Лако-красочные материалы — производство

The acidic compounds, which react with bases to form salts, may be divided into three main groups: phenols, carboxylic acids, and sul­fonic acids.

The phenols correspond to the aromatic amines. With caustic alka­lies, they give salts which are soluble in water without visible decom­position. Solutions of these salts, however, give a strongly alkaline re­action to litmus and are decomposed, with liberation of the free phenol, by the weakest acids, such as C02 or S02. The salts give a neutral reaction to thiazole paper or auramine paper, and if there is present an excess of alkali over that required for salt formation, thiazole paper is turned red and auramine paper is decolorized. Hence, if the solution reacts alkaline to these indicators, only the phenol salt will be obtained by evaporation or salting out. Analogous to the behavior of aromatic amines with acetic acid, the phenols do not form water-stable salts with ammonia. Aqueous solutions of phenol salts, therefore, can be treated with an ammonium salt to set the phenol free, and to precipi­tate it if the salt concentration is high enough.

Negative substituents enhance the acidic properties of phenols, an effect opposite to that produced with aromatic amines. o~ and p-Chloro — phenols are considerably stronger acids than phenol itself, and o — and p — nitrophenols are still stronger. Trinitrophenol, picric acid, is a strong acid whose salts are neutral and not decomposed by carbonic acid or by ammonium salts. These salts of picric acid can be salted out of neutral solutions by sodium or potassium chloride. With negatively sub­stituted phenols, it may be possible to separate the phenolate from solu­tions which are neutral or weakly alkaline to litmus. In doubtful cases, just as with the amines, the precipitated material must be studied to determine whether it is the free phenol or one of its salts. The color of the precipitate gives an indication in the case of the nitrophenols, since the free phenols have only a weak yellow color, whereas the alkali salts are deep yellow. Solubility tests with indifferent solvents may be used in the case of uncolored compounds. Only the free phenol can be sep­arated from acidic solutions.

In aminophenols, the amino and hydroxyl groups retain their indi — . vidual properties, each group having little influence on the other. It is possible, therefore, under certain conditions to salt out the sodium salt from a solution of the aminophenol containing excess alkali, or the hydrochloride from a solution containing hydrochloric acid. The free aminophenol itself can be separated from neutral, ammoniacal, bicar­bonate, or acetic acid solutions.

The carboxylic acids are considerably stronger acids than the phenols. They turn litmus red, and yield alkali metal salts which are neutral to litmus. They do not turn Congo red paper blue, however; at best only a violet coloration is formed. In contrast to the phenols, the carboxylic acids dissolve even in bicarbonate solutions, and very easily in carbonate and ammonia solutions. The free acids are regenerated from their salts by strong mineral acids.

The influence of amino and carboxylic groups on each other, when they are present in the same molecule, is quite small. Anthranilic acid dissolves as easily in carbonate solution as does benzoic acid, and as easily in aqueous hydrochloric acid as does aniline. Hence, an excess of either alkali or mineral acid must carefully be avoided if it is desired to isolate an aminocarboxylic acid in the free state. On the other hand, a slight excess of acetic acid does no harm.

The sulfonic acids make up the third group of acid compounds. These acids play a very important role in industrial chemistry. They are, however, less important in pure research, and therefore are often treated very briefly in textbooks and lectures on organic chemistry. The sulfonic acids behave quite differently from the carboxylic acids. They are as strongly acidic as the mineral acids, such as hydrochloric and sulfuric acids, and therefore are not liberated from their salts by the mineral acids. For example, if a dilute aqueous solution of sodium benzenesulfonate is treated with the equivalent quantity of hydrochloric acid, the solution then merely contains four ions: H+, Na+, Cl-, and C6H5S03-. If the dilution is not sufficiently high to give complete ionization, then the four possible combinations of these ions will be present — HC1, NaCl, CeH5S03H, and C6H5S03Na — all in equilib­rium with the four ions. If such a solution is concentrated by evaporation to the point where the solubility limit of one of the four compounds is reached, then the least soluble product begins to separate from the solu­tion. Continued evaporation leads to the exclusive formation of the insoluble compound, or more accurately, of one pair of compounds (e. g., sodium benzenesulfonate and HC1, or benzenesulfonic acid and NaCl). Unfortunately, the solubility relationship determines which compound will separate, and since this relationship varies widely with different sulfonic acids, no theoretical predictions can be made. Experience has shown, however, that the free sulfonic acids are, as a rule, very soluble in water, although often their solubilities are much smaller in strong hydrochloric or sulfuric acid. If, for example, naphthalene is sulfonated with ordinary sulfuric acid at high temperature in such a way that naphthalene-/J-sulfonic acid is the chief product (see page 187), and

