Cyanide

**Cyanide** refers to the CN⁻ anion (cyanide ion) or compounds containing it. The bonding is a carbon-nitrogen triple bond with a lone pair on carbon, which makes cyanide a strong Lewis base and an excellent ligand for transition metals — especially iron, copper, and cobalt. ## Common forms - **Hydrogen cyanide (HCN)**: volatile liquid/gas, boils at 26°C, faint bitter-almond odor (detectable by only ~40% of the population due to a genetic trait). Historically called prussic acid. - **Potassium cyanide (KCN)** and **sodium cyanide (NaCN)**: white crystalline salts. When they contact stomach acid or moisture, they release HCN gas. - **Cyanogenic glycosides**: plant compounds that release cyanide on digestion. Found in apple seeds (amygdalin), almonds (especially bitter almonds), cassava, lima beans, stone fruit pits, bamboo shoots. Traditional cassava processing (soaking, fermenting, cooking) exists specifically to expel these. - **Metal cyanides**: ferrocyanide, ferricyanide — inert chelated forms used in food additives (anti-caking in salt), pigments (Prussian blue, first synthetic pigment, 1706), and photography. - **Nitriles** (R-C≡N): organic compounds containing the cyano group; generally not toxic like inorganic cyanide because the C-C bond doesn't release CN⁻ under physiological conditions. ## Industrial use Global production of HCN is ~1.1 million tons/year, mostly captive (used on-site), made via the Andrussow process (methane + ammonia + oxygen over platinum) or the BMA process. Primary uses: - Adiponitrile production for nylon 66 - Gold mining: cyanide leaching dissolves gold from low-grade ore as soluble aurocyanide — the dominant extraction method worldwide, source of environmental controversy (Baia Mare 2000 spill, Romania) - Silver mining, electroplating, case hardening of steel (historically — phased out due to toxicity, replaced by salt bath nitriding with KNO3 or plasma nitriding) - Methyl methacrylate (plexiglass precursor) - Synthesis intermediates in pharma and agrochemicals ## Toxicology Cyanide's toxicity comes from its affinity for Fe³⁺ in the heme a3 site of Cytochrome c Oxidase: The Final Enzyme in Cellular Respiration (Complex IV of the electron transport chain). Binding blocks cellular respiration — cells can't use oxygen even when blood is fully saturated. See Cyanide Toxicity: How It Kills and Why Immunity Is Impossible for mechanism and The Movie Cyanide Myth: Why Hollywood Death Scenes Are Wrong for realistic timelines. Lethal dose (oral, adult): ~200-300 mg KCN or ~50 mg HCN inhaled. Treatment: Hydroxocobalamin: The Vitamin B12 Cyanide Antidote first-line, sodium thiosulfate adjunct. ## Biology At nanomolar concentrations cyanide is produced endogenously in mammals and may act as a signalling molecule — a PNAS 2021 paper showed it stimulates Complex IV at these levels. The body has a natural detoxification path: Rhodanese: The Cyanide Detoxification Enzyme That Can't Save You from Poisoning converts CN⁻ to thiocyanate using sulfur donors. This handles dietary exposure but is overwhelmed in acute poisoning. Some microbes use cyanide offensively: Chromobacterium violaceum, Pseudomonas aeruginosa, and many fungi (especially in millipedes' defensive secretions) excrete HCN. ## History - 1782: Swedish chemist Carl Wilhelm Scheele isolates HCN from Prussian blue, dies in 1786 likely from cumulative chemical exposures including cyanide. - 1811: Joseph Louis Gay-Lussac characterises CN⁻. - WWII: Zyklon B (HCN absorbed on diatomaceous earth) used in Nazi extermination camps. - 1978: Jonestown Mass Cyanide Poisoning (1978) — 918 deaths from potassium cyanide mixed in Flavor Aid. - 1982: Chicago Tylenol murders — potassium cyanide added to capsules, 7 deaths, triggered tamper-evident packaging. - 1980s onward: gradual phase-out of industrial cyanide case hardening due to fumes and waste; replaced by plasma nitriding and salt bath nitriding with KNO3. ## Cultural note The bitter-almond smell of HCN is often cited in mystery fiction, but genetic variation means ~60% of people physically cannot detect it. This is a dominant trait linked to the OR11A1 olfactory receptor locus.