Sodium cyanide

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Sodium cyanide
Sodium cyanide impure by NurdRage.jpg
Impure sodium cyanide made by NurdRage
Names
IUPAC name
Sodium cyanide
Other names
Cyanide of sodium
Properties
NaCN
Molar mass 49.0072 g/mol
Appearance White solid
Odor Faint almond-like
Density 1.5955 g/cm3
Melting point 563.7 °C (1,046.7 °F; 836.9 K)
Boiling point 1,496 °C (2,725 °F; 1,769 K)
48.15 g/100 ml (10 °C)
63.7 g/100 ml (25 °C)
Solubility Soluble in ammonia, methanol
Slightly soluble in dimethylformamide, ethanol, isopropanol, liq. SO2
Insoluble in DMSO, hydrocarbons
Solubility in ammonia 40.3 g/100 g (10 °C)
49.4 g/100 g (20 °C)
58.5 g/100 g (30 °C)
67.6 g/100 g (40 °C)
76.7 g/100 g (50 °C)
Solubility in ethanol 0.97 g/100 g (25 °C)
Solubility in ethanol 95% 1.92 g/100 g (25 °C)
Solubility in furfural 0.02% (25 °C)
Solubility in methanol 6.44 g/100 ml (15 °C)
7.8 g/100 ml (25 °C)
4.10 g/100 ml (67.4 °C)
Thermochemistry
115.7 J·mol-1·K-1
-91 kJ/mol
Hazards
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
6.44 mg/kg (rat, oral)
4 mg/kg (sheep, oral)
15 mg/kg (mammal, oral)
8 mg/kg (rat, oral)
Related compounds
Related compounds
Potassium cyanide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Sodium cyanide is an inorganic compound with the formula NaCN. It is a famous chemical historically used as poison, but it's immensely useful in chemistry.

Properties

Chemical

Sodium cyanide reacts with halocarbons to give nitriles. Iodine can be used as catalyst.

Physical

Sodium cyanide is a white crystalline solid, soluble in water, aq. alkali as well as methanol.

Availability

Sodium cyanide is sold by chemical suppliers, but due to it's toxicity, companies won't sell to the amateur chemist.

Preparation

Preparation of sodium cyanide is extremely dangerous and an inexperienced chemist may even die if proper safety protocols aren't in place. Do not attempt to synthesize NaCN if you don't know what to do. No seriously, don't!

A common route involves the reaction of molten sodium hydroxide with cyanuric acid. To do this, in a molten alkali crucible/can add 100 g of sodium hydroxide, 43 g of cyanuric acid and 12 g of carbon, which can be ordinary charcoal, though activated carbon may work better, due to its high surface area. Using a metal rod, mix the components. Slowly heat the mixture for 1 hour until it melts and raise it to 600 °C. The reaction produces trisodium cyanurate and water, which boils off. If you want to skip this step, you can simply make the salt by dissolving the cyanuric acid in an aq. solution of NaOH, then remove the water. Above 550 °C, trisodium cyanurate breaks down to sodium cyanate, which is further reduced by carbon to sodium cyanide. Carbon dioxide is formed, which boils off, producing lots of bubbles. Lots of sodium carbonate will be produced, as some of the carbon dioxide does not escape and instead it reacts with the molten sodium hydroxide. When the bubbling stops, turn off the heating and let it cool. Break apart the resulting solid and add methanol to dissolve sodium cyanide, as sodium carbonate and carbon are insoluble in methanol. After most has dissolved, add 100 g of sodium bicarbonate to neutralize the leftover sodium hydroxide, since NaOH is soluble in methanol, but sodium carbonate is insoluble. Filter the solution and remove the methanol to get solid sodium cyanide. The final product is impure, but can be further purified via recrystallization.[2]

Another route involves melting a mixture of sodium hydroxide with urea. Sodium cyanate is formed, which is ground, mixed with a powdered reducing agent like magnesium or zinc (but NOT iron) and ignited in a thermite-like reaction. This produces sodium cyanide and magnesium/carbon oxide. Dissolve the mixture in water and filter it to get rid of the magnesium oxide/hydroxide. This process gives crude sodium cyanide which needs to be purified. For this reaction, aluminium powder does not work, since aluminium is not reactive enough to initiate the reaction, while using iron powder will give Prussian blue. Also, while magnesium works because it removes the oxygen from the cyanate, magnesium is also reactive enough that it will also remove the nitrogen from the cyanate and cyanide molecule (possibly even carbon under the right conditions), leaving behind a very hot and very reactive sodium/potassium carbide which will immediately oxidize in air. Likewise, the resulting cyanide, which is also hot, will also oxidize in air to carbonate, meaning this reaction is best done in an air free container. The ignition of the mixture will also cause it to swollen up, and escape the crucible, where the hot cyanide will oxidize in air, further destroying your desired cyanide. While this method is much faster than the one mentioned above, the yields are pretty bad.

The NaCN obtained through these routes is extremely impure, and while can be used for general reactions (like dissolving gold and making Prussian blue or other Fe(CN) complexes), it is not pure enough for use in organic reactions. To further purify NaCN, there are two ways:

  • The safe, slow way: convert the impure NaCN into Prussian blue, filter it from the solution, calcinate the Prussian blue using molten sodium hydroxide to reconvert it back into NaCN, and purify it by recrystallizing it from ethanol and water.
  • The dangerous, quick way: convert the NaCN into Prussian blue, then add conc. sulfuric acid and gently heat the flask to distill the resulting HCN and bubble it in a cooled alcoholic solution of NaOH (methanol is a good choice). Since this route involves working with gaseous HCN, do not do this unless you have experience and this absolutely must be done outside or under a fumehood. Impure NaCN tends to have some sodium cyanate which in contact with a strong acid releases isocyanic acid, which may contaminate your distillate, but also has lots of sodium carbonate, which will release carbon dioxide upon reaction with an acid, which will react with your sodium hydroxide and form sodium carbonate again, contaminating your final product.

To accurately determine the amount of cyanide from the resulting mixture, you will have to titrate the cyanide solution using a solution of silver nitrate and using 5-(4-Dimethylaminobenzylidene)rhodanine as indicator.

Projects

  • Dissolve gold
  • Make nitriles
  • Make cyanogen and cyanuric chloride
  • Make Prussian blue
  • Make glycolic acid
  • Von Richter reaction

Handling

Safety

Sodium cyanide is highly toxic. Doses as small as 200 mg can kill a fully grown man.

Storage

Sodium cyanide should be stored in closed bottles, away from any acids, in a locked cabinet with a clear hazard label on the storage bottle. Alkali bases can be used as stabilizer.

Disposal

Sodium cyanide can be destroyed by oxidizing it with excess bleach or hydrogen peroxide to the less harmful sodium cyanate.

NaCN + NaClO → NaCNO + NaCl
NaCN + H2O2 → NaCNO + H2O

Decreasing the pH of the sodium cyanate in the presence of bleach, by adding an acid, will cause it to convert to sodium chloride.[3]

2 NaClO + 2 H+ → Cl2 + 2 Na+ + H2O
2 NaCN + 4 NaCNO + 3 Cl2 → 6 NaCl + 2 CO2 + N2 + 2 H2O

Sodium cyanate will also slowly hydrolyzes in water to harmless sodium carbonate and ammonium carbonate.

2 NaCNO + 4 H2O → Na2CO3 + (NH4)2CO3

References

  1. http://nist.gov/data/PDFfiles/jpcrd643.pdf
  2. https://www.youtube.com/watch?v=xz7i11XC9wk
  3. http://www.cipca.org/presentations/2013/parham_ww-treatment.pdf

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