then the sulfonation mixture is diluted with water (3 parts of water to 4 parts of sulfuric acid) and cooled, the naphthalene-/3sulfonic acid free from by-products[2] separates almost completely. The sulfonic acid can be purified further by dissolving it in a little water and adding concentrated hydrochloric acid; it is only very slightly soluble in about 10 per cent hydrochloric acid. Procedures such as this are successful, however, only if the solution contains no metal ions. If one starts with a salt of the sulfonic acid, or introduces metal ions into the solution by the addition of common salt or Glauber salt, then the product which separates is the corresponding sulfonate which is usually less soluble than the free sulfonic acid. Thus, if the sulfonation mixture of the fore­going example is poured into a salt solution, instead of diluted with water, a voluminous precipitate of sodium naphthalene-/Jsulfonate is formed in spite of the fact that the solution contains a large excess of free sulfuric acid. It is not possible, as a rule, to transform an alkali metal sulfonate into the free sulfonic acid by treatment with strong mineral acids. If such a transformation is ever required, the barium salt of the sulfonic acid must be prepared, for example, and this must be treated with the calculated amount of sulfuric acid. The conversion is accomplished quantitatively because the barium is removed as the in­soluble barium sulfate. Alternatively, the lead salt may be prepared and then decomposed with hydrogen sulfide to eliminate the metal as insoluble lead sulfide.

With di — and polysulfonic acids, the formation of acid salts is pos­sible, of course, by neutralizing only part of the sulfo groups, and it is conceivable that in some cases these acid salts would be sufficiently insoluble to separate out. However, this is the exception; as a rule, the precipitate formed by salting out consists of the neutral salt, even with polysulfonic acids.

All that has been said above about sulfonic acids holds true only if there are no amino or other basic groups present in the molecule along with the sulfo group. Sulfonic acids with amino groups behave quite differently. As was mentioned earlier, if water solutions of aniline hydrochloride and sodium naphthalenesulfonate are mixed, the diffi­cultly soluble aniline salt of naphthalenesulfonic acid is precipitated. An analogous situation is obtained if the amino and sulfo groups are in the same molecule; an intramolecular equalization of polarity results in the formation of an inner salt. Such inner salts are generally quite insoluble. For example, if an aqueous solution of sodium sulfanilate or sodium napthionate is acidified with a mineral acid, the inner salt

is precipitated almost completely. Usually, these inner salts are not formulated as such, but are referred to as free sulfanilic or naphthionic acid. These so-called free aminosulfonic acids retain the acidic char­acter of the sulfo group; they give an acid reaction to litmus, neutral to Congo red, and they dissolve in alkalis, alkali carbonates, or bi­carbonates with the formation of the corresponding salt, just as though the amino group were not present. They also form salts with organic bases, a reaction illustrated by the precipitation of aniline sulfanilate by mixing solutions of sodium sulfanilate and aniline hydrochloride. On the other hand, the basic properties of the amino group are completely absent in the aminosulfonic acids. They do not form salts with even a large excess of mineral acid. The hydrochloride and sulfate of sulfanilic acid do not exist. This holds even for strongly basic groups like the diazonium group. When sulfanilic acid is diazotized, for example, the precipitated, difficultly soluble, diazo compound is not the chloride, despite the presence of excess hydrochloric acid, but is the inner salt in which the diazonium group and the sulfo group are mutually neutralized.

With aminosulfonic acids which contain several amino groups, the rule is that each basic group neutralizes one sulfo group, anfl vice versa. If the same number of amino groups and sulfo groups is present, the properties of the compound are the same as those of the monoaminomonosulfonic acid. Thus, m-phenylenediaminedisulfonic acid and benzidinedisulfonic acid show the same behavior as sulfanilic acid. If more amino groups than sulfo groups are present, then the excess amino groups retain their ability to form salts with acids. Thus, m-phenylenediaminemonosulfonic acid dissolves in hydrochloric acid just as aniline does. In the separation of these aminosulfonic acids containing excess amino groups, therefore,-an excess of mineral acid must be avoided, just as in the case of aminocarboxylic acids, because excess acid would redissolve the product. If the reverse situation exists and the compound contains more sulfo groups than amino groups, acidification of the alkali metal salt will liberate as many sulfo groups as there are amino groups present. The other sulfo groups will retain the metal, behaving generally like the sulfo groups of benzene — and naphthalenesulfonic acids. That is, the free sulfonic acid is formed only in exceptional cases where the solubility relations are favorable. As a rule, an acid salt is precipitated, in which there is one free sulfo group for each amino group, and the remaining sulfo groups are attached to alkali metal atoms. These facts should be noted partic­ularly, because the compounds of this type are usually designated in

industry by the names of the free acids. Thus, S acid, amino G acid, and C acid are actually the monosodium salts of l-naphthylamine-4,8- disulfonic acid, 2-naphthylamine-6,8-disulfonic acid, and 2-naphthyl — amine-4,8-disulfonic acid, respectively; the so-called Koch acid is the disodium salt of l-naphthylamine-3,6,8-trisulfonic acid and H acid is the monosodium salt of lamino-8-naphthol-3,6-disulfonic acid. Hy­droxyl groups, such as that in the last example, have no significance except that they can be neutralized only in strongly alkaline solution. The basic salts formed in this way are usually much more soluble than the neutral salts, and hence, if it is desired to separate phenol — or naphtholsulfonic acids, conditions should be employed which ex­clude the possibility of salt formation on the hydroxyl group.