Primary and secondary amines

Chemistry A2 · Nitrogen compounds. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Amines are ammonia with hydrogen atoms replaced by alkyl groups. A primary amine is RNH₂ and a secondary amine is R₂NH.

A halogenoalkane heated with ammonia in ethanol under pressure gives a primary amine by nucleophilic substitution. The amine formed is also a nucleophile, so it can react again to give a secondary amine. A large excess of ammonia keeps this low. Amines are also made by reducing amides or nitriles.

Amines are bases because the lone pair on nitrogen accepts H⁺. Alkyl groups push electrons towards the nitrogen, so the lone pair is more available and the amine is a stronger base than ammonia.

Rules to remember

CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br (ethanol, heat, under pressure)
Halogenoalkane + primary amine (ethanol, sealed tube, heat) → secondary amine
Amide to amine: CH₃CONH₂ + 4[H] → CH₃CH₂NH₂ + H₂O (LiAlH₄)
Nitrile to amine: CH₃CN + 4[H] → CH₃CH₂NH₂ (LiAlH₄, or H₂ with Ni)
Amine + acyl chloride at room temperature → amide + HCl
In water: RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻; with acid: RNH₂ + HCl → RNH₃⁺Cl⁻
Base strength: ammonia < ethylamine < diethylamine

Common mistake

Students forget that the nitrile route adds one carbon atom. A halogenoalkane with two carbons gives a nitrile and then an amine with three carbons; ammonia with the same halogenoalkane gives an amine with two.

Worked example

Butanenitrile, CH₃CH₂CH₂CN, is reduced with hydrogen and a nickel catalyst. What is the product, and which bromoalkane could the nitrile be made from?

Reduction changes –C≡N into –CH₂NH₂; the number of carbon atoms stays at four.
Product: CH₃CH₂CH₂CH₂NH₂, a primary amine.
The nitrile has one more carbon than the bromoalkane it came from, so the bromoalkane has three carbons: 1-bromopropane, heated with KCN in ethanol.

Answer: Butylamine (butan-1-amine); the nitrile can be made from 1-bromopropane

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Watch: 8 | Electrophilic Substitution of Phenols and Phenyl Amine

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