Preparation (A-level only) — AQA A-Level Chemistry
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Preparation (A-level only) explained
Two routes exist, each with distinct features.
Read the full explanation
Heating a halogenoalkane with excess ammonia dissolved in ethanol in a sealed tube gives a primary amine. However, this amine is a nucleophile and attacks more halogenoalkane, yielding a mixture of primary, secondary, tertiary amines, and quaternary ammonium salts. A large excess of ammonia favours the primary amine. Alternatively, reducing a nitrile avoids this mixture: RCN + 4[H] gives RCH₂NH₂, using LiAlH₄ in dry ether, or hydrogen over a nickel catalyst. Note that nitrile reduction maintains the carbon count (RCN to RCH₂NH₂); the chain lengthening by one carbon occurs in the prior step when forming the nitrile from a halogenoalkane.
Aromatic amines, prepared by the reduction of nitro compounds, are used in the manufacture of dyes.
Aromatic amines cannot be made the aliphatic way, because the carbon-halogen bond in a compound such as chlorobenzene is strengthened by overlap with the ring and ammonia will not displace the halogen. Instead the ring is nitrated first and the nitro group is then reduced: nitrobenzene with tin and concentrated hydrochloric acid, heated under reflux, takes 6[H] and gives phenylamine and two molecules of water. In that acidic mixture the amine is protonated, so it exists as its salt, and excess sodium hydroxide is added at the end to release the free phenylamine. Aromatic amines are the feedstock for azo dyes: the amine is converted to a diazonium salt below 10 °C and then coupled to give a strongly coloured compound.
Your focus
- Write equations for both routes to a named primary aliphatic amine, using [H] for the reduction.
- Explain why the ammonia route produces a mixture and how the conditions are chosen to favour the primary amine.
- Recognise that the two-step synthesis via a nitrile lengthens the carbon chain, while the reduction step itself does not.
Show all 6 objectives
- Write a balanced equation using [H] for the reduction of a named nitro compound to an aromatic amine.
- Explain why excess sodium hydroxide is required at the end of the preparation.
- Outline the two-stage conversion of benzene into phenylamine, giving reagents and conditions for each stage.
Preparation (A-level only) exam tips
Marking Points
- one mark for stating a large excess of ammonia is used to favour the primary amine
- one mark for identifying further substitution products: secondary/tertiary amines and quaternary salts
- one mark for naming a reducing agent, LiAlH₄ in dry ether or hydrogen with a nickel catalyst
- one mark for the equation using 4[H], showing the amine has the same number of carbon atoms as the nitrile
- one mark for tin and concentrated hydrochloric acid as the reducing system, heated under reflux
- one mark for a balanced equation using 6[H] and producing two molecules of water
- one mark for adding excess sodium hydroxide to release the free amine from its salt
- one mark for the aromatic amine being used to make dyes
Examiner Tips
- 💡Remember that RCN and RCH2NH2 have the same number of carbons; the extra carbon was added when the nitrile was made.
- 💡Ethanolic ammonia in a sealed tube is the wording AQA expects; 'aqueous ammonia' changes the product.
- 💡Reduction questions want the reagent and the equation; 'reduce it' on its own scores nothing.
- 💡Say why the sodium hydroxide is needed, not just that it is added: the amine is a base and was protonated by the excess acid.
Common Mistakes
- claiming nitrile reduction adds a carbon atom, when the chain lengthening actually occurs during nitrile formation
- using aqueous sodium hydroxide with the halogenoalkane, which gives an alcohol instead
- claiming the ammonia route gives only the primary amine
- using lithium tetrahydridoaluminate in water, which would destroy the reagent
- omitting the sodium hydroxide stage, so the answer stops at the ammonium salt
- balancing the reduction with 4[H], which is the nitrile equation rather than the nitro one
- suggesting ammonia and chlorobenzene as a route to phenylamine
- naming the product of reducing nitrobenzene as an amide