Toxic hydrogen cyanide

In this demonstration students are introduced to the toxic hazard symbol and the effects of the toxic chemical hydrogen cyanide, derived from the leaves of cherry laurel (Prunus laurocerasus). Credit for this demonstration must go to my former, since retired, head of department Dr Tony Allen of Dawlish, Devon, who is a keen entomologist. 

HCN - a natural insecticide  

Hydrogen cyanide, HCN, is a colourless, toxic, volatile liquid (bp 26 ºC). The UK workplace exposure limit for HCN (short-term, ie  15 minute exposure) is 10 ppm (11 mg m^-3). The CN^- ion is toxic because it attaches to the Fe in cytochrome oxidase, an enzyme involved in cellular respiration.¹  The attached cyanide ion inhibits the enzyme, thus causing cell death.  

The shrub cherry laurel is common in most parts of the UK and Europe. Its leaves contain cyanogenic glycosides, molecules comprising a sugar residue attached to a cyanide-containing non-carbohydrate residue. When these compounds are broken down by enzymes they release HCN. Natural cyanide compounds are also found in apricot stones, cherry pips, peach and plum stones, corn, chickpeas, cashews and almonds. The Romans and Egyptians knew this all too well and extracted the prussic acid (hydrogen cyanide) from peach stones to use as a poison. 

Kit 

Two jam jars with lids; 
Cherry laurel leaves; 
An insect. 

Procedure

Collect a handful of fresh cherry laurel leaves. The young leaves are the best; they are easily recognised by their light green appearance and soft feel, and grow at the tips of stems. Rip the leaves into several pieces, rolling them in your fingers to crush them. Put the pieces in a small glass jam jar with a lid. Next trap an insect (eg  a fly) in another jam jar. Seal the jar and store in a dark cupboard until required. 

Before the demonstration, pass a pot of the leaves around the class and ask the students if they recognise the smell. Hopefully, someone will mention almonds or marzipan which is the characteristic odour of the HCN. Show the students the toxic hazard symbol, emphasising that HCN is toxic and in a confined space, where higher concentrations can accumulate, it can be lethal. 

Finally, carefully lift the lid of the jar containing the insect and add a few crushed leaves. Replace the lid and put the pot on a table in view of the class, or use a webcam and projector for larger classes. Watch as the insect rapidly dies in  ca  1 minute. 

Special tips

September is a good time to gather insects. Storing the insects in darkness calms them and they stay alive longer. Only the fresh young leaves of the cherry laurel contain sufficient cyanide-containing compounds to be effective. After storing in a sealed jar for a few days the crushed leaves turn brown, give off more HCN and work better.  

 

Teaching goals 

This demonstration forms part of a series of experiments I do in an introductory lesson on health and safety in the laboratory for year 7 pupils new to the school. The experiments are designed to illustrate, and help students understand, the dangers of laboratory chemicals, and to help them interpret hazard symbols.  

I start the lesson by highlighting the dangers of strong acids with a demonstration of the dehydration of sucrose using concentrated sulfuric acid. (Note this experiment must be done in a fume cupboard because quite a lot of sulfur dioxide is formed.) I tell the students that the acid is corrosive, identifying the hazard symbol. I then show them that citric acid (vitamin C) has an irritant symbol on the bottle yet some of us enjoy eating (but emphasising not when in the lab) 'acid drop' sweets, which are coated in this irritating compound. I include other demonstrations to illustrate the meaning of: 

• highly inflammable, eg by igniting a methanol-air mixture in a large bottle taken from an office water dispenser (great care needed); 
• oxidising, eg  the oxidation of glycerol using KMnO₄; and 
• explosive, eg  by igniting a balloon filled with hydrogen and oxygen.  

Finally, I use HCN to kill an insect to illustrate the meaning of toxic.  

References

1. D. F. Shriver, P. W. Atkins and C. H. Langford, Inorganic chemistry, 4th edn, p346. Oxford: OUP, 1990.