Hydrogen cyanide (HCN), also known as hydrocyanic acid or prussic acid, is a rapidly acting, highly toxic chemical. It can be a colorless gas or a pale-blue liquid and has a distinctive bitter almond odor, though a significant portion of the population (20-40%) cannot detect this scent. HCN interferes with the body’s ability to utilize oxygen, making it profoundly dangerous.
Naturally Occurring Sources
Hydrogen cyanide originates from various natural processes and environments. Many plants produce HCN as a defense mechanism, storing it in cyanogenic glycosides. When plant tissues are damaged, enzymes break down these glycosides, releasing HCN.
Common examples include cassava, bitter almonds, and the pits of fruits like apples, apricots, and cherries. Lima beans, bamboo shoots, and sorghum also contain these compounds. Beyond plants, HCN is released during natural phenomena like volcanic eruptions and large-scale biomass burning events, such as forest fires. Some microorganisms, including certain bacteria and fungi, also produce hydrogen cyanide.
Industrial Production and Applications
Humans deliberately produce hydrogen cyanide on a large scale for industrial applications. The most prevalent method is the Andrussow process, reacting methane and ammonia with oxygen over a catalyst at high temperatures. HCN is also obtained as a byproduct during acrylonitrile synthesis.
Most industrially produced HCN serves as a chemical intermediate. A significant portion converts into adiponitrile, a precursor for manufacturing nylon 6,6, used in textiles and automotive parts. It is also used to produce methyl methacrylate, a monomer for acrylic plastics.
HCN is used in the pharmaceutical industry for synthesizing various drugs, including sedatives and analgesics. It is also employed in the production of dyes, pesticides, and fumigants. In mining, hydrogen cyanide is used for extracting gold and silver from ore.
Formation from Combustion and Chemical Reactions
Hydrogen cyanide can form as an unintentional byproduct during combustion and chemical reactions. Fires, especially those involving nitrogen-containing materials, are a significant source of HCN release. When materials like wool, silk, polyurethane foams, and other synthetic polymers burn, high temperatures break down their nitrogenous components, forming HCN gas.
HCN in smoke from structural fires, particularly those involving common household furnishings, poses a substantial risk. This formation is intensified by high temperatures and limited oxygen. Hydrogen cyanide is also a component of cigarette smoke.
HCN can also arise from other chemical reactions. It may be an unintended byproduct in industrial or laboratory settings when carbon and nitrogen compounds react at elevated temperatures. Accidental HCN generation occurs when strong acids contact cyanide salts, converting the salt into toxic gaseous hydrogen cyanide.