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7.5Advanced Organic Chemistry

Amines — AQA A Level Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 5 of 12

📚 AQA A Level Chemistry Hub / Topical Revision
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Amines: Preparation, Basicity and Reactions — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Describe how amines are prepared, why they act as bases and nucleophiles, and the reactions leading to quaternary ammonium salts.Amines can be thought of as derivatives of ammonia, in whic…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Primary amine
  • Surfactant

Part 1 of 5Preparation of amines

Amines can be thought of as derivatives of ammonia, in which one or more of the hydrogens is replaced by an alkyl or aryl group (primary RNH2, secondary R2NH, tertiary R3N). Methods:

Preparation of amines
Halogenoalkane + excess ammonia in ethanol (heated in a sealed tube): RBr + 2NH3 → RNH2 + NH4Br.
  • Excess ammonia limits further substitution;
    • otherwise mixtures of primary, secondary, tertiary amines and quaternary salts form.
Reduction of nitriles: RCN + 4[H] → RCH2NH2 using LiAlH4 in dry ether or H2 with a nickel catalyst. This lengthens the carbon chain by one.
Aromatic amines: reduction of nitrobenzene with tin and concentrated HCl followed by NaOH gives phenylamine.

Part 2 of 5Basic properties

The nitrogen atom has a lone pair that can accept a proton: RNH2 + H+ → RNH3+.

1
Amines are therefore weak bases and react with acids to give salts, e.g. CH3NH2 + HCl → CH3NH3Cl.
2
Alkyl groups push electron density onto nitrogen (positive inductive effect),
  • so aliphatic amines are stronger bases than ammonia; secondary amines are stronger than primary.
3
In phenylamine the nitrogen lone pair is delocalised into the ring, making it less available, so phenylamine is a weaker base than ammonia.
4
Order of basicity: secondary aliphatic > primary aliphatic > ammonia > phenylamine.

Part 3 of 5Nucleophilic substitution and addition–elimination

The lone pair makes amines nucleophiles.

Nucleophilic substitution and addition–elimination
They attack halogenoalkanes (nucleophilic substitution), forming secondary amines and onwards: CH3CH2NH2 + CH3CH2Br → (CH3CH2)2NH2+Br-.
They also react with acyl chlorides and acid anhydrides by nucleophilic addition–elimination,
  • giving N-substituted amides: CH3COCl + 2CH3NH2 → CH3CONHCH3 + CH3NH3Cl.

Part 4 of 5Quaternary ammonium salts

If a tertiary amine reacts with a halogenoalkane, all four bonds on nitrogen are now to carbon, forming a quaternary ammonium salt, e.g. (CH3)3N + CH3Br → (CH3)4N+Br-.

Quaternary ammonium salts
They have no N-H bonds and no lone pair, so they are not basic.
With a long alkyl chain they act as cationic surfactants, used in fabric conditioners and hair products: the charged head group is hydrophilic and the long chain is hydrophobic.

Part 5 of 5Worked examples

  1. Write the equation for the preparation of propylamine from 1-bromopropane. [2]2 marks

  2. Explain why phenylamine is a weaker base than ethylamine. [3]3 marks

  3. Why is excess ammonia used in the preparation of a primary amine? [2]2 marks

  4. Suggest why quaternary ammonium salts with long chains are used in fabric softeners. [2]2 marks

Key terms

TermMeaning
AmineOrganic derivative of ammonia in which H atoms are replaced by alkyl or aryl groups.
Primary amineAmine with one carbon group on N.
NucleophileElectron pair donor.
Quaternary ammonium saltR4N+ X-, with four carbon groups on nitrogen.
SurfactantMolecule with hydrophilic head and hydrophobic tail.

Avoid these mistakes

1
Saying amines are acidic.
2
Believing phenylamine is a stronger base than ammonia.
3
Forgetting the second mole of amine/ammonia that neutralises HCl.
4
Calling a tertiary amine quaternary - quaternary salts have four C–N bonds.

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