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a large class of nitrogen-containing organic compounds in which one, two, or three hydrogen atoms in ammonia, NH3, are replaced by organic radicals R.
Primary amines, RNH2 secondary amines, R2NH; and tertiary amines, R3N, where R is CH3, C2H5, C6H11, C6H5, and so on, are distinguished according to the number of substituted hydrogen atoms. According to their radicals, amines are classified as aliphatic, such as methylamine, CH3NH2, and dimethylamine, (CH3)2NH; alicyclic, such as cyclohexylamine, C6H11NH2; aromatic, such as phenyl-amine or aniline, C6H5NH2; and heterocyclic, such as 2–aminopyridine, C5H4 N(NH2). Amines with two, three, or more amino groups -NH2 are called di-, tri-, and poly-amines and include ethylenediamine, H2NCH2CH2NH2, and hexamethylenediamine, H2N(CH2)6NH2.
Lower amines are gases with an ammoniacal odor; the higher ones are liquids or solids. Lower amines have been found in products of the vital activity of plants: trimethylamine, (CH3)3N, occurs in herring brine, to which it imparts its characteristic unpleasant odor, and in molasses. More complex amines, such as alkaloids, amino acids, and biogenic amines, are widely distributed in nature. Aliphatic amines are usually prepared by alkylating NH3; aromatic amines, by the reduction of nitro compounds.
Like ammonia, the amines are bases (aromatic amines with an NH2 side group are very weak bases). With acids, amines form salts of substituted ammonium—for example, C2H5NH2 + HC1 = [C2H5NH3]+ C1–. With alkyl halides, tertiary amines give quaternary ammonium salts: R3N + R’Cl = [R3 NR’]+ C1-. The reaction of amines with nitrous acid is very important. Primary aromatic amines form with HN0 2diazo compounds, which are widely used in laboratory and industrial synthesis. Primary aliphatic amines are converted by HN02 into alcohols—for example, C2H5 NNH2 + HN02 = C2H5 OH + N2 + H20; secondary aliphatic amines give nitrosoamines—(C2H5)2 NH + HN02 = H20 + (C2H5)2 NNO; tertiary amines do not react with HN02. This reaction is used in the identification of primary, secondary, and tertiary amines.
Amines are widely used in industry in the production of dyes and drugs, as well as of polyamides for the manufacture of synthetic fibers (kapron, nylon, and so on).
Aromatic amines were synthesized for the first time in 1842 by N. N. Zinin. In 1849, C. A. Wurtz discovered aliphatic amines.