Hydrogen cyanide

• 1. Introduction

  ›

  Hydrogen cyanide, also known as prussic acid, is a deadly compound that about half of all people cannot smell. A recessive genetic trait leaves them unable to detect its odor. Those who can detect it describe a bitter almond scent. Chemists write the compound as HCN, and it is both highly toxic and flammable. This documentary follows that small molecule through some very different settings. How did a blue pigment give it a name? Why is a substance that kills so quickly also a building block for plastics, medicines and mining? And why do astronomers keep finding it in distant places, while some scientists suspect it helped life begin?

  102866

• 2. The Blue Acid

  ›

  In 1752, the French chemist Pierre Macquer split the pigment Prussian blue into an iron oxide and a volatile component. He then found that the two pieces could be put back together to rebuild the pigment. That volatile piece is what we now call hydrogen cyanide. In 1782 the Swedish chemist Carl Wilhelm Scheele prepared it again from Prussian blue. It eventually received the German name Blausäure, meaning blue acid, because it acts as an acid in water and descends from the blue pigment. English speakers came to call it prussic acid.

  Claude Louis Berthollet delivered a jolt to chemical theory in 1787. He showed that prussic acid contains no oxygen. At the time, chemists believed every acid had to contain oxygen. The element's name even reflects that belief, since it comes from Greek elements meaning acid-former. German Sauerstoff, Czech kyslík and Russian кислород carry the same idea in their own languages. One acid without oxygen was enough to challenge the rule.

  Joseph Louis Gay-Lussac completed the picture. In 1811 he prepared pure, liquefied hydrogen cyanide, and in 1815 he deduced its chemical formula. He also built the French word cyanure for the radical, which gave English the word cyanide. His source was the Ancient Greek κύανος, meaning dark blue enamel or lapis lazuli. The same Greek root gave English the color name cyan. Once the formula was settled, the molecule itself turned out to have a strikingly simple shape.

  1516171920

• 3. A Simple Molecule

  ›

  The bond between carbon and nitrogen in HCN is a triple bond just 115 picometers long. It holds the molecule in a straight line. Its isomer, a molecule built from the same atoms in a different order, is hydrogen isocyanide, written HNC. Chemists even disagree about what category HCN belongs in. Tradition calls it inorganic, yet it can also be viewed as a nitrile, which explains the alternative names methanenitrile and formonitrile.

  Boiling at 25.6 degrees Celsius, the liquid turns to gas barely above room temperature. That volatility makes it more dangerous than solid cyanide compounds. Dissolved in water, it forms hydrocyanic acid, which has a pKa of 9.2 and so counts as a weak acid. It only partly ionizes, releasing the cyanide anion. The salts of that anion are called cyanides.

  Nickel complexes drive one of its most useful reactions. In hydrocyanation, HCN adds across the double bond of an alkene to give a nitrile. Metal cyanides are usually made by swapping partners in an alkali metal cyanide salt, a reaction called salt metathesis. Mercuric cyanide is the exception, since it forms directly when mercuric oxide meets aqueous HCN. The nickel-catalyzed reaction sits behind one of the compound's biggest industrial jobs.

  3451214

• 4. Factories and Uses

  ›

  Mining operations in the 1890s created a large demand for cyanides, and George Thomas Beilby met it in 1892. He patented a method that passes ammonia over glowing coal to make hydrogen cyanide. Hamilton Castner replaced that approach in 1894 with a synthesis starting from coal, ammonia and sodium. It yields sodium cyanide, and adding acid to that salt releases gaseous HCN.

  Leonid Andrussow, working at IG Farben, invented the most important modern route, now called the Andrussow oxidation. Methane and ammonia react with oxygen at about 1200 degrees Celsius over a platinum catalyst. The Degussa process, also called the BMA process, adds no oxygen, so energy must pass indirectly through the reactor wall. The Shawinigan process instead reacts hydrocarbons such as propane with ammonia. HCN is also recovered as a waste product from making acrylonitrile. In 2006, the United States produced between 500 million and 1 billion pounds, roughly 230,000 to 450,000 metric tons.

