34.1 Primary and secondary amines

Primary and secondary amines can be prepared by nucleophilic substitution of halogenoalkanes with ammonia or a primary amine, and by reducing suitable nitrogen-containing compounds such as nitriles or amides where specified.

The nitrogen substituent count determines the product: ammonia gives a primary amine after one alkylation, while a primary amine can give a secondary amine. Excess amine helps limit further substitution.

Bromoethane + excess NH₃ → ethylamine; ethylamine + bromoethane can then form diethylamine if further substitution is allowed.

Using ammonia does not guarantee only one product unless conditions control successive alkylation; primary, secondary and tertiary amines can form.

Ammonia or an amine reacts with an acyl chloride at room temperature to form an amide. The nitrogen nucleophile attacks the carbonyl carbon, chloride leaves, and HCl is produced.

Ammonia gives a primary amide; a primary amine gives an N-substituted secondary amide. A second equivalent of amine or another base can remove the HCl.

CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl overall. With methylamine, the product is N-methylacetamide rather than acetamide.

Do not call this simple acid–base neutralisation or write an alcohol product; the nitrogen remains attached to the acyl carbon.

An amine is a Brønsted–Lowry base: its nitrogen lone pair accepts a proton from water, producing an alkylammonium ion and hydroxide.

The equilibrium is partial, so aqueous amines are weak bases. The position depends on how available the lone pair is and on electron-donating or withdrawing groups.

Ethylamine + H₂O ⇌ ethylammonium ion + OH⁻. The solution is alkaline even though most ethylamine molecules remain unprotonated.

A weak base is not a base that cannot react; it is one whose protonation equilibrium is incomplete.