# 1911 Encyclopædia Britannica/Amines

See also Amine on Wikipedia; and our 1911 Encyclopædia Britannica disclaimer.
​AMINES, in chemistry, derivatives of ammonia in which one or more of the hydrogen atoms are replaced by alkyl or aryl groups. The replacement of one hydrogen atom by one alkyl or aryl group gives rise to primary amines; of two hydrogen atoms by two groups, to secondary amines; of three hydrogen atoms by three groups, to tertiary amines. The tertiary amines possess the power of combining with one molecular proportion of an alkyl iodide to form quaternary ammonium salts. The structural relations of these compounds may be shown thus: NH$_3$;  Ammonia; NH$_2$R; primary amine; NHR$_2$; secondary amine; NR$_3$ tertiary amine; NR$_4$I. quaternary ammonium iodide.
Aliphatic amines.—These compounds possess properties very similar to those of ammonia, the lowest members of the series being combustible gases readily soluble in water. The next higher members of the series are liquids of low boiling point also readily soluble in water, the solubility and volatility, however, decreasing with the increasing carbon content of the molecule, until the highest members of the series are odourless solids of high boiling point and are insoluble in water. They are all strong bases, readily forming salts with the mineral acids and double salts with the chlorides of gold, platinum and mercury. They are ionized in aqueous solution to a much greater extent than ammonia, the quaternary ammonium bases being the most ionized, and the secondary bases being more strongly ionized than the primary or tertiary bases. For data concerning the conductivity of the organic bases see G. Bredig (Zeit. für phys. Chem., 1894, 13, p. 289).
Many methods have been devised for the preparation of the amines, the first amine having been isolated in 1849 by A. Wurtz on boiling methyl isocyanate with caustic potash, CON·CH$_3$ + 2KHO=CH$_3$NH$_2$ + K$_2$CO$_3$. The primary amines may also be prepared by heating the alkyl iodides with ammonia (A. W. Hofmann); by the reduction of nitriles with alcohol and sodium (A. Ladenburg, Ber., 1886, 19, p. 783); by heating the esters of nitric acid with alcoholic ammonia at 100° C. (O. Wallach, Ber., 1881, 14, p. 421); by the action of reducing agents on nitroparaffins; by the action of zinc and hydrochloric acid on aldehyde ammonias (German Patent 73,812); by the reduction of the phenylhydrazones and oximes of aldehydes and ketones with sodium amalgam in the presence of alcohol and sodium acetate (J. Tafel, Ber., 1886, 19, p. 1925; 1889, 22, p. 1854; H. Goldschmidt, Ber., 1886, 19, p. 3232); by the action of dilute hydrochloric acid on the isonitriles, R·NC + 2H$_2$O=R·NH$_2$ + H$_2$CO$_2$; by heating the mustard oils with a mineral acid, by the hydrolysis of the alkyl phthalimides (S. Gabriel, Ber., 1887, 20, p. 2224; 1891, 24, p. 3104),
by distilling the amino-acids with baryta; by the action of bromine and caustic potash on the acid-amides (A. W. Hofmann, Ber., 1885, 18, p. 2734; 1886, 19, p. 1822); CH$_3$CONH$_2$→CH$_3$CONHBr→CH$_3$CONKBr→CH$_3$NCO→CH$_3$NH$_2$; and by the hydrolysis of substituted urethanes (Th. Curtius, Ber., 1894, 27, p. 779; 1896, 29, p. 1166),
The secondary amines are prepared, together with the primary and tertiary, by the action of ammonia on the alkyl iodides (see below), or by the hydrolysis of para-nitroso derivatives of tertiary aromatic amines, such as para-nitrosodimethylaniline, thus: NO·C$_6$H$_4$·N(CH$_3$)$_2$ + H$_2$O = NO·C$_6$H$_4$·OH + NH(CH$_3$)$_2$. By the action of ammonia on the alkyl iodides a complex mixture of primary, secondary and tertiary amines, along with a quaternary ammonium salt, is obtained, the separation of which is difficult. The method worked out by A. W. Hofmann is as follows:—the mixture is distilled with caustic potash, when the primary, secondary and tertiary amines distil over, and the quaternary ammonium salt remains behind unaffected. The aqueous solution of the amines is now shaken up with diethyl oxalate, when the primary amine forms a crystalline dialkyl oxamide and the secondary amine an insoluble liquid, which is an ethyl dialkyl oxamate, the tertiary amine not reacting: (CO$_2$C$_2$H$_5$)$_2$ + 2NH$_2$R = (CO·NHR)$_2$ + 2C$_2$H$_5$OH; (CO$_2$C$_2$H$_5$)$_2$ + NHR$_2$ = C$_2$H$_5$O$_2$C·CONR$_2$ + C$_2$H$_5$OH. The tertiary amine is then distilled off, the residual products separated by filtration and finally hydrolysed by a caustic alkali.
