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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:

\[CH_3CH_2CH_2Br + CN^- \rightarrow CH_3CH_2CH_2CN + Br^-\]

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

\[CH_3CH_2CH_2Br + KCN \rightarrow CH_3CH_2CH_2CN + KBr\]

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.


Jim Clark (