The alkaloids are precipitated out of their solutions by various solutions of chemical compounds, known as the "alkaloidal reagents": iodine dissolved in potassium iodide solution gives a chocolate-brown precipitate; tannic acid, phosphotungstic acid, phosphomolybdic acid, and mercuric iodide solutions give colorless, amorphous precipitates; while gold chloride and platinic chloride solutions give crystalline precipitates, many of which have sharp melting points and can be used for the identification of individual alkaloids. There are a great many specific color reactions for individual alkaloids, which are important to toxicologists and pharmacists, but which it would not be desirable to consider in detail here.
The alkaloids are conveniently divided into groups, according to the characteristic closed-ring arrangements which they contain. The several closed-ring arrangements which are found in common alkaloids, and upon which their grouping is based, may be illustrated by the following formulas:
The common alkaloids are distributed in the several groups as follows:
- Pyrridine—piperidine group; piperine, coniine, nicotine.
- Pyrrolidine group; hygrine and stachydrine.
- Tropane group; atropine, hyoscine, cocaine, lupinine.
- Quinoline group; quinine, cinchonine, strychnine, brucine.
- Isoquinoline group; papaverine, hydrastine, morphine, codeine, berberine.
The composition and properties of the individual alkaloids have been extensively studied, because of their medicinal uses. As they have no known metabolic use to the plants which elaborate them, it will not be worth while to consider all of these investigations in detail here. The following facts with reference to certain typical members of each group will serve to illustrate the general constitution and properties of the alkaloids.
Piperine, C17H19O3, is found in black peppers. Its constitution is represented by the following formula, the group which is united to the piperidine ring, in this case, being piperic acid:
