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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