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Reactions of Halogenoalkanes (Haloalkanes) and Cyanide Ions

This page looks at the reaction between halogenoalkanes (haloalkanes or alkyl halides) and cyanide ions from sodium or potassium cyanide solution.

You will find a link to a separate page about the mechanisms for the reaction.

Replacing a Halogen with –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.

Note: A solution of potassium cyanide in water is quite alkaline, and contains significant amounts of hydroxide ions from an interaction between the cyanide ions and water. These react with the halogenoalkane. This is discussed on the page about the reactions between halogenoalkanes and hydroxide ions.

For example, using 1-bromopropane as a typical primary halogenoalkane:

\text{CH}_3\text{CH}_2\text{CH}_2\text{Br} + \text{CN}^- \longrightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CN} + \text{Br}^-

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

\text{CH}_3\text{CH}_2\text{CH}_2\text{Br} + \text{KCN} \longrightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CN} + \text{KBr}

The bromine (or other halogen) in the halogenoalkane is simply replaced by a -CN group – hence a substitution reaction. In this example, butanenitrile is formed.

Note: When you are naming nitriles, you have to remember to include the carbon in the -CN group when you count the longest chain. In this example, there are 4 carbons in the longest chain – hence butanenitrile.

Secondary and tertiary halogenoalkanes behave similarly, although the mechanism will vary depending on which sort of halogenoalkane you are using.

Note: If you aren't sure what primary, secondary and tertiary halogenoalkanes are, you should read the beginning of the introduction to halogenoalkanes.

You will find the mechanisms for these reactions by following this link.

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 aren't 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.

Note: If you want to find out more about nitriles and reactions of the -CN group, you could explore the nitriles menu.

Questions to test your understanding

Questions on the reactions of halogenoalkanes with cyanide ions Answers