Cyanide is a chemical species that contains the cyano group (–C≡N). The cyanide ion has a −1 charge and is written as CN^–. It quickly combines with metal cations to form compounds, such as potassium cyanide (KCN) and sodium cyanide (NaCN). ^[1]

Cyanides are extremely toxic and can be deadly even in small amounts, so they must be handled and stored with great care. Despite this danger, they are widely used in industry. One key use is in gold mining, where sodium cyanide helps dissolve gold from ore, allowing it to be collected.

Structure

The Lewis structure shows that the cyanide ion (CN^–) is linear. The carbon and nitrogen atoms are connected by a triple bond (C≡N), which consists of one sigma (σ) bond and two pi (π) bonds. This triple bond is extremely strong, providing the cyano group with high stability and resistance to breaking apart.

The electron density in the ion is not evenly distributed. A greater share of the negative charge lies on the carbon atom. As a result, cyanide usually binds to metal ions through carbon, forming stable metal–cyanide complexes.

This ability to form strong metal–cyanide complexes makes cyanide useful in industrial processes such as gold extraction and electroplating. For the same reason, it can bind to metal-containing enzymes in living cells, which is responsible for its toxicity.

List of Common Cyanides ^[2]

Physical Properties ^[3]

• Physical state: Many inorganic cyanides, such as sodium cyanide and potassium cyanide, are white crystalline solids at room temperature. Hydrogen cyanide is a colorless, volatile liquid that readily vaporizes near room temperature.
• Odor: Hydrogen cyanide may have a faint bitter almond–like odor. Moist cyanide salts can release small amounts of hydrogen cyanide, which produces this smell. However, many people cannot detect it, making cyanide exposure dangerous because it may occur without warning.
• Solubility: Alkali metal cyanides are highly soluble in water. Hydrogen cyanide is also very soluble in water, where it forms a weak acid solution known as hydrocyanic acid.
• Melting and boiling points: Inorganic cyanide salts have high melting points due to strong ionic bonding between metal cations and the cyanide ion. In contrast, hydrogen cyanide has a low boiling point because it is held together by relatively weak intermolecular forces.
• Density: Solid cyanide salts are typically denser than water and therefore sink. Liquid hydrogen cyanide is lighter than water and floats.

Chemical Reactions ^[4]

1. Hydrolysis

At high temperature (170 °C), cyanide undergoes hydrolysis in the presence of water to produce formate and ammonia:

CN^– + 2 H₂O  → HCOO^– + NH₃

This process is sometimes used in industrial wastewater treatment to destroy cyanide.

2. Acid–Base Behavior

The cyanide ion is a weak conjugate base of the weak acid hydrogen cyanide (HCN). It reacts with water to produce hydrogen cyanide gas and hydroxide ion:

CN^–  + H₂O → HCN + OH^– 

This reaction explains why aqueous solutions of sodium cyanide or potassium cyanide are basic in nature.

3. Oxidation

Cyanide can be oxidized to the less toxic cyanate using oxidizing agents like hydrogen peroxide or sodium hypochlorite:

CN⁻ + H₂​O₂​ → OCN⁻ + H₂​O

The cyanate subsequently hydrolyzes into ammonia and carbonate. This reaction is also widely used in industrial wastewater treatment.

Toxicity of Cyanide 

Cyanide is highly toxic and can be fatal quickly, as it prevents cells from making energy by blocking a key enzyme in cellular respiration. This means oxygen in the blood cannot be used by cells.

As a result, the body is unable to produce the energy it needs. Symptoms such as rapid breathing, confusion, seizures, and loss of consciousness can appear within minutes. Without immediate treatment, cyanide poisoning can lead to death.

Because of its rapid and severe effects, cyanide is strictly regulated and requires immediate medical attention in cases of exposure.