Carbon monoxide, with the chemical formula CO, is a colorless, odorless, tasteless yet highly toxic gas. Its molecules consist of one carbon atom covalently bonded to one oxygen atom. There are two covalent bonds and a coordinate covalent bond between the oxygen and carbon atoms.

Carbon monoxide is produced from the partial combustion of carbon-containing compounds, notably in internal-combustion engines. Carbon monoxide forms in preference to the more usual carbon dioxide when there is a reduced availability of oxygen present during the combustion process. Carbon monoxide has significant fuel value, burning in air with a characteristic blue flame, producing carbon dioxide. Despite its serious toxicity, CO plays a highly useful role in modern technology, being a precursor to myriad products.

Production

Carbon monoxide is so fundamentally important that many methods have been developed for its production.

Producer gas is formed by combustion of carbon in oxygen at high temperatures when there is an excess of carbon. In an oven, air is passed through a bed of coke. The initially produced CO₂ equilibrates with the remaining hot carbon to give CO. The reaction of O₂ with carbon to give CO is described as the Boudouard equilibrium. Above 800 °C, CO is the predominant product:

O₂ + 2 C → 2 CO

ΔH = -221 kJ/mol

The downside of this method is if done with air it leaves a mixture that is mostly nitrogen.

Synthesis gas or Water gas is produced via the endothermic reaction of steam and carbon:

H₂O + C → H₂ + CO

ΔH = 131 kJ/mol

CO also is a byproduct of the reduction of metal oxide ores with carbon, shown in a simplified form as follows:

MO + C → M + CO

ΔH = 131 kJ/mol

Since CO is a gas, the reduction process can be driven by heating, exploiting the positive (favorable) entropy of reaction. The Ellingham diagram shows that CO formation is favored over CO₂ in high temperatures.

CO is the anhydride of formic acid. As such it is conveniently produced by the dehydration of formic acid, for example with sulfuric acid. Another laboratory preparation for carbon monoxide entails heating an intimate mixture of powdered zinc metal and calcium carbonate.

Zn + CaCO₃ → ZnO + CaO + CO

Another laborotory method to generate CO is reacting sucrose and sodium hydroxide in a closed system.

Structure

The CO molecule possesses a bond length of 0.1128 nm. Formal charge and electronegativity difference cancel each other out. The result is a small dipole moment with its negative end on the carbon atom. The reason for this, despite oxygen's greater electronegativity, is that the highest occupied molecular orbital has an energy much closer to that of carbon's p orbitals, meaning that greater electron density is found near the carbon. In addition, carbon's lower electronegativity creates a much more diffuse electron cloud, enhancing the dipole moment. This is also the reason that almost all chemistry involving carbon monoxide occurs through the carbon atom, and not the oxygen.