Nitriles and its Preparation

A nitrile is an organic chemical that contains a cyano functional group (subunit), CN^-, in which the carbon and nitrogen atoms have a triple bond i.e. C≡N^-. The general chemical formula of a nitrile is RCN, where R is the organic group.

Additional Information

Under the standard chemical naming convention used by chemists (IUPAC nomenclature), nitriles are named according to these rules: a.       Compounds in which the carbon atom of the CN group is attached to an acyclic hydrocarbon fragment are generally named using the suffix nitrile. b.      Nitriles in which the -CN group can be thought of as having replaced the -COOH group of a corresponding trivially named carboxylic acid are named by removing "ic acid" or "oic acid" and replacing it with onitrile. c.       A nitrile that is an analog of a compound that has "carboxylic acid" as its suffix, is named by subsitituting the suffix carbonitrile

In addition, the use of cyanide as a suffix is still common, and one nitrile may be known by a variety of different names. Some examples of these are shown below, with the most rigorous IUPAC name for each shown in red:

When there are other functional groups that take naming precedence over the cyanide/nitrile group, the prefix cyano is used. In this example, we have color-coded the parts of the name and the parts of the molecule that correspond to each other:

Physical properties

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 can't 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.

MAKING NITRILES

Making nitriles from halogenoalkanes

The 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 halogenoalkane:

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

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.

Making a nitrile by this method is a useful way of increasing the length of a carbon chain. Having made the nitrile, the -CN group can easily be modified to make other things - as you will find if you explore the nitriles menu (link a the bottom of the page).

Making nitriles from amides

Nitriles can be made by dehydrating amides.

Amides are dehydrated by heating a solid mixture of the amide and phosphorus(V) oxide, P₄O₁₀.

Water is removed from the amide group to leave a nitrile group, -CN. The liquid nitrile is collected by simple distillation.

For example, you will get ethanenitrile by dehydrating ethanamide.

Making nitriles from aldehydes and ketones

Aldehydes and ketones undergo an addition reaction with hydrogen cyanide. The hydrogen cyanide adds across the carbon-oxygen double bond in the aldehyde or ketone to produce a hydroxynitrile. Hydroxynitriles 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:

In every example of this kind, the -OH group will be on the number 2 carbon atom - the one next to the -CN group.

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.

These are useful reactions because they not only increase the number of carbon atoms in a chain, but also introduce another reactive group as well as the -CN group. The -OH group behaves just like the -OH group in any alcohol with a similar structure.

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 use of Nitriles

Nitriles have tremendous industrial importance. For example, through a process called hydrocyanation, HCN reacts with 1,4-butadiene (an alkene) to form adiponitrile (1,4-dicyanobutane, hexanedinitrile, tetramethylene cyanide, NC(CH₂)₄CN), a chemical precursor to hexamethylene diamine (1,6-diaminohexane, H₂N(CH₂)₆NH₂), one of the polymers used to make nylon. The wide range of chemical reactivity for nitriles is what makes them so useful, but this same feature means that they are incompatible with many substances (see MSDS relevance below).

~~forgiveness if there are mistakes~~

Problem

Cyanide is highly toxic. Cyanide consists of cyano, CN^-,  so does the nitrile.
whether nitrile is also highly toxic ??