The primary, secondary and tertiary amines may be readily distinguished by their behaviour with various reagents. Primary amines when heated with alcoholic potash and chloroform yield isonitriles, which are readily detected by their offensive smell. The secondary and tertiary amines do not give this reaction. With nitrous acid, the primary amines yield alcohols, the secondary amines yield nitrosamines and the tertiary amines do not react: R·NH$_2$ + ONOH = R·OH + N$_2$ + H$_2$O; R$_2$NH + ONOH = R$_2$N·NO + H$_2$O. With benzene sulphochloride in the presence of alkali, the primary amines yield compounds of the type C$_6$H$_5$SO$_2$NHR, soluble in alkalies, whilst the secondary amines ​yield compounds of the type C$_6$H$_5$SO$_2$NR$_2$, insoluble in alkalies (O. Hinsberg, Ber., 1890, 23, p. 2963). Primary amines heated with carbon bisulphide in alcoholic solution are converted into mustard oils, when the dithiocarbamate first produced is heated with a solution of mercuric chloride.
Methylamine, CH$_3$NH$_2$, occurs in Mercurialis perennis, in bone-oil, and herring brine. It is also a decomposition product of many alkaloids. At ordinary temperatures it is a gas, but may be condensed to a liquid which boils at −6° C. It has a strong ammoniacal smell, burns readily and is exceedingly soluble in water. Its critical temperature is 155° C. and critical pressure 72 atmos. (C. Vincent, J. Chappuis; Jahresb., 1886, p. 202). Dimethylamine, (CH$_3$)$_2$NH, is found in Peruvian guano. It is a heavy vapour which condenses at 7° C. to a liquid, having a pronounced fish-like smell. Trimethylamine, (CH$_3$)$_3$N, is very similar to dimethylamine, and condenses to a liquid which boils at 3·2–3·8° C. It is usually obtained from “vinasses,” the residue obtained from the distillation of beet sugar alcohol, and is used in the manufacture of potassium bicarbonate by the Solvay process, since its hydrochloride is much more soluble than potassium carbonate. Tetramethylammonium iodide, N(CH$_3$)$_4$I, is the chief product obtained by the action of methyl iodide on ammonia (Hofmann). It crystallizes in quadratic prisms and has a bitter taste. By warming its aqueous solution with an excess of silver oxide it is converted into tetramethylammonium hydroxide, N(CH$_3$)$_4$OH, which crystallizes in hygroscopic needles, and has a very alkaline reaction. It forms many crystalline salts and absorbs carbon dioxide. It precipitates many metallic hydroxides. On dry distillation it is resolved into trimethylamine and methyl alcohol. If the nitrogen atom in the quaternary ammonium salts be in combination with four different groups, then the molecule is asymmetrical, and the salt can be resolved into optically active enantiamorphous isomerides. W. J. Pope (Jour. Chem. Soc., 1901, 79, p. 828) has resolved benzyl-allyl-phenyl-methylamine iodide by boiling with silver d-camphorsulphonate in a nearly anhydrous mixture of acetone and ethyl acetate. The silver iodide is separated and the solvent distilled off. The residue crystallizes slowly, and the crystalline product is almost wholly d-benzyl-allyl-phenyl-ammonium-d-sulphonate, the corresponding l-compound remaining as a syrupy residue. The corresponding iodides are obtained by the addition of potassium iodide to solutions of the sulphonates, and are optically active antipodes.
