amino group, in chemistry, functional group that consists of a nitrogen atom attached by single bonds to hydrogen atoms, alkyl groups, aryl groups, or a combination of these three. An organic compound that contains an amino group is called an amine. Amines are derivatives of the inorganic compound ammonia, NH3. When one, two, or all three of the hydrogens in ammonia are replaced by an alkyl or aryl group, the resulting compound is known as a primary, secondary, or tertiary amine, respectively. Like ammonia, the amines are weak bases because the unshared electron pair of the nitrogen atom can form a coordinate bond with a proton (see chemical bond). Amines will react with a mineral acid to form an amine salt, e.g., with hydrochloric acid to form an amine hydrochloride. A water-insoluble amine can be made to dissolve by adding acid to form its water-soluble amine salt. Amines react similarly with alkyl halides to form alkyl ammonium salts. Amines can be synthesized by reacting ammonia with an alkyl halide and neutralizing the resulting alkyl ammonium salt with an alkali, e.g., sodium hydroxide. This procedure yields a mixture of primary, secondary, and tertiary amines that is easily separated into its three components by fractional distillation. Amines can also be prepared by the reaction of ammonia with an alcohol or by the reduction of any of a variety of compounds containing nitrogen in a higher oxidation state. Amines take part in many kinds of chemical reactions; in particular, they can react with an acid chloride, acid anhydride, or ester to form an amide. All reactions of amines involve bonding of an electron-deficient atom to the amino nitrogen through its unshared electron pair. One of the most important amines is aniline, an aromatic amine.
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