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Formation of ammonium salts

Ammonia reacts with dilute acids to form ammonium salts that contain the ammonium ion, NH₄⁺. Hydrochloric acid gives ammonium chloride and sulfuric acid gives ammonium sulfate:

Amines react in a similar way. The nitrogen atom picks up an extra hydrogen atom by forming a dative bond with a proton. The ionic salts that form are generally soluble in water, as they contain the very polar ammonium group. Be careful to get the right number of hydrogen atoms in the ammonium group. It is an NH₄ group in which one or more of the hydrogen atoms have been replaced by alkyl groups. For example, butylamine forms butylammonium chloride and butylammonium sulfate respectively with hydrochloric and sulfuric acids:

We can regenerate the original amine by adding a strong base to the salt. This is the standard method for making ammonia gas, by adding sodium hydroxide to an ammonium salt like ammonium chloride. It also gives a good way to purify an amine. It is relatively simple to purify the ionic salt by recrystallisation, whereas it is much more difficult to purify the non-crystalline amines. The salts are generally purified by recrystallisation, and then the original amine is regenerated by adding sodium hydroxide solution to the purified salt.

As the amine salts are ionic they have a very high boiling point and are therefore unlikely to have a smell. when the amines are added to the hydrochloric acid they are converted to the salt and the original fishy amine smell disappears. The very polar ionic grouping also significantly increases the solubility of the original amine in water. Consequently, while the ethyl 4-aminobenzoate was sparingly soluble in even the hot water, it did dissolve in the dilute hydrochloric acid on warming.