Diamimes.—The diamines contain two amino groups and bear the same relation to the glycols that the primary monamines bear to the primary alcohols. They are of importance, since the higher homologues are identical in many cases with the ptomaines produced by the putrefactive action of some bacteria on albumen and other related substances. Ethylene diamine, C$_2$H$_4$(NH$_2$)$_2$, may be prepared by heating ethylene dibromide with alcoholic ammonia to 100° C. (F. S. Cloez, Jahresb., 1853, p. 468); or by the action of tin and hydrochloric acid on cyanogen (T. Fairley, Ann. Suppl., 3, 1864, p. 372). It is an alkaline liquid, which, when anhydrous boils at 116·5° C. Nitrous acid converts it into ethylene oxide. It combines directly with many metallic salts. (See S. F. Jörgensen, Jour. pr. Chem., 1889 (2), 39, p. 8.) Trimethylene diamine, NH$_2$·(CH$_2$)$_3$·NH$_2$, is prepared by the action of ammonia on trimethylene bromide (E. Fischer, Ber., 1884, 17, p. 1799). It is a liquid which boils at 135–136° C., and is readily soluble in alcohol, ether, chloroform and benzene. Tetramethylene diamine (putrescine), NH$_2$·(CH$_2$)$_4$·NH$_2$, is prepared by reducing ethylene dicyanide (succinonitrile) with sodium in absolute alcoholic solution (A. Ladenburg, Ber., 1886, 19, p. 780). It melts at 27° C., and is easily soluble in water. Pentamethylene diamine (cadaverine), NH$_2$·(CH$_2$)$_5$·NH$_2$, is prepared by reducing trimethylene cyanide in ether solution by zinc and hydrochloric acid (A. Ladenburg, Ber., 1883, 16, p. 1151). J. v. Braun (Ber., 1904, 37, p. 3583) has prepared pentamethylene derivatives from piperidine by the action of phosphorus pentachloride. On heating piperidine with phosphorus pentachloride to 200° C. in a sealed tube pentamethylene dichloride is obtained, and this on treatment with potassium phthalimide gives a condensation product of composition, C$_6$H$_4$[CO]$_2$N(CH$_2$)$_5$N[CO]$_2$C$_5$H$_4$, which is finally hydrolysed by hydrochloric acid. Cadaverine is a syrup at ordinary temperatures, and boils at 178–179° C. It is readily soluble in water and alcohol, but only slightly soluble in ether.
Aromatic Amines.—The aromatic amines in some respects resemble the aliphatic amines, since they form salts with acids, and double salts with platinum chloride, and they also distil without decomposition. On the other hand, they are much weaker bases than the aliphatic amines, their salts undergoing hydrolytic dissociation in aqueous solution. The primary aromatic amines may be prepared by the reduction of the nitro-hydrocarbons, the reducing agents used being either alcoholic-ammonium sulphide (N. Zinin), zinc and hydrochloric acid (A. W. Hofmann), an alcoholic solution of stannous chloride (containing hydrochloric acid) (R. Anschutz, Ber., 1886, 19, p. 2161), tin and hydrochloric acid, or, on the manufacturing scale, iron and hydrochloric acid. They may also be obtained by the reduction of nitroso compounds and of hydrazo compounds and of hydrazones (J. Tafel, Ber., 1886, 19, p. 1924), by distilling the amido-acids with lime, by heating phenols with zinc chloride ammonia (V. Merz, Ber., 1880, 13, p. 1298), and by heating the secondary and tertiary bases with concentrated hydrochloric acid to about 180° C.
At a temperature of about 300–400° C. the alkyl chloride formed in this reaction attacks the benzene nucleus and replaces hydrogen by an alkyl group or groups, forming primary amines homologous with the original amine; thus methylaniline hydrochloride is converted into para- and ortho-toluidine hydrochloride, and trimethyl phenyl ammonium iodide is converted into mesidine hydriodide. It is to be noted that only traces of the aromatic amines are produced by heating the halogen substituted benzenes with ammonia, unless the amino group be situated in the side chain, as in the case of benzylamine.
