Chemguide: Support for CIE A level Chemistry

Learning outcome 17: Carbonyl compounds

17.1: Aldehydes and ketones

Statement 17.1.3:

This statement talks about the mechanism for the reaction between HCN and aldehydes and ketones. Before you go on, find and read this statement together with the preceding one 17.1.2(b).

You will find what you need on the page about the mechanism for the reactions between carbonyl compounds and hydrogen cyanide. You will probably want to follow the link to the "Talk me through . . ." page as well.

CIE want a simplified version of all this, and so there are bits of those pages which you can ignore, or just skim through quickly if you are interested.

You will be asked about this mechanism using hydrogen cyanide in the presence of some cyanide ions. You do not need to worry about exactly how this is achieved, or the reasons for the pH chosen for the reaction. Just learn that you use HCN together with some cyanide ions (from, say, sodium cyanide) as a catalyst.

So show the first stage of the reaction with the cyanide ion attacking the C=O double bond.

Then you must show the second stage of the mechanism properly as a reaction between the intermediate ion and an HCN molecule regenerating a cyanide ion.

You will not be asked about the optical isomerism parts of these pages - ignore them.

A common follow-on from questions involving this statement

Quite a lot of questions follow this up by looking at the hydrolysis of the -CN group in the product molecule to give a carboxylic acid. This is dealt with in statement 19.2.3. Bear this in mind when you come to do your revision.

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© Jim Clark 2020