Difference between revisions of "Hydrogen cyanide"

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Hydrogen cyanide, also known as prussic acid, is a chemical compound with the chemical formula HCN. It is a colorless, extremely poisonous and flammable liquid that boils just above room temperature, at 25.6 °C.

The exact classification of HCN as organic or inorganic remains unresolved. Going by the C-H bond definition, HCN would be organic, but in aqueous solution it dissociates into H⁺ and CN^-, and since the cyanide ion is a pseudohalogen, this makes it inorganic. Hydrogen cyanide is considered a borderline chemical compound.

Properties

Chemical

Hydrogen cyanide reacts with sodium hydroxide to yield sodium cyanide.

HCN + NaOH → NaCN + H₂O

Hydrogen cyanide produces the nucleophilic cyanide ion CN, which can attack the carbonyl carbon of an aldehyde or ketone, forming a hydroxyl nitrile or cyanohydrin.

R-CH=O + HCN → R-HC(OH)-CN

Physical

Hydrogen cyanide is a highly toxic colorless liquid with a bitter almond smell. It is miscible with water and ethanol, and slightly soluble in diethyl ether. HCN melts at -13.4 °C and boils at 25.6 °C. Its density is 0.687 g/cm³.

When hydrogen cyanide is frozen using liquid nitrogen, luminous sparks appear as it freezes. This is caused by a pyroeletric effect, probably induced by the phase transition from a centrosymmetric to a non-centrosymmetric space group.^[1]

Availability

Anhydrous hydrogen cyanide is only available to large chemical entities and cannot be purchased by individuals or small companies.

HCN is is listed on schedule 3 of the Chemical Weapons Convention, meaning all transactions are monitored.

HCN absorbed by diatomaceous earth is somewhat easier to come by: it is sometimes sold as an industrial-strength pesticide. The most well known brand is Uragan D2 (Czech Republic). Another, very infamous brand under which this product was sold was Zyklon-B (the agent used to kill people in Nazi gas chambers).

Preparation

Due to its extreme toxicity, HCN production is generally avoided and instead sodium cyanide or other cyanide salts are used for reactions where a cyanide group is required. The procedures below should only performed if you have proper installations and experience.

Adding any non-oxidizing strong acid to sodium or potassium cyanide will liberate hydrogen cyanide, which will rapidly evaporate due to its low boiling point.

2 NaCN + H₂SO₄ → 2 HCN + Na₂SO₄

2 KCN + H₂SO₄ → 2 HCN + K₂SO₄

Adding a dilute acid to Prussian blue and heating the flask will also yield hydrogen cyanide gas.

Fe₇(CN)₁₈ + 18 H⁺ → HCN + Fe²⁺

The thermal decomposition of amides in the presence of a catalyst will yield hydrogen cyanide gas. An approximate reaction is given below:

2 R-CONH₂ → 2 HCN + H₂O + R-R

A closely related method is the removal of water from formamide, which can be done in a number of ways. Distillation with anhydrous phosphoric acid, as well as passing it over activated alumina at 250-300 °C, as well as passing it over various metal catalysts such as brass, iron or copper-alloys under mild vacuum (in the range 50-150 Torr) and at temperatures of 400-500 °C, sometimes in the presence of ammonia gas, gives HCN in high yields.^[2]

HCONH₂ → HCN + H₂O

Moderate heating of potassium ferrocyanide, K₄[Fe(CN)₆], and ~35% H₂SO₄ will produce produce HCN gas which may be dried with calcium chloride.^[3] The reaction is:^[4]

2 K₄[Fe(CN)₆] + 6 H₂SO₄ → 6 KHSO₄ + K₂Fe[Fe(CN)₆] + 6 HCN

HCN may be prepared quantitatively without the use of acids from Hg(CN)₂ by treatment with aluminium metal in water:^[5][2]

Hg(CN)₂ + Al + 3 H₂O → 2 HCN + Al(OH)₃ + ½ H₂ + Hg

Very pure HCN may be obtained by the dehydration of ammonium formate with phosphorus pentoxide in a molar ratio of 1:2.5 with heating.^[2][6]

3 HCOONH₄ + P₂O₅ → 3 HCN + 2 H₃PO₄

A crude method of obtaining hydrogen cyanide involves adding acid to ground cherry, bitter almond, peach or apricot seed kernels. The acid breaks down the amygdalin releasing HCN gas.

The combustion of nitrile containing plastic materials will give off very impure hydrogen cyanide gas.

Projects

Seriously, if you really want to make experiments with cyanides, it's MUCH safer to use cyanide salts. Experimenting with hydrogen cyanide is not worth the risk.

Handling

Safety

Hydrogen cyanide is lethal. Air concentrations of 100–200 ppm in air will kill a human within 10-60 minutes, while 2000 ppm (about 2380 mg/m³) will kill a human in about 1 minute. Since it boils slightly above room temperature, open containers are a big no-no, and metal cylinders are instead employed, which severely limit its evaporation.

Contact of HCN with bases and amines may cause violent polymerization and even explosion.

Hydrogen cyanide will also attack certain plastic and rubber materials.

In case of poisoning, glucose, methylene blue or sodium thiosulfate can be used as antidotes.

Storage

DO NOT STORE HCN! EVER! If you absolutely want to, you can make your own Uragan D2 and store that.

Disposal

Hydrogen cyanide can be neutralized with a variety of chemicals, such as hydrogen peroxide, sodium hypochlorite (bleach), which will convert it to the less toxic isocyanate species. Sodium thiosulfate will convert it to thiocyanate, which is less toxic. These two products can be further oxidized to nitrogen, carbon dioxide and water with a strong oxidizer.

References

1. ↑ https://www.youtube.com/watch?v=MghUSkNKjYc
2. ↑ ^{2.0 2.1 2.2} Gmelins Handbuch der anorganischen Chemie, Kohlenstoff Teil D1, Verlag Chemie GmbH, Weinheim/Bergstraße, 8th edition 1971, p. 156, 159
3. ↑ L. Gattermann, Justus Liebigs Ann. Chem. 357, 318-319 (1907). https://doi.org/10.1002/jlac.19073570209
4. ↑ Gmelins Handbuch der anorganischen Chemie, Eisen Teil B, Verlag Chemie GmbH, Berlin, 8th edition 1932, p. 659
5. ↑ G. Denigès, Bull. Trav. Soc. Pharm. Bordeux 85, 36 (1947)
6. ↑ M. Lebl, Chemicke Listy 64, 877-879 (1970)

Relevant Sciencemadness threads

• Preparation of cyanides
• Distilling HCN from potassium ferrocyanide...
• What does HCN or H2S poisoning feels like?