Nitriles

INTRODUCING NITRILES

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

What are nitriles?

Nitriles contain the -CN group, and used to be known as cyanides.

Some simple nitriles

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.

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

---

Note:  Compounds like this are formed when aldehydes react with hydrogen cyanide. This is therefore the sort of branched nitrile that you are most likely to come across at this level.

Physical properties

Boiling points

The small nitriles are liquids at room temperature.

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

Note:  The majority of the data sheets I have looked at quote this boiling range for butanenitrile. I don't know why it doesn't seem to have a precise boiling point.

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.

Hydrolysis of nitriles with aqueous acid to give carboxylic acids

Description: Addition of water and acid to a nitrile leads to formation of a carboxylic acid.

Notes:

• This reaction is referred to as “acidic hydrolysis”.
• The reaction is generally used with water as solvent, so an excess of water is present. The acid used is often written as “H3O(+)”

Mechanism:

Protonation of the nitrile nitrogen by acid (Step 1, arrows A and B) makes the nitrile carbon a better electrophile. Attack at the carbon by water (Step 2, arrows C and D) followed by proton transfer (Step 3, arrows E and F) gives a species that is inesonance with a protonated amide (arrows G and H). Addition of water to the protonated amide (Step 4, arrows I and J) followed by proton transfer (Step 5, arrows K and L) result in formation of NH3(+) which is an excellent leaving group. Expulsion of NH3 through 1,2-addition (Step 6, arrows M and N) followed by deprotonation (Step 7, arrows O and P) give the carboxylic acid.

Reaction type:  Nucleophilic Acyl Substitution then Nucleophilic Addition

Summary:

• Nitriles, RCºN, react with Grignard reagents or organolithium reagents to give ketones.
• The strongly nucleophilic organometallic reagents add to the CºN bond in a similar fashion to that seen for aldehydes and ketones.
• The reaction proceeds via an imine salt intermediate that is then hydrolyzed to give the ketone product.

• Since the ketone is not formed until after the addition of water, the organometallic reagent does not get the opportunity to react with the ketone product.

Dehydration of amides to give nitriles

Description: Primary amides can be converted to nitriles with a dehydrating reagent such as P₂O₅ .

Notes: Note that the net effect of this reaction is to remove two H atoms and one O from the amide. For this reason this is called a “dehydration”.

Only primary amides work for this reaction. Other reagents can be used for this, however, such as thionyl chloride (SOCl₂)

Examples:

Notes:

Mechanism:

The reaction begins with the oxygen of the amide attacking phosphorus (through a resonance form) forming an O–P bond (Step 1, arrows A, B, and C). After a proton transfer (Step 2, arrows D and E) a lone pair from nitrogen forms a new C–N bond, expelling oxygen (Step 3, arrows F and G). Finally the nitrogen is deprotonated (Step 4, arrows H and I) to give the neutral nitrile.

Notes:

There are certainly other reasonable ways to draw proton transfer (Step 2) as well as other bases to use for deprotonation (Step 4) besides phosphate. This is just one reasonable possibility.

It’s also reasonable to show fragmentation of the P–O–P bond in step 3, although for simplicity’s sake this was not drawn.