  In a laboratory, adding acids to alkali metal cyanide salts releases small amounts of HCN. That reaction turns a nonvolatile salt into a gas, which is why it sometimes causes accidental poisonings. Potassium ferricyanide and acid will also yield the gas.

  Sodium cyanide and potassium cyanide, both made from HCN, serve mainly in gold and silver mining and in electroplating those metals. Through intermediates called cyanohydrins, HCN leads to methyl methacrylate from acetone, the amino acid methionine by the Strecker synthesis, and the chelating agents EDTA and NTA. Adding it to butadiene gives adiponitrile, a precursor of Nylon-6,6.

  Worldwide, HCN also fumigates food production facilities against many species of pest insects. Very small amounts do the job compared with other toxic fumigants. Its environmental impact is lower than that of sulfuryl fluoride and methyl bromide. Nature supplies the same compound in humbler places, though, such as fruit seeds.

  2122232628

• 5. Nature's Cyanide

  ›

  One hundred grams of crushed apple seeds can give off about 70 mg of hydrogen cyanide. Cherries, apricots and apples, along with nuts such as bitter almonds, hold small amounts of cyanohydrins named mandelonitrile and amygdalin. These slowly release the gas. Almond oil and extract come from bitter almonds. Cassava roots hold a cyanogenic glycoside called linamarin, which breaks down into HCN at up to 370 mg per kilogram of fresh root.

  The millipedes Harpaphe haydeniana, Desmoxytes purpurosea and Apheloria release hydrogen cyanide as a defense. So do burnet moths and the larvae of Paropsisterna eucalyptus. People add to the supply too, since vehicle exhaust, smoke from burning nitrogen-containing plastics and tobacco smoke all carry it.

  Mammalian neurons may make it as well. Some researchers report that neurons produce HCN when opioids activate their opioid receptors. That HCN switches on NMDA receptors and appears to help pass signals between nerve cells. Opioid painkilling weakened when HCN scavengers were present, so the researchers regarded HCN made inside the body as a neuromodulator. Stimulating muscarinic cholinergic receptors in cultured pheochromocytoma cells raises HCN production. Yet in a living organism, the same stimulation lowers it.

  White blood cells generate HCN while engulfing invaders, a process called phagocytosis. It is one of several toxic chemicals they use to kill bacteria, fungi and other pathogens.

  The blood vessel widening caused by sodium nitroprusside depends on nitric oxide and also on cyanide the body generates. That cyanide adds toxicity and some extra blood-pressure-lowering effect, compared with nitroglycerine and other nitrates that do not raise blood cyanide levels. Those raised levels hint at what far larger exposures do.

  2930323340

• 6. Poison and Weapon

  ›

  A concentration of 100-200 parts per million in breathing air kills a person within 10 to 60 minutes. At 2000 ppm, about 2380 mg per cubic meter, death comes in roughly one minute. The cyanide ion is responsible. It halts cellular respiration by acting as a non-competitive inhibitor of cytochrome c oxidase, an enzyme in mitochondria. For this reason hydrogen cyanide is commonly listed among chemical weapons as a blood agent. In air, the gas is also explosive at concentrations above 5.6 percent.

  From the middle of the 18th century, prussic acid appeared in a number of poisoning murders and suicides. Whalers used it as a killing agent in harpoons, but quickly abandoned it because it endangered the crew. Some US states used it for judicial execution, producing the gas during the procedure by reacting sulfuric acid with sodium cyanide or potassium cyanide.

  France began using hydrogen cyanide as a chemical weapon against the Central Powers in 1916, and the United States and Italy followed in 1918. It proved not effective enough, partly because of weather conditions. The gas is lighter than air, so it rises and disperses rapidly. Denser agents such as phosgene or chlorine stayed low and sank into the trenches of the Western Front. HCN also had to reach higher concentrations to be fatal. To make it linger, it was mixed with smoke-producing compounds. The French mixture Vincennite combined 50 percent hydrogen cyanide with 30 percent arsenic trichloride, 15 percent stannic chloride and 5 percent chloroform.

