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Why is cyanide so interesting to my readers

Some time ago I wrote about cyanide in fruit, while the C word does excite all manner of “mighty dread” I think that the public think about the C word because of war crimes and literature.

I had a long think about this subject, I have to confess that while cyanide is not exactly a health tonic it has a lot of very interesting chemistry. In modern chemistry people try to avoid using it but sometimes it does get used to make things happen.

One nice reaction is that of p-benzoquinone and cyanide in DMSO, this is the basis of a wounderful test for cyanide. If cyanide is added to a solution of the benzoquinone then an aromatic compound which is fluorescent is formed. This test was described in a paper by George G. Guilbault and David N. Kramer, Analytical Chemistry, 1965, volume 37, issue 11, pages 1395-1399. Below is shown the chemical reaction by which is works.

Reaction of benzoquinone with cyanide

Figure 1. The benzoquinone reaction.

Another nice reaction is the condensation reaction of two molecules of benzaldehyde to form a hydroxyketone, this is a reaction which known as the benzoin condensation. In this reaction a cyanide anion attacks the aldehydes as a nucelophile and then forms a new nucleophile which attacks another molecule of the aldehyde. Then to regenerate the cyanide anion the cyanide is lost as a leaving group thus forming the product. This is a nice example of carbonyl chemistry, I stress “example”. The most important thing which I have learn in organic chemistry is that the same reactions keep coming back again and again but dressed up in slightly different outfits.

The first step is the reaction of the cyanide with the benzaldehyde to form the addition product. This is a single step process which is likely to go via the transition state shown. Now I have been kind to you and drawn in the curved arrow, you will need in the future to use the curved arrow to understand organic chemistry and do you grades the world of good. Always keep this golden rule in mind, the curved arrow moves electrons and NEVER atoms or atomic nuclei. Now here it is.

The first step in the benzoin condensation

Now the second step is a transfer of a proton from the carbon onto the oxygen, the cyanide group helps by withdrawing electron density from the carbon bearing the hydrogen which is to be transferred. The cyanide will pull electron density by means of a resonance effect and a little bit of an inductive pull. The inductive pull is due to a tug of war for electron density in the sigma bonds.

As the reaction is normally run in a protic medium (mixture of water and alcohol) I have drawn a mechanism where a proton from the solvent attaches to the alkoxide oxygen before the carbon is deprotonated.

Second stage, the proton transfer

 The next step is the one which forms the carbon carbon bond, here the carbon centred anion attacks a second molecule of benzaldehyde to form a new intermediate.

 

The step which forms the carbon carbon bond

Now what is needed is a proton transfer step to move the minus charge, this should be an easy one.

A proton transfer step to set up the molecule for the last step

Now the last step is where the cyanide is used as a leaving group. The important thing about this reaction is that the cyanide is both a nucleophile and a leaving group. Now we will see the final step.

Final step of the reaction

 

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