The primary amines are colourless liquids or crystalline solids, which are insoluble in water, but readily soluble in the common organic solvents. When heated with alkyl or aryl iodides, they are converted into secondary and tertiary amines. Concentrated nitric acid attacks them violently, producing various oxidation products, but if the amino group be “protected” by being previously acetylated, then nitro derivatives are obtained. When heated with concentrated sulphuric acid for some time, they are sulphonated. They form condensation products with aldehydes, benzaldehyde and aniline forming benzylidene aniline, C$_6$H$_5$N: CHC$_6$H$_5$, and when heated with acids they form anilides. They give the isonitrile reaction (see above) when warmed with chloroform and a caustic alkali, and form alkyl thioureas when heated with an alcoholic solution of carbon bisulphide. When warmed with a solution of nitrous acid, they are converted into phenols; if, however, nitrous acid be added to an ice-cold solution of a primary amine in excess of mineral acid, a diazonium salt is formed (see Azo Compounds and Diazo Compounds), or in absence of excess of acid, a diazoamine is produced.
The secondary amines may be of two types—namely, the purely aromatic amines, and the mixed secondary amines, which contain an aromatic residue and an alkyl group. The purely aromatic amines result upon heating the primary amines with their hydrochlorides, and, in some cases, by heating a phenol with a primary amine and anhydrous zinc chloride. The mixed secondary amines are prepared by the action of alkyl iodides on the primary amines, or by heating salts of the primary amine with alcohols under pressure. The mixed secondary amines have basic properties, but the purely aromatic secondary amines are only very feeble bases. Both classes readily exchange the imide hydrogen for acid radicals, and give nitrosamines with nitrous acid. The secondary amines do not give the isonitrile reaction.
The tertiary amines may also be of two types, the purely aromatic and the mixed type. The mixed tertiary amines are produced by the action of alkyl halides on the primary amines. The simplest aromatic tertiary amine, triphenylamine, is prepared by the action of brombenzene on sodium diphenylamine (C. Heydrich, Ber., 1885, 18, p. 2156). The simplest aromatic monamine is aniline (q. v.), and the simplest mixed amines are ​mono- and di-methyl aniline. These substances are treated in the article Aniline.
The aromatic amine resembling the aliphatic amines is benzylamine, C$_6$H$_5$·CH$_2$·NH$_2$, which may be prepared by reducing benzonitrile in alcoholic solution by means of zinc and acetic acid (O. Mendius, Ann. 1862, 121, p. 144), or by metallic sodium (E. Bamberger, Ber., 1887, 20, p. 1709). It can also be obtained by the action of ammonia on benzyl chloride (S. Cannizzaro, Ann., 1865, 134, p. 128), but di- and tri-benzylamines are simultaneously formed. It is a liquid, which boils at 183° C., and is miscible in all proportions with water, alcohol and ether. It is basic in character, and has a strongly alkaline reaction. Diphenylamine, (C$_6$H$_5$)$_2$NH, is the simplest representative of the true aromatic secondary amines. It is prepared by heating aniline and aniline hydrochloride for some hours to 210–240° C, (Ch. Girard and G. de Laire, Zeit für Chem., 1866, p. 438). It crystallizes in white plates, which melt at 45° C. and boil at 302° C. It is almost insoluble in water, but readily volatilizes in steam. When heated with monobasic saturated acids and zinc chloride it yields acridines.
Aromatic Diamines.—The diamines are prepared by reducing the nitranilines or the dinitrohydrocarbons. They crystallize in plates, and for the most part distil without decomposition. Orthophenylene diamine, C$_6$H$_4$ (NH$_2$)$_2$, crystallizes from water in plates, which melt at 102–103° C. and boil at 256–258° C. When heated with 10% hydrochloric acid to 180° C. it yields pyrocatechin (Jacob Meyer, Ber., 1897, 30, p. 2569). The orthodiamines are characterized by the large number of condensation products they form. (See Imidazoles, Quinoxalines, &c.). Metaphenylene diamine crystallizes in rhombic plates which melt at 63° C. and boil at 287° C. It is easily soluble in water and alcohol. When heated with 10% hydrochloric acid to 180° C. it yields resorcin (J. Meyer). Paraphenylene diamine may be prepared as above, and also by the reduction of amidoazobenzene. It crystallizes in tables which melt at 140° C. and boil at 267° C. When heated with 10% hydrochloric acid to 180° C. it yields hydroquinone (J. Meyer). Manganese dioxide and dilute sulphuric acid oxidize it to quinone. The three classes of diamines may be distinguished by their behaviour towards nitrous acid. The ortho-compounds condense to azimido benzenes, the meta-compounds yield azo-dyestuffs, and the para-compounds yield bis-diazo compounds of the type XN$_2$·C$_6$H$_4$·N$_2$X.