Quinine and the allied alkaloids have not been met with in any appreciable amount in other parts of the cinchonas than the bark, nor has their presence been ascertained in other plants than those of the tribe Cinchoneæ.
Characters of the Cinchona Alkaloids.
1. Quinine.—It is obtained from alcoholic solutions, in prisms of the composition C₂₀H₂₄N₂O₂ + 3 OH₂, fusing at 57° C. The crystals may be deprived of water by warming or exposure over oil of vitriol, and they fuse at 177° C. The anhydrous alkaloid is likewise crystallizable; it requires about 21 parts of ether for solution, but dissolves more readily in chloroform or absolute alcohol. These solutions deviate the ray of polarized light to the left, and so do likewise solutions of the salts of quinine. Yet one and the same quantity of alkaloid exhibits a very different rotatory power according to the solvent used, though the volume of the solution remain the same. Even the common sulphate differs in this respect from the two other sulphates of quinine. The same remark applies to the optical power of the other alkaloids.
If ten volumes of a solution of quinine, or of one of its salts, are mixed in a test tube with one volume of chlorine water, and a drop of ammonia is added, a brilliant green colour makes its appearance. In solutions rich in quinine, a green precipitate, Thalleioquin or Dalleiochine is produced; in solutions containing less than ¹/₁₀₀₀ of quinine, no precipitate is formed, but the fluid assumes a green even more beautiful than in a stronger solution. The test succeeds with a solution containing only one part of quinine in 5,000, and in a solution containing not more than ¹/₂₀₀₀ of quinine, if bromine is used instead of chlorine.[1358]
The bitter taste of quinine is not appreciable in solutions containing less than one part in 100,000. The blue fluorescence displayed by a solution of quinine in dilute sulphuric acid is observable in solutions containing much less than one part in 200,000 of water; yet it is not apparent in very strong solutions.
Besides the common medicinal sulphate, 2 C₂₀H₂₄N₂O₂ + SO₄H₂ + 8 OH₂, quinine forms two other crystallizable sulphates, namely the sulphate, C₂₀H₂₄N₂O₂ + SO₄H₂ + 7 OH₂, and a third having the composition C₂₀H₂₄N₂O₂ + 2 SO₄H₂ + 7 OH₂.
Herapath, at Bristol, showed in 1852 that quinine forms with sulphuric acid and iodine a peculiar compound, Iodo-sulphate of Quinine, having the composition (C₂₀H₂₄N₂O₂)₄ + 3 (SO₄H₂) + 2 HI + 4 I + 3 OH₂. As this substance possesses optical properties analogous to those of tourmaline, it was called by Haidinger, Herapathite. It may be easily obtained by dissolving sulphate of quinine in 10 parts of weak spirit of wine containing 5 per cent. of sulphuric acid, and adding an alcoholic solution of iodine until a black precipitate is no longer formed. This precipitate is collected on a filter and washed with alcohol; then dissolved in boiling spirit of wine and allowed to crystallize. The tabular crystals thus obtained are extremely remarkable on account of their dichroism and polarizing power, as well as for the sparing solubility, since they require 1000 parts of boiling water for solution; their sparing solubility in cold alcohol may be utilized for separating quinine from the other cinchona alkaloids and estimating its quantity.
2. Quinidine or Conquinine—forms crystals having the composition, C₂₀H₂₄N₂O₂ + 2 OH₂; the anhydrous alkaloid melts at 168° C., and requires about 30 parts of ether for solution. Its solutions are strongly dextrogyre; it agrees with quinine as regards bitterness, fluorescence and the thalleioquin test, and forms a neutral and an acid sulphate. The most striking character of quinidine is afforded by its hydriodate, the crystals of which require for solution at 15° C., 1250 parts of water or 110 parts of alcohol sp. gr. ·834. Quinidine may therefore be separated from the other alkaloids of bark by a solution of iodide of potassium which will precipitate the hydriodate. According to Hesse (1873), quinidine is further characterized by the fact that its sulphate is soluble in 20 parts of chloroform at 15° C., the sulphates of the other cinchona alkaloids being far less soluble in that liquid. The common medicinal sulphate of quinine, e.g., requires for solution 1000 parts of chloroform.
3. Cinchonine.—This alkaloid forms crystals which are always anhydrous; they fuse at 257° C., and require about 400 parts of ether and 120 of spirit of wine for solution. Cinchonine further differs from quinine by its dextrogyre power, its want of fluorescence, and its non-susceptibility to the thalleioquin test. Its hydriodate is readily soluble in water, and still more so in alcohol whether dilute or strong.
4. Cinchonidine.—forms anhydrous crystals melting at 206° C., soluble in 76 parts of ether, or 20 of spirit of wine, then affording levogyre liquids, devoid of fluorescence, and not acquiring a green colour (thalleioquin) by means of chlorine water and ammonia. Hydrochlorate of cinchonidine forms pyramidal crystals of the monoclinic system, very different from the hydrochlorates of the allied alkaloids.