HYDROCYANIC ACID: MOLECULAR STRUCTURE, PROPERTIES, USES - CHEMISTRY - 2025

The hydrocyanic acid or hydrogen cyanide is an organic compound whose chemical formula is HCN. It is also known as methanonitrile or formonitrile and, until several years ago, as prussic acid, although this is actually another compound.

Hydrocyanic acid is an extremely poisonous, colorless gas that is obtained by treating cyanides with acids. This acid is found inside the seed of peaches, also known in many places as peaches.

Peach seed, which contains hydrocyanic acid or hydrogen cyanide, HCN. An.ha. Source: Wikipedia Commons.

At an ambient temperature lower than 25 ºC it is a liquid and above that temperature it is a gas. In both cases it is extremely toxic to humans, animals and even most microorganisms not acclimated to it. It is a good solvent for ions. It is very unstable as it tends to polymerize easily.

It is found in the plant kingdom incorporated in the molecules of some glycosides, because when these are hydrolyzed by enzymes of the plant, HCN, glucose and benzaldehyde are obtained.

These glycosides are found within the seeds of certain fruits such as peaches, apricots, cherries, plums, and in bitter almonds, so they should never be ingested.

It is also found in plant glycosides such as some types of sorghum. Also, some bacteria produce it during their metabolism. It is used mainly in the production of polymers and in some metallurgical processes.

HCN is a deadly poison by inhalation, ingestion and contact. It is present in cigarette smoke and in the smoke from the fires of plastics and materials that contain carbon and nitrogen. It is considered an atmospheric pollutant because it is produced during the combustion of organic material in large areas of the planet.

Molecular structure and electronic configuration

Hydrogen cyanide or hydrogen cyanide is a covalent, molecular compound with one hydrogen, one carbon, and one nitrogen atom.

The carbon atom and the nitrogen atom share 3 pairs of electrons, so they form a triple bond. Hydrogen is bound to carbon, which with this bond has its valence of four and its full electron byte.

Nitrogen has a valence of five and to complete its octet it has a pair of unpaired or solitary electrons located laterally.

HCN is therefore a completely linear molecule, with an unpaired pair of electrons located laterally on the nitrogen.

Lewis representation of hydrocyanic acid, where the electrons shared in each bond and the lone electron pair of nitrogen are observed. Author: Marilú Stea.

Structure of hydrocyanic acid or hydrogen cyanide where the triple bond between carbon and nitrogen is observed. Author: Marilú Stea.

Nomenclature

- Hydrocyanic acid

- Hydrogen cyanide

- Methanonitrile

- Formonitrile

- Hydrocyanic acid

Properties

Physical state

Below 25.6 ºC, if it is anhydrous and stabilized, it is a colorless or pale blue liquid that is very unstable and toxic. If it is above that temperature, it is an extremely poisonous colorless gas.

Molecular weight

27.03 g / mol

Melting point

-13.28 ºC

Boiling point

25.63 ºC (note that it boils just above room temperature).

Flashpoint

-18 ºC (closed cup method)

Auto-ignition temperature

538 ºC

Density

0.6875 g / cm ³ at 20 ºC

Solubility

Fully miscible with water, ethyl alcohol and ethyl ether.

Dissociation constant

K = 2.1 x 10 ^-9

pK _a = 9.2 (it is a very weak acid)

Some chemical properties

HCN has a very high dielectric constant (107 to 25 ºC). This is because its molecules are very polar and associate through hydrogen bonds, as in the case of water H ₂ O.

Because it has such a high dielectric constant, HCN turns out to be a good ionizing solvent.

Liquid anhydrous HCN is very unstable, it tends to polymerize violently. To avoid this, stabilizers are added, such as a small percentage of H ₂ SO ₄.

In aqueous solution and in the presence of ammonia and high pressure, it forms adenine, a compound that is part of DNA and RNA, that is, a biologically important molecule.

It is a very weak acid, as its ionization constant is very small, so it only partially ionizes in water, giving the cyanide anion CN ^-. It forms salts with the bases but not with the carbonates.

Its aqueous solutions not protected from light decompose slowly generating ammonium formate HCOONH ₄.

In solution it has a faint almond odor.

Corrosiveness

As it is a weak acid, it is generally not corrosive.

However, aqueous solutions of HCN that contain sulfuric acid as a stabilizer strongly attack steel at temperatures above 40ºC and stainless steel at temperatures above 80ºC.

Furthermore dilute aqueous solutions of HCN can cause stress to carbon steel even at room temperature.

It can also attack some types of rubbers, plastics, and coatings.

Location in nature

It is found relatively abundant in the plant kingdom as part of glycosides.

For example, it is generated from amygdalin C ₆ H ₅ -CH (-CN) -O-Glucose-O-Glucose, a compound present in bitter almonds. Amygdalin is a cyanogenic beta-glucoside, as when hydrolyzed it forms two glucose molecules, one of benzaldehyde and one of HCN. The enzyme that releases them is beta-glucoxidase.

Amygdalin can be found in the seeds of peaches, apricots, bitter almonds, cherries, and plums.

Some types of sorghum plants contain the cyanogenic glucoside called durrin (ie, p-hydroxy- (S) -mandelonitrile-beta-D-glucoside). This compound can be degraded by a two-step enzymatic hydrolysis.

First, the enzyme durrinase that is endogenous in sorghum plants hydrolyzes it to glucose and p-hydroxy- (S) -mandelonitrile. The latter is then rapidly converted to free HCN and p-hydroxybenzaldehyde.

Sorghum plant with a high content of durrin. No machine-readable author provided. Pethan assumed (based on copyright claims).. Source: Wikipedia Commons.

HCN is responsible for the resistance of sorghum plants to pests and pathogens.

