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Nature & Preparation of Amines (A2 Only)

Chemistry: AQA A Level, topic 6.5.1

Nature of Amines

Amines are organic compounds that contain nitrogen. They can be thought of as alkylated forms of ammonia (e.g. NRH2). You can have both aliphatic and aromatic amines.

Aliphatic amines

Aliphatic amines

  • Aliphatic amines are amines containing aliphatic -R groups, such as alkyl chains.
  • The alkyl chain may be as short as methyl, or much longer.
Aromatic amines

Aromatic amines

  • Aromatic amines share the same general formula as aliphatic amines.
  • But aromatic amines must have an aromatic -R group.
Amides

Amides

  • Amides are different to amines but we must also be able to identify them - they crop up throughout this course.
  • An amide contains a -RCONH2 group, as shown above.

Classification of Amines

Amines, both aliphatic and aromatic, can be classified into primary, secondary and tertiary amines.

Primary amines

Primary amines

  • Primary amines contain ONE -R group and TWO -H groups.
  • NRH2.
Secondary amines

Secondary amines

  • Secondary amines contain TWO -R groups and ONE -H group.
  • NR2H.
Tertiary amines

Tertiary amines

  • Tertiary amines contain THREE -R groups and NO -H groups.
  • NR3.
Ammonium ions

Ammonium ions

  • Under certain conditions, there is a possibility of a fourth group being added through a dative covalent bond.
  • This is possible because of the lone pair on the nitrogen.
  • An example is methylamine, N(CH3)H2 in acid; N(CH3)H3+
  • These molecules are called ammonium ions and are often found as ammonium salts. This is because the ion is much more stable in this form.

Preparation of Amines

We will consider ways to prepare aliphatic and aromatic amines.

Halogenoalkane and ammonia

Halogenoalkane and ammonia

  • We can produce amines by heating halogenoalkanes with excess ammonia.
  • Above shows the mechanism of how this happens.
  • The overall reaction is:
  • NH3 + CH3Cl → CH3NH2 + HCl
Halogenoalkane and ammonia

Halogenoalkane and ammonia

  • There is one issue with this method.
  • The primary amine produced is also nucleophilic because it too has a lone pair of electrons and can react with the halogenoalkane itself to produce a secondary amine.
  • This process can happen again to produce a tertiary amine and a quaternary ammonium ion - as shown above.
Reducing a nitrile

Reducing a nitrile

  • You can form an amine by the reduction of a nitrile in two ways.
  • First method:
  • LiAlH4.
  • Dilute acid.
  • In the equation above the [H] term represents the reducing agent which is LiAlH4!
Reducing a nitrile

Reducing a nitrile

  • The previous method is good but LiAlH4 is expensive.
  • Instead, the second method is catalytic hydrogenation.
  • You need:
  • Hydrogen gas.
  • Metal catalyst e.g. platinium or nickel.
  • High temperature and pressure.
Preparing aromatic amines

Preparing aromatic amines

  • These are produced by reducing nitrobenzene, a two step process.
  • First step:
  • Tin.
  • Conc. HCl.
  • Under reflux.
  • This forms an ionic salt. To obtain the amine we carry out the second step:
  • Add NaOH, a base.

Test yourself on Nature & Preparation of Amines (A2 Only)

  • How many bonds does the nitrogen in amine molecules have?

  • What does the functional group of amides contain that makes them very different to amines?

  • Any tertiary amine contains no -H groups. True or false?

  • In the reaction between halogenoalkanes and ammonia, why is it hard to keep the product as a monosubstituted amine?

  • What is the issue with LiAlH4?

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