HCN — Hydrogen Cyanide

Hydrogen cyanide (HCN) is a colorless, highly poisonous, and volatile chemical compound that has been used both as an important industrial reagent and a chemical weapon. It has a faint bitter almond odor, although not everyone can detect it due to genetic variability in smell perception.

Hydrogen cyanide is a covalent compound composed of hydrogen, carbon, and nitrogen, represented by the molecular formula \(HCN\). It is known for its strong triple bond between carbon and nitrogen, which contributes to its stability and chemical reactivity. In aqueous solution, it acts as a weak acid, dissociating partially to form cyanide ions (CN⁻).

Despite its toxicity, hydrogen cyanide plays a crucial role in industrial chemistry, particularly in the production of acrylonitrile, methyl methacrylate, and synthetic fibers like nylon. In small amounts, it also occurs naturally in plants such as bitter almonds, cassava, and apricot pits as part of cyanogenic glycosides.

The structure of hydrogen cyanide consists of a linear arrangement of atoms: hydrogen attached to carbon, which is triple-bonded to nitrogen. The Lewis structure of HCN is:

The carbon atom forms a single bond with hydrogen and a triple bond with nitrogen. This triple bond includes one sigma (σ) bond and two pi (π) bonds, resulting in a bond length of about 1.16 Å between carbon and nitrogen. The linear geometry (bond angle of 180°) gives HCN its non-polar structure, though the individual bonds are polar due to electronegativity differences.

Hydrogen cyanide is slightly soluble in water, forming hydrocyanic acid when dissolved:

Since it only partially ionizes, it behaves as a weak acid with a pKa of about 9.2. The cyanide ion formed is a strong nucleophile and plays an important role in organic synthesis reactions like the formation of nitriles and amino acids.

Hydrogen cyanide is produced both naturally and synthetically. It was first prepared by Swedish chemist Carl Wilhelm Scheele in 1782 by heating ferrocyanides with acids. In industry, it is manufactured by several important methods:

• Andrussow Process: This is the most common industrial method where methane, ammonia, and oxygen react in the presence of a platinum catalyst at high temperature:

• Blausäure Process: Involves the reaction between ammonia and carbon at high temperature:

• From Sodium Cyanide and Acid: Laboratory preparation can be done by reacting sodium cyanide with dilute sulfuric acid:

In nature, HCN is released in small amounts by volcanoes and biomass burning, and can also be formed in interstellar space, making it a molecule of astrobiological interest.

Physical Properties:

• Hydrogen cyanide is a colorless, highly volatile liquid that boils slightly above room temperature (25.6°C).
• It has a faint almond-like odor and is lighter than water.
• It is highly soluble in water, ethanol, and ether.
• Due to its volatility, it easily forms vapors that are extremely toxic.

Chemical Properties:

• Hydrogen cyanide exhibits weakly acidic behavior, forming cyanide ions in aqueous solution:

\( HCN \leftrightharpoons H^+ + CN^- \)

• It reacts with bases like sodium hydroxide to form sodium cyanide:

\( HCN + NaOH \rightarrow NaCN + H_2O \)

• As a nitrile precursor, HCN reacts with aldehydes and ketones to form cyanohydrins:

\( R_2C=O + HCN \rightarrow R_2C(OH)CN \)

• It undergoes polymerization in the presence of alkalis, producing paracyanogen (a brownish-black polymer).
• It burns in air with a blue flame to produce carbon dioxide, water, and nitrogen:

\( 4HCN + 5O_2 \rightarrow 2H_2O + 4CO_2 + 2N_2 \)

Despite being highly toxic, hydrogen cyanide is used in several essential industrial processes due to its strong reactivity:

• In the Production of Synthetic Fibers: Used to produce acrylonitrile, a key monomer for making polyacrylonitrile fibers (used in textiles and plastics).
• Manufacture of Plastics and Rubbers: Serves as a precursor for adiponitrile, which is further converted into nylon-6,6.
• In Mining: Cyanide salts derived from HCN are used in gold and silver extraction processes.
• Organic Synthesis: Acts as a key reagent in the synthesis of amino acids, nitriles, and other nitrogen-containing compounds.
• Fumigation and Pest Control (historically): Hydrogen cyanide was once used as a fumigant, but due to its toxicity, safer alternatives are now preferred.

Hydrogen cyanide is one of the most lethal poisons known to humans. It inhibits cellular respiration by binding to the ferric ion (Fe³⁺) in cytochrome oxidase enzymes, preventing oxygen utilization in cells. This leads to cellular asphyxiation even when oxygen is available in the blood.

Symptoms of Exposure: include headache, dizziness, nausea, rapid breathing, and in severe cases, convulsions, loss of consciousness, and death.

Safety Measures:

• Work only in well-ventilated or controlled industrial environments.
• Use protective equipment like respirators, gloves, and eye protection.
• Store in tightly sealed, corrosion-resistant containers away from acids and oxidizing agents.
• In case of poisoning, the antidotes include sodium thiosulfate, hydroxocobalamin, or amyl nitrite under medical supervision.

Due to its high lethality, hydrogen cyanide and its salts are tightly regulated under chemical safety and anti-terrorism conventions.