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Key Chemicals Month
This month we feature chemicals that have important applications in industry and the lab.—Ed.

Carbon monoxide (CO) is the simplest carbon oxide, an important industrial and laboratory chemical, and a deadly toxin.

The molecular structure of CO is unusual. It is variously written with a carbon–oxygen triple, double, or single bond; with or without unshared electrons on the atoms; and with or without positive or negative charges on each atom. In actuality, it consists of resonance structures among the individual proposed structures. In his seminal work, , 1954 Nobel Prize chemistry laureate Linus Pauling discussed the resonance energies of the different forms of CO extensively.

Humans have known about the effects of CO since before recorded history. In the 1770s, early notable chemists Joseph Priestley, Carl Wilhelm Scheele, and others synthesized and isolated the molecule. Early reports in the chemical literature from 1878 included such titles as “�, “�, and “Method to protect iron or steel against rust� (British and German patents).

In industry, CO is manufactured via the combustion of carbon or organic substances in insufficient oxygen to burn the carbon all the way to carbon dioxide. If CO is not used immediately in subsequent reactions, it is sold in compressed gas cylinders. In the laboratory, CO is conveniently made by dehydrating formic or oxalic acid with concentrated sulfuric acid.

CO’s uses are many and varied. An important one in the petroleum and chemical industries is the water–gas shift reaction in which CO reacts with water to produce hydrogen

CO + H₂O â†� H₂ + CO₂

In an oil refinery or chemical plant, the hydrogen is used to hydrogenate unsaturated hydrocarbons or to manufacture ammonia, methanol, or other basic chemicals.

CO is widely used in metallurgy to reduce ores to metals, as a complexation agent to make metal carbonyls, and as a reagent to convert alkenes to aldehydes. Global annual production figures are difficult to access, but the market value in 2024 is estimated at US$4�5 billion.

As shown in the hazard information table, CO is an extremely toxic gas, especially when inhaled. CO binds to hemoglobin preferentially to oxygen, resulting in oxygen deprivation and frequently to death, which is why it is recommended that residences have CO monitors. Sadly, CO inhalation is one of the .