This is explained by the fact that durrin and the enzyme durrinase have different locations in these plants, and they come into contact only when the tissues are injured or destroyed, releasing the HCN and protecting the plant from infections that could penetrate through the injured part..

Durrin molecule where the triple CN bond is observed, which by enzymatic hydrolysis produces HCN. Edgar181. Source: Wikipedia Commons.

Additionally, some human pathogenic bacteria such as Pseudomonas aeruginosa and P. gingivalis produce it during their metabolic activity.

Applications

In the preparation of other chemical compounds and polymers

The use that involves most of the HCN produced at the industrial level is the preparation of intermediates for organic synthesis.

It is used in the synthesis of adiponitrile NC- (CH ₂) ₄ -CN, which is used to prepare nylon or nylon, a polyamide. It is also used to prepare acrylonitrile or cyanoethylene CH ₂ = CH-CN, used to prepare acrylic fibers and plastics.

Its derivative sodium cyanide NaCN is used for the recovery of gold in the mining of this metal.

Another of its derivatives, the cyanogen chloride ClCN, is used for pesticide formulas.

HCN is used for the preparation of chelating agents such as EDTA (ethylene-diamine-tetra-acetate).

It is used for the manufacture of ferrocyanides and some pharmaceutical products.

Various uses

HCN gas has been used as an insecticide, fungicide and disinfectant, for fumigation of ships and buildings. Also to fumigate furniture in order to restore it.

HCN has been used in metal polishing, metal electrodeposition, photographic processes, and metallurgical processes.

Due to its extremely high toxicity, it was designated as a chemical warfare agent.

In agriculture

It has been used as a herbicide and pesticide in orchards. It was used to control scales and other pathogens on citrus trees, but some of these pests have become resistant to HCN.

It has also been used to fumigate grain silos. The HCN gas prepared on site has been used in the fumigation of wheat grains to preserve them from pests such as insects, fungi and rodents. For this use it is essential that the seeds to be fumigated tolerate the pesticidal agent.

Testing has been done by spraying wheat seeds with HCN and it has been found that it does not negatively affect their germination potential, rather it seems to favor it.

However, high doses of HCN can significantly reduce the length of the small leaves that sprout from the seed.

On the other hand, due to the fact that it is a potent nematicide and that some sorghum plants have it in their tissues, the potential of sorghum plants to be used as biocidal green manure is being investigated.

Its use would serve to improve soils, suppress weeds and control diseases and damage caused by phytoparasitic nematodes.

Risks

For humans, HCN is a lethal poison by all routes: inhalation, ingestion and contact.

Author: Clker-Free-Vector-Images. Source: Pixabay.

Inhaled can be fatal. It is estimated that about 60-70% of the population can detect the bitter almond odor of HCN when it is in air at a concentration of 1-5 ppm.

But there is a 20% of the population that cannot detect it even at lethal concentrations because they are genetically unable to do so.

Ingested it is an acute and immediate action poison.

If their solutions come into contact with the skin, the associated cyanide can be lethal.

HCN is present in cigarette smoke and that generated when plastics containing nitrogen are burned.

Mechanism of lethal action within the body

It is a chemical asphyxiator and is rapidly toxic, frequently leading to death. Upon entering the body, it binds to metalloenzymes (enzymes that contain a metal ion), inactivating them. It is a toxic agent for various organs of the human body

Its main toxic effect consists in the inhibition of cellular respiration, since it deactivates an enzyme that influences phosphorylation in mitochondria, which are organelles that intervene, among other things, in the respiratory function of cells.

Risk of cigarette smoke

HCN is present in cigarette smoke.

Although many people know the poisoning effect of HCN, few people realize that they are exposed to its harmful effects through cigarette smoke.

HCN is one of the causes of the inhibition of several cellular respiratory enzymes. The amount of HCN present in cigarette smoke has a particularly damaging effect on the nervous system.

HCN levels in cigarette smoke have been reported between 10 and 400 μg per cigarette for directly inhaled smoke and 0.006 to 0.27 μg / cigarette for secondary inhalation (secondhand smoke). HCN produces toxic effects from 40 µM onwards.

Author: Alexas Fotos. Source: Pixabay.

When inhaled, it quickly enters the bloodstream, where it is released into plasma or binds to hemoglobin. A small part is converted to thiocyanate and is excreted in the urine.

Risks of heating HCN

Prolonged exposure to heat of liquid HCN in closed containers can cause unexpected violent rupture of the containers. It can polymerize explosively at 50-60ºC in the presence of traces of alkali and in the absence of inhibitors.

Presence of HCN in fire smoke

HCN is released during the combustion of nitrogen-containing polymers, such as wool, silk, polyacrylonitriles, and nylon, among others. These materials are present in our homes and in most places of human activity.

For this reason, during fires HCN can potentially be the cause of death by inhalation.

Pollutant of the atmosphere

HCN is a pollutant of the troposphere. It is resistant to photolysis and under ambient atmospheric conditions it does not undergo hydrolysis.

The photochemically produced hydroxyl OH • radicals can react with HCN, but the reaction is very slow, thus the half-life of HCN in the atmosphere is 2 years.

When biomass, especially peat, is burned, HCN is released into the atmosphere, and also during industrial activities. However, the combustion of peat is 5 to 10 times more polluting than the burning of other types of biomass.

Some researchers have found that the high temperatures and drought caused by the El Niño phenomenon in certain areas of the planet exacerbate seasonal fires in areas with a high content of decomposed plant matter.

Author: Steve Buissinne. Source: Pixabay.

This leads to intense biomass burning in dry seasons.

These events are the source of high concentrations of HCN in the troposphere, which are eventually transported to the lower stratosphere, remaining for a very long time.

References

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