Amines: basicity and reactions

Naming and solubility

Primary amines have the -NH2 group: CH3CH2CH2CH2NH2 is butylamine (butan-1-amine); C6H5NH2 is phenylamine. Short-chain amines are miscible with water because the N-H groups and the nitrogen lone pair form hydrogen bonds with water. Phenylamine is only slightly soluble, because of its large benzene ring.

Basicity

A base accepts a proton using the nitrogen lone pair. Order of base strength:

primary aliphatic amine > ammonia > primary aromatic amine

• Alkyl groups release electrons (positive inductive effect), increasing the electron density on N, so butylamine is a stronger base than ammonia.
• In phenylamine the lone pair is partly delocalised into the benzene ring, so it is less available, and phenylamine is a weaker base than ammonia.

Reactions of butylamine and phenylamine (spec 19.2)

• Water: an alkaline solution: CH3CH2CH2CH2NH2 + H2O ⇌ CH3CH2CH2CH2NH3+ + OH-
• Acids: salts: C6H5NH2 + HCl -> C6H5NH3+Cl- (phenylammonium chloride, soluble in water)
• Halogenoalkanes: the amine is a nucleophile, giving secondary and tertiary amines and quaternary ammonium salts
• Ethanoyl chloride: N-substituted amides: CH3COCl + C4H9NH2 -> CH3CONHC4H9 + HCl
• Copper(II) ions: with butylamine, as with ammonia, a pale blue precipitate forms, then dissolves in excess to give a deep blue complex ion.

Worked example

Put ammonia, butylamine and phenylamine in order of increasing base strength and explain.

1. Phenylamine: N lone pair delocalised into the ring, least available.
2. Ammonia: no electron-releasing groups.
3. Butylamine: alkyl group pushes electrons onto N, most available.

Answer: Phenylamine < ammonia < butylamine

Check yourself

1. Which list shows increasing base strength?
   • Phenylamine, ammonia, ethylamine
   • Ethylamine, ammonia, phenylamine
   • Ammonia, phenylamine, ethylamine
   • Phenylamine, ethylamine, ammonia
   Show answer

   Phenylamine, ammonia, ethylamine. Aromatic amines are weakest, aliphatic amines strongest.

2. Why is phenylamine a weaker base than ammonia?
   • Phenylamine has no lone pair
   • The benzene ring releases electrons onto N
   • The nitrogen lone pair is partly delocalised into the benzene ring
   • Phenylamine is insoluble in acids
   Show answer

   The nitrogen lone pair is partly delocalised into the benzene ring. Less available lone pair, weaker base.

3. Why is a solution of butylamine in water alkaline?
   • Butylamine accepts a proton from water, forming OH- ions
   • Butylamine releases H+ ions
   • Butylamine contains OH- ions
   • Butylamine reacts with dissolved CO2
   Show answer

   Butylamine accepts a proton from water, forming OH- ions. C4H9NH2 + H2O ⇌ C4H9NH3+ + OH-.

4. What is formed when phenylamine reacts with hydrochloric acid?
   • Chlorobenzene
   • Benzene and ammonium chloride
   • Phenol
   • Phenylammonium chloride
   Show answer

   Phenylammonium chloride. The amine accepts H+ to form a soluble salt.

5. What is the organic product of butylamine with ethanoyl chloride?
   • Butanamide
   • N-butylethanamide
   • Butyl ethanoate
   • Ethylbutylamine
   Show answer

   N-butylethanamide. The amine nitrogen bonds to the acyl carbon: CH3CONHC4H9.

6. What is seen when excess butylamine solution is added to aqueous copper(II) sulfate?
   • A white precipitate forms
   • An orange-brown precipitate forms
   • The solution turns yellow
   • A pale blue precipitate forms, then dissolves to give a deep blue solution
   Show answer

   A pale blue precipitate forms, then dissolves to give a deep blue solution. Like ammonia: Cu(OH)2 first, then a deep blue complex with amine ligands.

7. Why are short-chain amines soluble in water?
   • They are ionic
   • They form hydrogen bonds with water molecules
   • They have no polar bonds
   • They react to form carboxylic acids
   Show answer

   They form hydrogen bonds with water molecules. N-H and the N lone pair hydrogen bond with water.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson Edexcel IAL Chemistry (2018) scheme of work

Last updated 7 November 2026. Spotted a mistake? Tell us.