After studying this section you should be able to:

• explain the relative basicities of ethylamine and phenylamine
• describe the reactions of primary amines with acids to form salts
• describe the formation of phenylamine by reduction of nitrobenzene
• describe the synthesis of an azo-dye from phenylamine
• describe the reactions of amines with halogenoalkanes and acyl chlorides

Aliphatic and aromatic amines

• Notice the similarity with ammonia.

Physical properties of amines

•  higher melting and boiling points than alkanes of comparable relative molecular mass
• solubility in water – amines with fewer than six carbons mix with water in all proportions.
• As with other polar functional groups, the solubility of amines in water decreases with increasing carbon chain length as the non-polar contribution to the molecule becomes more important.

Amines as bases

• Amines are the organic bases.

The basicity of aliphatic and aromatic amines

Aliphatic amines are stronger bases than ammonia; aromatic amines are substantially weaker.

Organic ammonium salts

• Amines react by the usual ‘base reactions’ producing organic ammonium salts
• A proton, H+, is added to the amino nitrogen atom.

Preparation of amines

Other reducing agents can also be used:

•  H2/Ni catalyst
• LiAlH4 in dry ether
• LiAlH4 is an almost universal reducing agent and works in most examples of organic reduction.

Preparation of amines from amides (Edexcel only)

• This process is often called the Hofmann degradation and can be used to move down a homologous series. 

The preparation of dyes from aromatic amines

Aromatic amines, such as phenylamine, are important industrially for the production of dyes. Modern dyes are formed in a two-stage synthesis:

• the aromatic amine is converted into a diazonium salt
• the diazonium salt is coupled with an aromatic compound such as phenol, forming an azo dye.

Formation of diazonium salts

Using benzene for the synthesis of dyes

• benzene is nitrated to nitrobenzene
• nitrobenzene is reduced to phenylamine (see above)
• phenylamine is converted into a diazonium salt
• the diazonium salt is coupled with an aromatic compound such as phenol, forming an azo dye.

Reactions of amines with halogenoalkanes

Key points from AS - Nucleophilic substitution reactions of halogenoalkanes

• At each stage, a hydrogen atom is replaced by an alkyl group.
• Quaternary ammonium salts are used as cationic surfactants in fabric softeners.

Reactions of amines with acyl chlorides

PROGRESS CHECK