SIMPLE ADDITION TO ALDEHYDES AND KETONES

This page looks at the addition of hydrogen cyanide and sodium hydrogensulphite (sodium bisulphite) to aldehydes and ketones.

Addition of hydrogen cyanide to aldehydes and ketones

The reactions

Hydrogen cyanide adds across the carbon-oxygen double bond in aldehydes and ketones to produce compounds known as hydroxynitriles. These used to be known as cyanohydrins.

For example, with ethanal (an aldehyde) you get 2-hydroxypropanenitrile:

With propanone (a ketone) you get 2-hydroxy-2-methylpropanenitrile:

The reaction isn't normally done using hydrogen cyanide itself, because this is an extremely poisonous gas. Instead, the aldehyde or ketone is mixed with a solution of sodium or potassium cyanide in water to which a little sulphuric acid has been added. The pH of the solution is adjusted to about 4 - 5, because this gives the fastest reaction. The reaction happens at room temperature.

The solution will contain hydrogen cyanide (from the reaction between the sodium or potassium cyanide and the sulphuric acid), but still contains some free cyanide ions. This is important for the mechanism.

Uses of the reaction

The product molecules contain two functional groups:

• the -OH group which behaves like a simple alcohol and can be replaced by other things like chlorine, which can in turn be replaced to give, for example, an -NH₂ group;

• the -CN group which is easily converted into a carboxylic acid group -COOH.

For example, starting from a hydroxynitrile made from an aldehyde, you can quite easily produce relatively complicated molecules like 2-amino acids - the amino acids which are used to construct proteins.

The second step is the reaction between a halogenoalkane and ammonia.

The final step is hydrolysis of a nitrile.

Addition of sodium hydrogensulphite to aldehydes and ketones

Sodium hydrogensulphite used to be known as sodium bisulphite, and you might well still come across it in organic textbooks under this name - or using the bisulfite spelling.

The reactions

This reaction only works well for aldehydes. In the case of ketones, one of the hydrocarbon groups attached to the carbonyl group needs to be a methyl group. Bulky groups attached to the carbonyl group get in the way of the reaction happening.

The aldehyde or ketone is shaken with a saturated solution of sodium hydrogensulphite in water. Where the product is formed, it separates as white crystals.

In the case of ethanal, the equation is:

. . . and with propanone, the equation is:

These compounds are rarely named systematically, and are usually known as "hydrogensulphite (or bisulphite) addition compounds".

Uses of the reaction

The reaction is usually used during the purification of aldehydes (and any ketones that it works for). The addition compound can be split easily to regenerate the aldehyde or ketone by treating it with either dilute acid or dilute alkali.

If you have an impure aldehyde, for example, you could shake it with a saturated solution of sodium hydrogensulphite to produce the crystals. These crystals could easily be filtered and washed to remove any other impurities. Addition of dilute acid, for example, would then regenerate the original aldehyde.

It would, of course, still need to be separated from the excess acid and assorted inorganic products of the reaction - but that is beyond the scope of this page!