Amines and amidesEdexcel A-Level Chemistry: Revision notes

An amine contains the –NH₂ group (primary), –NH– (secondary) or a nitrogen bonded to three carbons (tertiary). Primary aliphatic amines such as butylamine, CH₃(CH₂)₃NH₂, have the NH₂ group on an alkyl chain, while aromatic amines such as phenylamine, C₆H₅NH₂, have it on a benzene ring.

An amide contains –CONH₂ (primary amide, e.g. ethanamide CH₃CONH₂) or –CONH– (N-substituted, e.g. N-butylethanamide). The nitrogen is bonded directly to a carbonyl carbon.

Butylamine has a lone pair on nitrogen, so it acts as a base and as a nucleophile.

• With water: CH₃(CH₂)₃NH₂ + H₂O ⇌ CH₃(CH₂)₃NH₃⁺ + OH⁻, giving an alkaline solution.
• With acids: forms a salt, CH₃(CH₂)₃NH₂ + HCl → CH₃(CH₂)₃NH₃⁺Cl⁻ (butylammonium chloride).
• With ethanoyl chloride: nucleophilic addition–elimination gives N-butylethanamide, CH₃CONH(CH₂)₃CH₃, and HCl.
• With a halogenoalkane: nucleophilic substitution, e.g. with CH₃Br gives a secondary amine; further reaction can give tertiary amines and quaternary ammonium salts.
• With copper(II) ions: a pale blue precipitate of Cu(OH)₂ forms first; in excess amine this dissolves to a deep blue solution of a complex ion, with the amine acting as a ligand.

Base strength depends on how available the lone pair on nitrogen is to accept H⁺. A lower pKb means a stronger base.

Order of increasing base strength: phenylamine < ammonia < butylamine (pKb about 9.1, 4.75, 3.4)

• Butylamine is stronger than ammonia because the alkyl group is electron-releasing, which increases the electron density on N.
• Phenylamine is weaker than ammonia because the nitrogen lone pair is delocalised into the benzene ring, so it is less available to accept a proton.
• Amides are not basic: the lone pair is delocalised into the C=O group.

From a halogenoalkane: heat the halogenoalkane with a large excess of concentrated ammonia in ethanol in a sealed tube (nucleophilic substitution):

CH₃(CH₂)₃Br + 2NH₃ → CH₃(CH₂)₃NH₂ + NH₄Br

The excess ammonia stops the amine formed reacting further to give secondary and tertiary amines and quaternary ammonium salts.

By reducing a nitrile: use LiAlH₄ in dry ether (then dilute acid), or hydrogen with a nickel catalyst:

CH₃CH₂CH₂CN + 4[H] → CH₃CH₂CH₂CH₂NH₂

This route lengthens the carbon chain when the nitrile is made from a halogenoalkane and cyanide.

Aromatic amines are made by reducing aromatic nitro-compounds. Nitrobenzene is heated under reflux with tin and concentrated hydrochloric acid:

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

In the acid the product is protonated and exists as phenylammonium chloride, C₆H₅NH₃⁺Cl⁻. Excess sodium hydroxide is then added to liberate the free amine:

C₆H₅NH₃⁺ + OH⁻ → C₆H₅NH₂ + H₂O

The phenylamine is separated by steam distillation (or solvent extraction), dried and distilled.

Acyl chlorides react readily with ammonia and amines to give amides. The N lone pair attacks the δ+ carbon of C=O (nucleophilic addition–elimination).

With ammonia: CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl (ethanamide)

With butylamine: CH₃COCl + 2CH₃(CH₂)₃NH₂ → CH₃CONH(CH₂)₃CH₃ + CH₃(CH₂)₃NH₃⁺Cl⁻

Two moles of amine are used because the second mole reacts with the HCl formed. The reaction is vigorous at room temperature and gives white fumes of HCl.