Reactions of amines

Acylation. Primary and secondary amines react with acid chlorides, anhydrides or esters: the H on the nitrogen is replaced by an acyl group (RCO–), giving an amide. With benzoyl chloride this is called benzoylation. Tertiary amines have no N–H, so they do not react.

The reaction is carried out in the presence of a base stronger than the amine, such as pyridine. It removes the HCl formed, which would otherwise turn the amine into its salt, and so it pushes the reaction forward. Example: CH₃NH₂ + CH₃COCl → CH₃CONHCH₃ + HCl.

Three tests tell amines apart:

• Carbylamine test, for primary amines only (aliphatic or aromatic). Warm with chloroform and alcoholic KOH. A primary amine gives an isocyanide (carbylamine) with a very unpleasant smell. Secondary and tertiary amines do not.
• Hinsberg test, which separates all three. Shake with benzenesulphonyl chloride, C₆H₅SO₂Cl (Hinsberg's reagent). A primary amine gives a product that dissolves in alkali (it still has an acidic H on N). A secondary amine gives a product that does not dissolve in alkali (no H left on N). A tertiary amine does not react at all.
• Nitrous acid (made from NaNO₂ and HCl). Aliphatic primary amines give nitrogen gas (bubbles) and an alcohol. Aromatic primary amines, kept cold at 273-278 K, give a diazonium salt instead (next lesson).

The ring of aniline. The –NH₂ group pushes electrons strongly into the ring, so aniline reacts very easily with electrophiles, at the ortho and para positions.

• With bromine water, it reacts at all three positions at once, giving a white precipitate of 2,4,6-tribromoaniline.
• To put on just one group, first protect the NH₂ by acetylation. The acetyl group tones down the activation. After the reaction, hydrolysis gives back the –NH₂.
• Nitration with the usual acid mixture also gives some meta product: in strong acid, much of the aniline becomes the anilinium ion, C₆H₅NH₃⁺, which is meta-directing.
• Sulphonation: heating anilinium hydrogensulphate gives sulphanilic acid (4-aminobenzenesulphonic acid).