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Introduction to Nitriles

This page explains what nitriles are and looks at their simple physical properties such as solubility and boiling points.

Introduction

Nitriles contain the -CN group, and used to be known as cyanides. The smallest organic nitrile is ethanenitrile, CH3CN, (old name: methyl cyanide or acetonitrile - and sometimes now called ethanonitrile). Hydrogen cyanide, HCN, doesn't usually count as organic, even though it contains a carbon atom.

ethanenitrile.gif

Notice the triple bond between the carbon and nitrogen in the -CN group. The three simplest nitriles are:

CH3CN ethanenitrile
CH3CH2CN propanenitrile
CH3CH2CH2CN butanenitrile

When you are counting the length of the carbon chain, don't forget the carbon in the -CN group. If the chain is branched, this carbon usually counts as the number 1 carbon.

branched.gif

Physical properties

Boiling points

The small nitriles are liquids at room temperature.

nitrile boiling point (°C)
CH3CN 82
CH3CH2CN 97
CH3CH2CH2CN 116 - 118

These boiling points are very high for the size of the molecules - similar to what you would expect if they were capable of forming hydrogen bonds. However, they don't form hydrogen bonds - they don't have a hydrogen atom directly attached to an electronegative element. They are just very polar molecules. The nitrogen is very electronegative and the electrons in the triple bond are very easily pulled towards the nitrogen end of the bond. Nitriles therefore have strong permanent dipole-dipole attractions as well as van der Waals dispersion forces between their molecules.

Solubility in water

Ethanenitrile is completely soluble in water, and the solubility then falls as chain length increases.

nitrile solubility at 20°C
CH3CN miscible
CH3CH2CN 10 g per 100 cm3 of water
CH3CH2CH2CN 3 g per 100 cm3 of water

The reason for the solubility is that although nitriles cannot hydrogen bond with themselves, they can hydrogen bond with water molecules. One of the slightly positive hydrogen atoms in a water molecule is attracted to the lone pair on the nitrogen atom in a nitrile and a hydrogen bond is formed.

There will also, of course, be dispersion forces and dipole-dipole attractions between the nitrile and water molecules. Forming these attractions releases energy. This helps to supply the energy needed to separate water molecule from water molecule and nitrile molecule from nitrile molecule before they can mix together. As chain lengths increase, the hydrocarbon parts of the nitrile molecules start to get in the way. By forcing themselves between water molecules, they break the relatively strong hydrogen bonds between water molecules without replacing them by anything as good. This makes the process energetically less profitable, and so solubility decreases.

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