Salts similar to ammonium salts in which the hydrogen atoms attached to the nitrogen are replaced by one or more organic groups. Amines readily form salts by reaction with acids, gaining a proton to form a positive ammonium ion, They are named as if they were substituted derivatives of ammonium compounds; for example, dimethylamine (CH3)2NH) will react with hydrogen chloride to give dimethylammonium chloride, which is an ionic compound [(CH3)2NH2]+Cl−. When the amine has a common nonsystematic name the suffix -ium can be used; for example, phenylamine (aniline) would give [C6H5NH3]+Cl−, known as anilinium chloride. Formerly, such compounds were sometimes called hydrochlorides, e.g. aniline hydrochloride with the formula C6H5NH2.HCl.
Salts formed by amines are crystalline substances that are readily soluble in water. Many insoluble alkaloids (e.g. quinine and atropine) are used medicinally in the form of soluble salts (‘hydrochlorides’). If alkali (sodium hydroxide) is added to solutions of such salts the free amine is liberated.
If all four hydrogen atoms of an ammonium salt are replaced by organic groups a quaternary ammonium compound is formed. Such compounds are made by reacting tertiary amines with halogen compounds; for example, trimethylamine (CH3)3N) with chloromethane (CH3Cl) gives tetramethylammonium chloride, (CH3)4N+Cl−. Salts of this type do not liberate the free amine when alkali is added, and quaternary hydroxides (such as (CH3)4N+OH−) can be isolated. Such compounds are strong alkalis, comparable to sodium hydroxide.