Amine Reactions & Quaternary Ammonium Salts (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about the reactions of amines and quaternary ammonium salts.

These parts go together because the lone pair on the nitrogen atom makes amines good nucleophiles, and that one property is behind everything here.

The lone pair on the nitrogen atom in amines makes them good nucleophiles, just like ammonia. That lone pair attacks halogenoalkanes in nucleophilic substitution, and it attacks acyl chlorides and acid anhydrides in nucleophilic addition–elimination. When the substitution runs all the way and a fourth alkyl group goes onto the nitrogen, the product is a quaternary ammonium salt, and the positive charge left on that nitrogen is what makes it useful.

We start with amines as nucleophiles and the substitution series with halogenoalkanes. Then nucleophilic addition–elimination with acyl chlorides and acid anhydrides. Then the quaternary ammonium salt itself, and its use as a cationic surfactant.

The lone pair on the nitrogen atom in amines makes them good nucleophiles, just like ammonia. When ammonia reacts with a halogenoalkane a nucleophilic substitution takes place, forming a primary amine. It happens in two steps: in the first the ammonia acts as a nucleophile, and in the second it acts as a base.

That primary amine is also a good nucleophile, so it can undergo further substitution to give a secondary amine, then a tertiary amine. The final substitution occurs when the tertiary amine reacts with the halogenoalkane to make a quaternary ammonium salt.

Since all these multiple substitutions occur, it is not a very efficient way to synthesise amines. If you want just the primary amine, a large excess of ammonia is used to ensure it is the dominant nucleophile in the reaction vessel.

Ammonia and amines also undergo nucleophilic addition–elimination with acyl chlorides and acid anhydrides. The delta positive carbon atom is the site of the nucleophilic attack by the lone pair on the nitrogen, so it is the same lone pair doing the same job as in substitution.

Chlorine is more electronegative than oxygen, so it creates a stronger dipole along the carbon to chlorine bond, making that carbon more susceptible to attack from nucleophiles. That is why acyl chlorides are more reactive than acid anhydrides.

The reaction produces HCl as an elimination product, which is where the name addition–elimination comes from. Both ammonia and amines are basic, so the HCl reacts to form ammonium chloride or the amine salt, and two moles of ammonia or amine are needed in the overall equation. With ammonia the product is an amide and white fumes of ammonium chloride are formed. With an amine the product is a N-substituted amide and a white organic ammonium salt.

Quaternary ammonium salts are chemical analogues of ammonium chloride, in which the hydrogens have been replaced by four alkyl groups. The alkyl groups can be the same or different. That is where the substitution series ends: once a fourth group is on the nitrogen there is nothing left to substitute.

They are very useful in the manufacture of cleaning products such as conditioners for washing hair and in fabric softeners, and they are cationic because of the positive charge on the nitrogen atom.

In that role the salts act as cationic surfactants. Surfactants are chemicals that lower the surface tension between immiscible liquids and allow wetting to take place.

The hydrocarbon groups are hydrophobic and the amine group is hydrophilic. This allows the molecules to cluster on the surface of water with their hydrophilic charged ends in the water and their hydrophobic tails on the surface.

When hair or clothing is wet it can pick up negative charges that facilitate the build up of static electricity when dry. The conditioner or softener is attracted to the wet surfaces by the positive charge on the surfactant, forms a smooth coating, and prevents the build up of static. The charge doing that job sits on the same nitrogen the lone pair started on.

This is the only nucleophilic substitution reaction which needs two moles of the nucleophile. And when you draw the addition–elimination mechanism, the intermediate must show both formal charges: a negative charge on the oxygen and a positive charge on the nitrogen.

Name the mechanism as nucleophilic addition–elimination, because writing electrophilic addition–elimination loses the mark. Do not show the chlorine leaving in the same step as the nucleophilic attack, and do not use the eliminated chloride ion to remove the hydrogen ion from the intermediate: use a second molecule of amine or ammonia instead.

The lone pair on the nitrogen makes amines good nucleophiles, just like ammonia.

With halogenoalkanes that lone pair drives nucleophilic substitution, and the substitution keeps going through primary, secondary and tertiary amines to a quaternary ammonium salt.

With acyl chlorides and acid anhydrides it drives nucleophilic addition–elimination, giving an amide and an ammonium or amine salt, with two moles of the nucleophile needed.

The quaternary ammonium salt keeps a positive charge on that nitrogen, which is what makes it work as a cationic surfactant in conditioners and fabric softeners.