  The most infamous use came with Zyklon B, German for Cyclone B. The B stands for Blausäure, the prussic acid, and distinguishes it from an earlier product later known as Zyklon A. Nazi Germany used it in the extermination camps of Majdanek and Auschwitz-Birkenau during World War II. It killed Jews and other persecuted minorities en masse as part of the genocide known as the Final Solution. The camps also used hydrogen cyanide to delouse clothing, in attempts to eradicate diseases carried by lice and other parasites. One of the original Czech producers kept making Zyklon B under the trademark Uragan D2 until around 2015.

  During World War II, the United States considered using hydrogen cyanide, along with cyanogen chloride and mustard gas, in Operation Downfall, the planned invasion of Japan. President Harry Truman decided against it. He used the atomic bombs developed by the secret Manhattan Project instead.

  Today the Chemical Weapons Convention lists the compound under Schedule 3, for potential weapons that have large-scale industrial uses. Signatory countries must declare plants making more than 30 metric tons per year. Those plants are open to inspection by the Organisation for the Prohibition of Chemical Weapons.

  6364656768

• 7. Origins of Life

  ›

  Four molecules of HCN will link into one molecule of diaminomaleonitrile, because HCN is unstable and prone to condensing with itself. Chemists write that four-unit product as (HCN)4. The three-unit product, aminomalononitrile, is written (HCN)3. These two compounds draw special interest from researchers.

  As a precursor to amino acids and nucleic acids, hydrogen cyanide has been proposed as a player in the origin of life. How these reactions connect to that theory remains speculative. Even so, studies in this area uncovered new routes to organic compounds built by condensing HCN, adenine among them.

  A cascade of asteroids known as the Late Heavy Bombardment is thought to have battered the young Earth, set off by the interaction of Jupiter and Saturn. One postulate holds that carbon from those asteroids reacted with nitrogen in the atmosphere and formed HCN. That idea raises an obvious question about other worlds, and telescopes have been asking it.

  1344453946

• 8. Beyond Earth

  ›

  Six separate instruments have measured hydrogen cyanide in the atmosphere of Titan. Four were on the Cassini space probe, one was on Voyager, and one was on Earth. In one measurement, Cassini dipped into the atmosphere to gather gas for mass spectrometry. Sunlight-driven chemistry there makes methane and nitrogen radicals, which react through an intermediate called H2CN to give HCN. Ultraviolet radiation splits HCN into CN and hydrogen. But CN reacts with methane to rebuild HCN efficiently.

  Carbon stars and the gas between stars both show hydrogen cyanide. Because it leads toward nucleic acids, which are critical for life on Earth, astronomers are motivated to hunt for it and its derivatives. Ground-based telescopes can detect it through several atmospheric windows.

  Two main routes form HCN in interstellar clouds. In one, CH2 meets a nitrogen atom. In the other, an HCNH+ ion recombines with an electron. This second route dominates by 30 percent, but only when the ion is in its linear form. Its structural isomer H2NC+ produces only hydrogen isocyanide. Destruction depends on location. Light breaks HCN apart in photon-dominated regions, and cosmic rays do so at greater depth. In the dark core, it reacts mostly with HCO+ ions, which win out over reactions with H+ by a factor of about 3.5.

  Astronomers use HCN as a tracer of dense molecular gas and of stellar inflow in high-mass star-forming regions. The ratio of HNC to HCN also separates photon-dominated regions from X-ray-dominated regions.

  In studies released on the 11th of August 2014, astronomers used the Atacama Large Millimeter/Submillimeter Array for the first time to map HCN, HNC, H2CO and dust. They looked inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON). In February 2016, NASA's Hubble Space Telescope found traces of hydrogen cyanide in the atmosphere of the hot super-Earth 55 Cancri e.

  On the 14th of December 2023, astronomers reported the first discovery of hydrogen cyanide in the plumes of Enceladus, Saturn's sixth-largest moon. Other organic molecules turned up there too, some still to be better identified and understood. According to the researchers, "these newly discovered compounds could potentially support extant microbial communities or drive complex organic synthesis leading to the origin of life." Whether they do so remains for future observations to show.

  3635585962