Key Reactions and Biological Effects of Hydrogen Cyanide
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 Explained: Discovery, Properties & Uses
1. What is Hydrogen Cyanide (HCN) and what are its key physical properties?
Hydrogen Cyanide, with the chemical formula HCN, is a chemical compound also known as prussic acid. It is a highly volatile and poisonous substance. Its key physical properties include:
- State: It exists as a colorless or pale blue liquid below 25.6°C and a colorless gas above this temperature.
- Odor: It has a faint, bitter almond-like smell, though not everyone can detect it due to a genetic trait.
- Solubility: It is readily soluble in water and alcohol.
- Acidity: It is a very weak acid in aqueous solution.
2. What are the most important industrial uses of hydrogen cyanide?
Despite its toxicity, hydrogen cyanide is a crucial chemical intermediate in many manufacturing processes. Its primary importance lies in its use for:
- Polymer Production: It is a key ingredient in making acrylonitrile and adiponitrile, which are precursors to acrylic plastics, synthetic fibres like nylon, and synthetic rubber.
- Mining: It is used in the process of extracting gold and silver from low-grade ores.
- Chemical Synthesis: It serves as a starting material for producing various chemical compounds, including chelating agents like EDTA and pharmaceuticals.
- Fumigation: It is used as a fumigant to control pests in shipping, warehousing, and agriculture.
3. What are the typical symptoms of exposure to hydrogen cyanide?
Exposure to hydrogen cyanide can be rapidly fatal. The symptoms depend on the concentration and duration of exposure. Initial symptoms of mild exposure include headache, dizziness, nausea, vomiting, and rapid breathing. In cases of severe poisoning, symptoms progress quickly to convulsions, loss of consciousness, respiratory failure, and cardiac arrest. A characteristic sign in victims can be unusually pink or cherry-red skin, as the body's cells are unable to use the oxygen present in the blood.
4. How is hydrogen cyanide applied in organic chemistry reactions?
In organic chemistry, hydrogen cyanide is an important source of the cyanide nucleophile (CN⁻). Its most significant application is the addition to carbonyl compounds like aldehydes and ketones. This reaction, known as cyanohydrin formation, involves the nucleophilic attack of the cyanide ion on the carbonyl carbon. The resulting cyanohydrins are versatile intermediates, which can be further hydrolysed to create alpha-hydroxy acids, an important class of organic compounds.
5. Why is hydrogen cyanide so extremely toxic at the cellular level?
Hydrogen cyanide's extreme toxicity is due to its ability to disrupt cellular respiration. The cyanide ion (CN⁻) binds irreversibly to the iron atom in cytochrome c oxidase, an essential enzyme in the mitochondria. This binding action blocks the final step of the electron transport chain, which means cells can no longer use oxygen to produce ATP (the body's main energy molecule). This leads to a condition called histotoxic hypoxia, where tissues starve for energy and die, even when there is sufficient oxygen in the blood, leading to rapid systemic failure.
6. What is the key difference between hydrogen cyanide (HCN) and a cyanide salt like potassium cyanide (KCN)?
The main difference lies in their chemical bonding and physical state. Hydrogen cyanide (HCN) is a covalent molecule that exists as a volatile liquid or gas at room temperature. It is a weak acid. In contrast, potassium cyanide (KCN) is an ionic compound, a stable, white crystalline solid composed of potassium ions (K⁺) and cyanide ions (CN⁻). While both are toxic because they provide the cyanide ion, their physical form and chemical nature are distinct. KCN releases the toxic CN⁻ ion when dissolved in water or when it reacts with acid, such as stomach acid.
7. If hydrogen cyanide is a poison, why is it found naturally in foods like apple seeds and almonds?
Hydrogen cyanide in plants serves as a natural chemical defence mechanism against herbivores. It is not stored in its free, toxic form. Instead, it is locked within larger, inactive molecules called cyanogenic glycosides (like amygdalin in apple seeds and bitter almonds). When the plant's tissues are crushed or chewed, enzymes are released that break down these glycosides, producing hydrogen cyanide as a toxic deterrent. The amount in a few apple seeds is negligible and not harmful to humans, but consuming large quantities can be dangerous.
8. How did hydrogen cyanide gas acquire its notorious historical reputation?
Hydrogen cyanide's notorious reputation stems primarily from its use as a chemical weapon and an agent of mass murder. While it was used sporadically in World War I, its infamy was cemented during World War II. The Nazi regime used a commercial preparation of hydrogen cyanide adsorbed onto a porous solid, branded as Zyklon B, as their primary killing agent in the gas chambers of extermination camps like Auschwitz. Its high volatility and rapid, lethal effects in an enclosed space made it an effective tool for mass murder, forever linking it to these historical atrocities.
