The Alkaloids of Coffee
In addition to caffein, the main alkaloid of coffee, trigonellin—the methylbetaine of nicotinic acid—sometimes known as caffearine, has been isolated from coffee.[132] This alkaloid, having the formula C14H16O4N2, is also found in fenugreek, Trigonella fœnum-græcum, in various leguminous plants, and in the seeds of strophanthus. When pure it forms colorless needles melting at 140° C., and, as with all alkaloids, gives a weak basic reaction. It is very soluble in water, slightly soluble in alcohol, and only very slightly soluble in ether, chloroform or benzol, so that it does not contaminate the caffein in the determination of the latter. Its effects on the body have not been studied, but they are probably not very great, as Polstorff obtained only 0.23 percent from the coffee which he examined.
Caffein, thein, trimethylxanthin, or C5H(CH3)3N4O2, in addition to being in the coffee bean is also found in guarana leaves, the kola nut, maté, or Paraguay tea, and, in small quantities, in cocoa. It is also found in other parts of these plants besides those commonly used for food purposes.
A neat test for detecting the presence of caffein is that of A. Viehoever,[133] in which the caffein is sublimed directly from the plant tissue in a special apparatus. The presence of caffein in the sublimate is verified by observing its melting point, determined on a special heating stage used in connection with a microscope.
The chief commercial source of this alkaloid is waste and damaged tea, from which it is prepared by extraction with boiling water, the tannin precipitated from the solution with litharge, and the solution then concentrated to crystallize out the caffein. It is further purified by sublimation or recrystallization from water. Coffee chaff and roaster-flue dust have been proposed as sources for medicinal caffein, but the extraction of the alkaloid from the former has not proven to be a commercial success. Several manufacturers of pharmaceuticals are now extracting caffein from roaster-flue dust, probably by an adaptation of the Faunce[134] process. The recovery of caffein from roaster-flue gases may be facilitated and increased by the use of a condenser such as proposed Ewé.[135]
Pure caffein forms long, white, silky, flexible needles, which readily felt together to form light, fleecy masses. It melts at 235–7° C. and sublimes completely at 178° C., though the sublimation starts at 120°. Salts of an unstable nature are formed with caffein by most acids. The solubility of caffein as determined by Seidell[136] is given in Table I.
| Table I—The Solubility of Caffein | ||||
| Solvent | Sp. Gr. of Solvent | Temperature of Solution | Solubility: Grm. Caffein per 100 Grm. of Saturated Solution | Sp. Gr. of Saturated Solution |
| Water | 0.997 | 25 | 2.14 | —— |
| Ether | 0.716 | 25 | 0.27 | —— |
| Chloroform | 1.476 | 25 | 11.0 | —— |
| Acetone | 0.809 | 30–1 | 2.18 | 0.832 |
| Benzene | 0.872 | 30–1 | 1.22 | 0.875 |
| Benzaldehyde | 1.055 | 30–1 | 11.62 | 1.087 |
| Amylacetate | 0.860 | 30–1 | 0.72 | 0.862 |
| Aniline | 1.02 | 30–1 | 22.89 | 1.080 |
| Amyl alcohol | 0.814 | 25 | 0.49 | 0.810 |
| Acetic acid | 1.055 | 21.5 | 2.44 | —— |
| Xylene | 0.847 | 32.5 | 1.11 | 0.847 |
| Toluene | 0.862 | 25 | 0.57 | 0.861 |
The similarity between caffein and theobromin (the chief alkaloid of cocoa), xanthin (one of the constituents of meat), and uric acid, is shown by the accompanying structural formulæ.
These formulæ show merely the relative position occupied by caffein in the purin group, and do not in any wise indicate, because of its similarity of structure to the other compounds, that it has the same physiological action. The presence and position of the methyl groups (CH3) in caffein is probably the controlling factor which makes its action differ from the behavior of other members of the series. The structure of these compounds was established, and their syntheses accomplished, in the course of various classic researches by Emil Fischer.[137]