7 сентября, 2015 
Pokraskin The aliphatic amines are, in general, volatile liquids soluble in water. They are strong bases which turn litmus blue, and react with mineral acids to form neutral salts. The reaction to litmus — apart from the physical form — shows immediately whether the base or a salt is at hand. For the isolation and identification of aliphatic amines, the salts formed with picric acid and similar acids are of value. These salts are usually nicely crystalline and have characteristic melting points.
The aromatic amines, which are much more important technically, have quite different properties. Let us take the simplest compound of the series, aniline, as an example. Free aniline is only slightly soluble in water, about 3 per cent. The aqueous solution does not turn litmus blue; therefore aniline is a weak base. Aniline gives easily soluble salts with strong mineral acids — hydrochloric, sulfuric, and nitric. Solutions of these salts react strongly acid to litmus; hence, the salts are hydrolyzed in solution, but not sufficiently to cause free aniline to precipitate. The hydrogen ion concentration of these salt solutions is not high enough to turn Congo red paper blue. Congo red paper can be used, therefore, to determine the presence of free mineral acids over that required for salt formation. Evaporation of a solution of an aniline salt
yields the unchanged salt. It follows that if a precipitate forms, or crystallization occurs, in an aniline salt solution which reacts acid to Congo red, then the separated material must be a salt of aniline and not the free base. A separation such as this may occur, for example, when a concentrated solution of aniline hydrochloride is treated with sodium sulfate, because aniline sulfate is considerable less soluble that the hydrochloride. Precipitation occurs also when concentrated hydrochloric acid is added to the aniline hydrochloride solution, because the salt is much less soluble in strong hydrochloric acid than it is in water. Precipitation is especially easily accomplished with the alkali metal salts of sulfonic acids (e. g., sodium naphthalenesulfonate), which often form difficultly soluble salts with aromatic amines. With aniline itself, of course, there is no difficulty in telling whether a precipitate consists of the free base or of a salt, because the former is a liquid at ordinary temperatures and the latter a solid. With solid amines, on the other hand, the appearance of the precipitate is not a sufficient indication.
Aniline does not give water-stable salts with weak acids like acetic acid. Some aromatic amines do give crystalline acetates with acetic acid, but these are decomposed by water with separation of the free base. This property is utilized industrially in the isolation of m-xylidine from a mixture containing its isomers. Hence, free aniline separates out of an aqueous solution of an aniline salt with a mineral acid if the mineral acid anion is replaced by the acetate ion, for example, by the addition of sodium acetate in sufficient concentration.
Homologs of aniline and naphthylamines have properties similar to those of aniline, as do polynuclear compounds which have only one amino group in each nucleus, such as benzidine, diaminodiphenylme- thane, and di — and triaminotriphenylmethanes. If two or more amino groups are present in one nucleus, the basicity is somewhat higher, and the water solubility is greatly increased, although the essential character of the compound is not changed. Nuclear substitution products of these amines behave similarly, provided that the substituents have no appreciable effect on the base strength. Alkoxy groups (—OCH3, —OC2H5, etc.) and acylamino groups (—NHCOCH3, —NHCOC6H5, etc.) are examples of such indifferent substituents, and hence anisidine, pheneti — dine, and monoacetyl — and monobenzoyl-p-phenylenediamines behave like aniline with respect to salt formation. Also, the introduction of alkyl groups into the amino group effects no appreciable change in salt-forming properties; mono — and dimethylaniline, mono — and diethylaniline, ethylbenzylaniline, etc., are all quite similar to aniline. On the other hand, the introduction of a second aromatic residue into the amino group
completely removes the basicity. Diphenylamine, phenylnaphthylamine, and the like, and carbazole, do not form salts with aqueous acids. Also, the introduction of certain substituents, called negative or acidic substituents, into the nucleus, lowers the basicity and finally removes it completely. Nitro groups show this effect most strongly, halogens to a smaller extent. Still less effective are the carbonyl groups in aldehydes, ketones, carboxylic esters and amides, etc. The sulfone group acts similarly to the carbonyl group. The azo group is also slightly negative. Not only the nature of the substituent group, but also its position, is an important factor. Groups in the ortho position have the greatest effect, para groups have somewhat less effect, and those in the meta position, much less. Thus, m-nitroaniline gives a hydrochloride which is stable in aqueous solution, while a concentration of hydrochloric acid of at least 10 per cent is necessary to prevent hydrolysis of p-nitroaniline hydrochloride. o-Nitroaniline forms a hydrochloride only with concentrated hydrochloric acid; the addition of only a small amount of water precipitates the free base. Dinitroanilines give no salts with aqueous acids, and neither do trichloroaniline and dichloronitroaniline. With aminoanthraquinones, the two carbonyl groups lower the basicity of the amino group to such an extent that 75-80 per cent sulfuric acid is necessary for salt formation; the free base separates from more dilute acids.
Amines having strongly negative substituents, therefore, may separate as the free base from solutions which react strongly acid to Congo red. With colored compounds, the color usually shows whether the free base or a salt has separated. For example, the nitrated amines are deep yellow but their salts are colorless. Similarly, the salts of aminoanthraquinones are colorless, while the free bases are orange to red. With aminoazo compounds, salt formation is accompanied by a color change from yellow to red, or from orange to violet. Nitrosated amines, such as p-nitrosodimethylaniline, are green and their salts are yellow. With colorless compounds, the character of the precipitate is most easily recognized by its behavior toward indifferent solvents, such as ether or benzene. The free bases are usually quite easily soluble in these solvents while the salts are insoluble.
It is to be noted that diazo compounds are much more strongly basic than the amines from which they are derived. A solution of diazotized p-nitroaniline, for example, can be diluted with water to any desired extent, or even neutralized with acetate, without setting the diazonium base free. The stability of the diazonium compound is much less under these conditions, however, than in strongly acid solution.
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