# The Reaction of Alkyl Halides with Cyanide Ions

Halogenoalkanes (haloalkanes or alkyl halides) reaction with cyanide ions from sodium or potassium cyanide solution to generate nitriles.

## Replacing a halogen by -CN

If a halogenoalkane is heated under reflux with a solution of sodium or potassium cyanide in ethanol, the halogen is replaced by a -CN group and a nitrile is produced. Heating under reflux means heating with a condenser placed vertically in the flask to prevent loss of volatile substances from the mixture.

The solvent is important. If water is present you tend to get substitution by -OH instead of -CN. For example, using 1-bromopropane as a typical primary halogenoalkane:

$\ce{CH_3CH_2CH_2Br + CN^{-} \rightarrow CH_3CH_2CH_2CN + Br^{-}}$

You could write the full equation rather than the ionic one, but it slightly obscures what's going on:

$\ce{CH_3CH_2CH_2Br + KCN \rightarrow CH_3CH_2CH_2CN + KBr}$

The bromine (or other halogen) in the halogenoalkane is simply replaced by a -CN group - hence a substitution reaction to generate a nitrile. In this example, butanenitrile is formed. Secondary and tertiary halogenoalkanes behave similarly, although the mechanism will vary depending on which sort of halogenoalkane you are using.

## Why these reactions matter

This reaction with cyanide ions is a useful way of lengthening carbon chains. For example, in the equations above, you start with a 3-carbon chain and end up with a 4-carbon chain. There are not very many simple ways of making new carbon-carbon bonds. It is fairly easy to change the -CN group at the end of the new chain into other groups.