Quaternary Ammonium Salts

AQA A-level ChemistryAminesLesson 9 of 10

A quaternary ammonium ion is a positive ion in which a nitrogen atom is bonded to four alkyl (or aryl) groups, R₄N⁺. Together with a negative ion, such as a halide, it forms a quaternary ammonium salt, for example tetraethylammonium bromide, (CH₃CH₂)₄N⁺Br⁻.

These salts are the end point of the reaction between ammonia and a haloalkane. The lone pair on nitrogen makes ammonia a nucleophile, and every amine formed still has a lone pair, so substitution carries on: primary amine → secondary amine → tertiary amine → quaternary ammonium salt. The last step is a tertiary amine attacking another haloalkane molecule:

More in Amines

• Naming Amines
• Amines as Bases
• Relative Base Strength of Amines
• Preparing Primary Aliphatic Amines
• Reduction of Nitriles
• Preparing Aromatic Amines
• Amines as Nucleophiles
• Cationic Surfactants

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