Carbon Monoxide

Facts about carbon monoxide

Carbon monoxide (chemical formula CO) is a colurless, odourless, poisonous gas produced by burning carbon in insufficient oxygen. In the laboratory, carbon monoxide is produced by dehydrating formic acid with concentrated sulfuric acid.

The mixture of CO and hydrogen is called synthesis gas. It is used for the production of various organic compounds.

Carbon monoxide is a very weak Lewis base but it acts as a strong ligand to transition metals. Nickel forms volatile Ni(CO)₄ at ordinary temperature. Many other transition metals from a wide variety of carbonyls under different conditions.

Bonding in carbon monoxide

Carbon monoxide has a total of ten electrons in the outer orbitals. On the hybridization model, we assumed both carbon and oxygen to have sp hybridized.

Carbon	↑↓	↑	↑		hybridized	↑↓	↑	↑
	2s	2px	2py	2pz		2spx	2spx	py	pz

Oxygen	↑↓	↑↓	↑	↑	hybridized	↑↓	↑	↑	↑↓
	2s	2px	2py	2pz		2spx	2spx	py	pz

The pairs of electrons in sp orbitals of carbon and oxygen make the lone pairs of CO. The sp orbitals of carbon and oxygen overlap to form the σ covalent bond in CO. The two p_y orbitals overlap to form one π bond.

Another electron pair on the p_z orbital of oxygen overlaps with the empty p_z orbital of carbon to form the second π bond. The last one is the coordinate type bond.

Molecular orbital diagram of carbon monoxide

The chemical bonding in carbon monoxide is best represented by the molecular orbital diagram given below the picture,

• On the molecular orbital model of carbon monoxide, the two sp_x hybrid orbitals of oxygen and carbon combine to give two molecular orbitals. One is low energy sigma bonding type that contains two electrons and the other is high energy sigma antibonding type.
• The two p_y orbitals will combine to give two π-type molecular orbitals. One is the bonding type being occupied by two electrons and the other is the antibonding type.
• Similarly, the two p_x orbitals of carbon and oxygen will give two molecular orbitals. The bonding type orbitals contain two electrons but the antibonding type is empty.

All the bonding molecular orbital in CO are filled. There are three antibonding orbitals of which sp_x is much higher energy. This particular molecular orbital is placed along C-O bond axis and is not accessible for back donation.

The other two molecular orbitals are available to metal for back donation. Since oxygen is more electronegative than carbon. Therefore, the carbon lone pair functions as a donner.

Properties of carbon monoxide

Carbon monoxide is an electron-deficient molecule and has π-acceptor properties. It is an ideal ligand for a low oxidation number or state of metal. CO is sparingly soluble in water and burns in air to form carbon dioxide with a blue flame.

It is extraordinarily poisonous. CO combines with the hemoglobin of blood. Therefore, it renders to carry oxygen by red blood cells. Some basic physical properties of carbon monoxide are given below the table,

Properties of carbon monoxide
Chemical formula	CO
Appearance	Colouless gas
Molar mass	28.010 g/mol
Density	Liquid	Gas
	789 kg/m3	1.250 kg/m3 at 0° C and 1 atm pressure
Melting point	−205.02 °C
Boiling point	−191.5 °C
Solubility in water	27.5 mg/L at 25°C
Dipole moment	0.122 D
Heat capacity	29.1 J mol−1 K−1

Production Process

• Controlled oxidation or catalytic steam reforming of methane or natural gas gives CO.
• The gasification of coal with oxygen at 1500 °C gives carbon monoxide.
  C + H₂O → CO + H₂
  CO₂ formed by this process may be removed by washing monoethanolamine. Sulfur compounds can be removed before the synthetic use of carbon monoxide.

Chemical reaction

CO is burned in the air to form a blue flame.
2CO + O₂ → 2CO₂ (ΔH⁰ = −565 kJ mol⁻¹)

It reduces ammoniacal silver nitrate to silver and palladium chloride to palladium.
PdCl₂ + H₂O + CO → Pd + 2 HCl + CO₂

Alkali metals in liquid ammonia react with CO to produce colorless crystalline solid salts containing [OC≡CO]⁻² ion.
2 Na + 2 CO → NaOC≡CONa

It reduces many metal oxides. I₂O₅ reduces by CO to form iodine. This reaction may be used for the detection of carbon monoxide.
I₂O₅ + 5 CO → I₂ + 5 CO₂

Uses of carbon monoxide

• Carbon monoxide uses in industry for the production of water gas and synthesis gas. Pure hydrogen may be obtained by the water gas shift (WGS) reaction.
  CO + H₂O → CO₂ + H₂ (ΔH⁰ = −41.2 kJ mol⁻¹)
• Carbon monoxide is a major constituent of gaseous fuels including coal gas. Coal gas is also used as a domestic fuel through pipeline supply.
• It is catalytically reduced by hydrogen to give a variety of products under varying conditions.
• Heterogeneous reduction of CO by hydrogen catalyzed by metal gives liquid hydrocarbon. The liquid hydrocarbon is used as a substitute for natural petroleum. The process is called Fischer Tropsch synthesis. Through this process, one may obtain a variety of products like methane, waxy hydrocarbon, methanol, and higher alcohol.
• It is used in the packaging of food products like fish, beef, etc.
• It is used in infrared lasers and removes rust from the metal surface.
• Carbon monoxide and hydrogen are added to olefinic bonds to produce aldehydes and alcohols.