Reactions of Amines

State the reactions of amines you need to know.

• Ethanoylation of amines

• Reaction with nitric (III) acid / nitrous acid

• Coupling reactions with benzenediazonium salts.

What is ethanoylation?

When a hydrogen atom in an amine is replaced with the ethanoyl group (CH₃CO).

An acid chloride e.g. ethanoyl chloride is used; the product formed is an amide.

How can you determine if a compound is an aliphatic or aromatic amine?

By reactions of nitrous / nitric (III) acid.

Briefly describe nitrous / nitric (III) acid.

Nitric(III) acid (or nitrous acid), HNO₂, is unstable and is always prepared in situ.

in the reaction mixture (as it is needed).

How is nitric (III) acid formed?

By reacting sodium nitrite (NaNO₂) with hydrochloric acid.

NaNO₂ + HCl = HNO₂ + NaCl

Describe the reaction of nitrous / nitric (III) acid with aliphatic amines.

Aliphatic amines react with nitrous acid at any temperature to form:

• Alcohol

• Water

• Nitrogen gas

Within this reaction, effervescence is observed.

Describe the reaction of aromatic amines (e.g. phenylamine) with nitrous acid at room temperature (above 10°C).

At room temperature, aromatic amines react similarly to aliphatic amines, producing:

• Phenol

• Water

• Nitrogen gas

Effervescence is observed.

Describe the reaction of aromatic amines (e.g. phenylamine) with cold nitrous acid at room temperature (below 10°C).

A stable benzenediazonium ion is formed.

• No effervescence is observed.

Summarise what coupling reactions are.

• Benzenediazonium compounds are extremely reactive and, below 10°C, they react with phenols and aromatic amines in alkaline solution.

• The compounds formed contain the -N=N- azo group, which links the two benzene rings together.

Coupling reactions are used to form what?

Azodyes.

Azodyes contain the -N=N- azo group, which links the two benzene rings together.

What is a chromophore?

A structural unit of a molecule which is responsible for the absorption of a certain wavelength in the visible spectrum.

Chromophores are responsible for giving a molecule its colour.

What rule of thumb can be used to determine the colour of a compound using its absorption?

The colour seen of a compound is OPPOSITE to the colour absorbed.

E.g. Red and blue are on opposite sides of the visible spectrum.

if a compound shows a maximum absorption at the blue section of the spectrum, the colour seen will be red / orange/ yellow.