From Perkin's Organic Chemistry
Tartaric acid, or dihydroxysuccinic acid, is vegetable acids, and is contained in grapes, in the berries of one of the most commonly occurring species, the mountain ash, and in other fruits; during the secondary fermentation of grape-juice, which takes place in the casks, a considerable quantity of argol or impure potassium hydrogen tartrate, is deposited, and it is from this salt that the tartaric acid of commerce is obtained.
Tartaric acid can be obtained from succinic acid, and, therefore, from its elements, by reactions similar to those employed in the synthesis of malic acid; dibromosuccinic acid is first prepared by heating succinic acid with bromine (2 mols.) and water, and two hydroxyl-groups are then substituted for the two atoms of bromine in the usual way-namely, by heating the dibromo-derivative with water and silver hydroxide.
Tartaric acid may also be obtained synthetically from glyoxal, which, like other aldehydes, combines directly with hydrocyanic acid, the dicyanohydrin thus produced is decomposed by mineral acids, giving tartaric acid, just as cyanoacetic acid yields malonic acid, Tartaric acid is prepared on the large scale from argol. This crude, dark-red deposit is partially purified by recrystallisation from hot water, and its aqueous solution is then boiled with chalk, when insoluble calcium tartrate is precipitated, neutral potassium tartrate remaining in solution, the calcium salt is separated, and the solution treated with calcium chloride, when a second precipitate of calcium tartrateis obtained, The calcium tartrate from these two operations is washed with water, and decomposed with the theoretical quantity of dilute sulphuric acid; finally, the filtered solution of the tartaric acid is evaporated to crystallisation.
The tartaric acid obtained in this way is optically inactive and is a mixture of racemic acid and mesotartaric acid. This product is also optically inactive, and consists of racemio acid only. Tartaric acid forms large transparent crystals, and is readily soluble in water and alcohol, but insoluble in ether; it melts at about 167° but not sharply, owing to decomposition taking place. When heated for a long time at about 150°, it is converted into tartaric anhydride, and several other compounds, and on dry distillation it yields a variety of products, among others, pyruvic acid and pyrotartaric acid.
Tartaric acid, like other dicarboxylic acids, forms both neutral and acid salts, some of which are of considerable importance. Normal potassium tartrate is readily prepared by neutralising the acid, or the acid potassium salt, with potash; it is readily soluble in cold water, in which respect it differs from potassium hydrogen tartrate, which is only sparingly soluble. The latter is precipitated, the precipitation is much hastened by shaking or stirring with a glass rod. On adding excess of tartaric acid to a concentrated neutral solution of a potassium salt (test for potassium), and also on treating an aqueous solution of normal potassium tartrate with one equivalent of a mineral acid, it is known in commerce as argol or cream of tartar. Potassium sodium tartrate, or Rochelle salt, is obtained when potassium hydrogen tartrate is neutralised with sodium carbonate and then concentrated; it forms large transparent crystals, and is employed in the preparation of Fehling's solution.
Calrium tartrate, being insoluble in water, is precipitated on adding a soluble calcium salt to a neutral solution of a tartrate; it is readily soluble in potash, but is reprecipitated on boiling the solution, a behaviour which is made use of in testing for tartaric acid.
Tartar emetic, or potassium antimonyl tartrate, is prepared by boiling potassium hydrogen tartrate with antimonious oxide and water; it is readily soluble in water, and is used in medicine as an emetic, and in calico-printing as a mordant.
The detection of tartaric acid or of a tartrate is based (a) on the behaviour of the neutral solution with calcium chloride (in the cold), and on the solubility of the precipitate in potash; (b) on the behaviour of the neutral solution with an ammoniacal solution of silver nitrate, from which a mirror of silver is deposited on warming; (c) on the fact that the solid compound rapidly chars when heated alone, giving an odour of burnt sugar; it also chars when heated with concentrated sulphuric acid, sulphur dioxide and the two oxides of carbon being evolved.
That the constitution of tartaric acid is expressed by the formula given above is shown by the methods of formation of the acid; it is a dihydroxy-derivative of succinic acid, just as malic acid is a monohydroxy-derivative of the same compound.
On reduction with hydriodic acid, tartaric acid is converted first into malic, then into succinic acid, whereas, when heated with concentrated hydrobromic acid, it, yields dibromosuccinic acid, as was to be expected, It is a remarkable fact that four distinct modifications of tartaric acid are known-namely, dextrotartaric acid (the compound just described), levotartaric acid, racemic acid, and mesotartaric acid. These four compounds have the same constitution-that is to say, they are all dihydroxy-derivatives of succinic acid, they differ, however, in certain physical properties, as, for example, in crystalline form or solubility, but more especially in their behaviour towards polarised light; the salts of the four acids exhibit similar differences. This point is referred to later part
Dextrotartaric acid rotates the plane of polarisation to the right, levotartaric acid to an equal extent to the left.
Racemic acid is optically inactive; it is produced when equal quantities of the dextro- and levo-acids are dissolved in water, and the solution of the mixture allowed to crystallise. It may be obtained synthetically by heating an aqueous solution of dibromo-succinic acid with silver hydroxide, as described above; also from glyoxal. Racemic acid may be resolved into dextrî- and levo-tartaric acids.
Mesotartaric acid, like racemic acid, is optically inactive, but it cannot be resolved into the two optically active modifications; it is formed, together with racemic acid, when dextrotartaric acid is heated for a long time with a small quantity of water at about 165°, and when dibromosuccinic acid is heated with silver hydroxide.