Amines
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
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- Notice the similarity with ammonia.
Physical properties of amines
The amino group dominates the physical properties of short-chain amines.
Hydrogen bonding takes place between amine molecules, resulting in:
- 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
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- Amines are the organic bases.
The basicity of aliphatic and aromatic amines
Aliphatic amines are stronger bases than ammonia; aromatic amines are substantially weaker.
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Organic ammonium salts
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- Amines react by the usual ‘base reactions’ producing organic ammonium salts
- A proton, H+, is added to the amino nitrogen atom.
Preparation of amines
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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)
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- 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
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Using benzene for the synthesis of dyes
Benzene is used as the raw material for the synthesis of dyes.
A four-stage synthesis is shown below:
- 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
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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
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PROGRESS CHECK
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