Quaternary Ammonium Salts

What is the general formula of a quaternary ammonium salt?

• R₄N⁺ X⁻, where R are alkyl groups and X⁻ is a halide ion.

How is a quaternary ammonium salt formed?

• By the nucleophilic substitution reaction between a tertiary amine and a halogenoalkane.

Write an equation for the formation of a quaternary ammonium salt from trimethylamine and iodomethane.

• (CH₃)₃N + CH₃I → (CH₃)₄N⁺ I⁻
  (The product is tetramethylammonium iodide)

What is the key structural feature that distinguishes a quaternary ammonium salt from a primary, secondary, or tertiary amine?

• The nitrogen atom is bonded to four alkyl groups and therefore carries a permanent positive charge.

What is the main commercial use of quaternary ammonium salts with one long alkyl chain?

• As cationic surfactants, used in fabric softeners and hair conditioners.

Describe the two parts of a cationic surfactant molecule.

1. A hydrophilic head containing the positively charged N⁺ group.

2. A hydrophobic tail consisting of a long hydrocarbon chain.

Why are quaternary ammonium salts soluble in water?

• Because they are ionic compounds, and the charged head group can form strong ion-dipole bonds with water molecules.

Describe the preparation of a quaternary ammonium salt.

1. Gently heat a tertiary amine with a halogenoalkane.

2. The reaction occurs via nucleophilic substitution.

3. The quaternary ammonium salt will often precipitate out of solution as it forms, or can be crystallised from the mixture.