What is Hydrogen Cyanide Gas?
Hydrogen Cyanide, also called prussic acid and formonitrile, is a highly toxic, colourless gas or liquid that has a smell similar to bitter almonds. The chemical formula of hydrogen cyanide gas is HCN. In the form of a liquid, hydrogen cyanide is inflammable and has a boiling point slightly above room temperature. Hydrogen cyanide is mildly acidic. It has a wide range of uses, especially in the polymer and pharmaceutical industries. It is also used to produce potassium cyanide, another toxic chemical compound that has varied usage.
Due to its highly poisonous nature, cyanide gas has long been used as a pesticide and rodenticide. It has also been the method of choice to inflict capital punishment for a long time. Fatal doses of HCN are often administered to the convict in such cases.
Structure and General Properties of HCN
Hydrogen cyanide is a linear molecule with a triple bond between carbon and nitrogen.
It is weakly acidic with a pKa of 9.2. It partially ionizes in H2O solution to provide the cyanide anion,CN-. A solution of hydrogen cyanide in water, denoted as HCN, is called hydrocyanic acid. The salts of the cyanide anion are known as cyanides.
HCN has a low acrid almond-like odour which people often fail to detect due to a passive genetic trait. The active compound has been used as inhalation rodenticide, human poison, and in the slaughter of whales.
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Discovery of Hydrogen Cyanide
Prussian blue, a dark blue chemical compound, was first used to isolate hydrogen cyanide. French chemist Pierre Macquer showed that a volatile gas was formed when Prussian blue was converted to iron oxide. Following this discovery, William Scheele isolated cyanide gas and called it prussic acid. Later, French chemist Gay Lussac formed liquified hydrogen cyanide and also deduced its chemical nature and formula.
Towards the end of the 19th century, the demand for HCN started growing for mining operations. Large-scale hydrogen cyanide could be produced by using glowing coal and ammonia. At present, HCN is synthesized from sodium cyanide with sulphuric acid and decomposition of formamide.
Occurrence of Hydrogen Cyanide
Hydrogen cyanide naturally occurs in fruits that have a pit, like apricots, peaches, cherries, and bitter almonds. Some insects like millipedes and moths release cyanide gas as a defense mechanism. Hydrogen cyanide is present in large quantities in Titan, one of Saturn’s moons. This gas is also found in automobile exhaust, burning plastics, and tobacco smoke.
Chemical Nature of Hydrogen Cyanide
Hydrogen cyanide is a colourless pungent-smelling gas that inhibits cellular respiration. The HCN structure consists of one carbon and a nitrogen atom triple bonded to a methine group. Hydrogen cyanide has a tautomer called hydrogen isocyanide where the HCN structure becomes HNC. Cyanide gas has a bitterly burning taste and is soluble in water, ethanol, and ethyl ether. As a gas, it gets easily absorbed on porous surfaces and produces vapors that are lighter than air. When dissolved in water, hydrogen cyanide acts as a weak acid.
Hydrogen Cyanide Uses
Although potentially hazardous and environmentally polluting, there are many hydrogen cyanide uses in several industries. This gas is used in the production of many industrially important chemicals, like sodium cyanide, adiponitrile, acetone cyanohydrin, etc. It is used to synthesize cyanuric chloride, which is an effective insecticide and is also used for other agricultural purposes. Hydrogen cyanide gas is an important compound in coordination chemistry and is used to produce chelating agents like EDTA. Polymerized hydrogen cyanide salts are used in ore extraction, to treat steel, and in electrolytic processes.
Hydrogen cyanide gas has long been used in chemical warfare due to its highly toxic nature. Hydrogen cyanide is a blood agent, which implies that it gets absorbed into the bloodstream rapidly and can cause death within a few minutes. This gas acts as a non-competitive inhibitor of cytochrome c oxidase, an enzyme of the mitochondria. Human beings can tolerate 50-60 ppm of hydrogen cyanide in the air for a couple of hours without experiencing any serious problems. But above 100-200 ppm, hydrogen cyanide effects are lethal. High concentrations of the gas can kill a human being within 10 to 60 minutes. During World War 1, hydrogen cyanide was used as a chemical weapon by the French, Italians, and Americans. However, it did not have the desired results. This gas, being lighter than air, was needed in greater concentration to have the lethal hydrogen cyanide effects. In contrast, other poisonous gasses like phosgene and chlorine were more popular as they are denser and sink in trenches more easily.
During World War 2, Nazi Germany committed mass atrocities against the Jewish population of Europe. In what is today known as the Holocaust, mass concentration camps were established throughout German-occupied territory, where people deemed unfit to live by the Nazis were rounded up. These camps had gas chambers, which were used for mass murder. The gas used in these chambers was Zyklon B, whose primary component was hydrogen cyanide gas. Despite being lightweight, this gas was the perfect weapon of choice in closed gas chambers where victims had no escape. Witnesses who oversaw this mass killing have later said that death by hydrogen cyanide gas is painful, as the victims would be screaming and shouting for help, and some of the victims were found foaming at the mouth and bleeding through their ears after death. Cyanide gas has thus gained a notorious reputation.
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FAQs on Hydrogen Cyanide
1.What are the symptoms of cyanide poisoning?
Although hydrogen cyanide gas has many beneficial uses and is widely used to make fertilizers and to manufacture other important chemical compounds, it has a notorious reputation due to its lethality. This gas can cause death within a few minutes if one is exposed to high doses of it. The initial symptoms include nausea, vomiting, rapid pulse, breathing difficulties, and headache. Gradually, one might experience acute symptoms like loss of consciousness, seizure, and cardiac arrest. The skin of potential victims can turn unusually pink as cellular uptake of oxygen is reduced. A fire outbreak or an accident at a metallurgical factory can cause potential cyanide poisoning.
2.How did hydrogen cyanide gas become a notorious chemical?
Cyanide gas has always been the weapon of choice for chemical warfare due to its ability to cause death quickly. There had been some sporadic uses of this gas during the First World War in trench warfare but quickly became unpopular as it was lighter than air and needed in large quantities to be lethal in open trenches. However, the gas found its active use during the Second World War, when the Nazis used it in the form of pellets under the brand name Zyklon B in their extermination camps. It became the perfect tool for mass murder in closed gas chambers where there was no path for the gas to escape.
