosed areas. The American Association of Poison Control Centers (AAPCC) reported 15,769 cases of carbon monoxide poisoning resulting in 39 deaths in 2007. In 2005, the CPSC reported 94 generator-related carbon monoxide poisoning deaths. Forty-seven of these deaths were known to have occurred during power outages due to severe weather, including Hurricane Katrina. Still others die from carbon monoxide produced by non-consumer products, such as cars left running in attached garages. The Centers for Disease Control and Prevention estimates that several thousand people go to hospital emergency rooms every year to be treated for carbon monoxide poisoning.
Carbon monoxide is also a constituent of tobacco smoke.
Production
Many methods have been developed for carbon monoxide's production.
Laboratory preparation
Carbon monoxide is conveniently produced in the laboratory by the dehydration of formic acid, for example with sulfuric acid. Another method is heating an intimate mixture of powdered zinc metal and calcium carbonate, which releases CO and leaves behind zinc oxide and calcium oxide:
Industrial production
A major industrial source of CO is producer gas, a mixture containing mostly carbon monoxide and nitrogen, formed by combustion of carbon in air at high temperature when
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