Alkaloids

Alkaloids form an important class of substances discovered by modern chemistry. They are divided into two classes—namely, natural and artificial. The natural alkaloids are found in plants and animals, and are often designated organic bases. Those obtained from plants are frequently their active principles; but it must not therefore be assumed that when a plant contains an alkaloid it is of necessity the active principle, which may rather be a resin, glucoside, volatile oil, or vegetable acid. Most of the natural alkaloids consist of carbon, hydrogen, nitrogen, and oxygen, and are solid bodies at the ordinary temperature. A few, however, only contain carbon, hydrogen, and nitrogen, and these are for the most part liquids which can be distilled without decomposition. The more important of this class are coniine and nicotine. The alkaloids have generally an energetic action on the animal system, and hence are every day employed in small doses as medicine; whilst in comparatively large doses they are powerful poisons. Many of them have an alkaline reaction on vegetable colours, and are hence termed vegetable alkalies; but in the greater number this property is only possessed in a very faint degree, and it is by analogies, based on other properties, that they are all classed under one title. There is only one property common to all alkaloids, natural and artificial—namely, that they combine directly with acids to form more or less stable salts, capable of undergoing double decomposition; as, for example, sulphate of quinine, muriate of morphine, &c. Most of the alkaloids have an acrid, bitter taste, and are sparingly soluble in water, more freely so in alcohol. To describe the methods of obtaining alkaloids from plants or animal matters would be beyond the scope of this article; but, in general, it may be stated that they are precipitated from solutions by tannin, and the double iodides of potassium and mercury, or cadmium and bismuth; and by treating these precipitates with an alkali, the bases may be obtained. The following list contains the names of the chief alkaloids, with the plants from which they are obtained:

The number of animal alkaloids which has been examined is small, the better-known ones being urea, found in the urine of the mammalia, and kreatine and kreatinine, two of the constituents of the juice of flesh. Besides these, however, there are many substances, answering to the alkaloidal tests, which are found in flesh, both in the fresh and putrefied state, and which are classed under the title Ptomaines (q. v.).

The artificial alkaloids are those organic bases which are not found in any known plant or animal, but of which the later researches of chemists have contrived to form a large number. As the artificial alkaloids do not differ from the natural alkaloids in composition, structure, or properties, it is confidently believed that the day is not far distant when all of the alkaloids will be prepared artificially; indeed, recently, several of the natural alkaloids have been manufactured on the small scale without the intervention of the living plant or animal. For instance, urea can be formed by heating carbonate of ammonia to 234° F. (140° C.) in a sealed tube, while coniine, the alkaloid of hemlock, has been prepared artificially by a German chemist. Hitherto, the more important alkaloids—quinine, morphine, &c.—have defied all attempts at artificial production.

The following remarks on the artificial alkaloids refer (1) to the classification of organic bases, and (2) to their formation.

(1) From the fact that nearly all artificial organic bases are (as will be afterwards shown) actually constructed from ammonia, and that, whether artificially or naturally formed, they exhibit the property of basicity, which is a leading characteristic of ammonia, chemists have been led to refer organic bases generally to the typical body ammonia, and to regard them as being constructed upon or derived from the simple type $\backslash text\{NH\}\_3$. Berzelius believed that all the alkaloids actually contained ammonia as an ingredient of their composition, a view which is now untenable; and it is to Liebig that we are indebted for the idea that they are derivatives of ammonia, or, in other words, amidogen bases or ammonia in which an equivalent of hydrogen is replaced by an organic radical. These bases are classified under the general term amines—the word amines being applied to all organic bases that are derived from ammonia ($\backslash text\{NH\}\_3$). The amines may be (1) monamines, (2) diamines, (3) triamines, (4) tetramines, or (5) pentamines, according as they are constructed upon a single, double, treble, quadruple, or quintuple atom of $\backslash text\{NH\}\_3$. We shall confine our illustrations of the meaning of these terms to the monamines, both because they form the most important group, and because they are much more readily elucidated than the other groups, which are extremely complicated in their composition. Monamines are constructed upon the simple type of ammonia, $\backslash text\{H\}\_3\backslash text\{N\}$. In primary monamines one of the atoms of hydrogen is replaced by an organic radical, R; and hence their general formula is $\backslash text\{RH\}\_2\backslash text\{N\}$. Ethyl-amine ($\backslash text\{C\}\_2\backslash text\{H\}\_5\backslash text\{H\}\_2\backslash text\{N\}$, or $\backslash text\{C\}\_2\backslash text\{H\}\_5\backslash text\{-N\}$, is an example. In secondary monamines two of the atoms of hydrogen are replaced by two atoms of either the same or of different radicals. Hence their general formula is $\backslash text\{RR\text{'}HN\}$, where R and R' may be the same or different radicals. ($\backslash text\{C\}\_2\backslash text\{H\}\_5\backslash text\{HN\}$, diethyl-amine, and $(\backslash text\{CH\}\_3)(\backslash text\{C\}\_2\backslash text\{H\}\_5)\backslash text\{HN\}$, are examples of these. In tertiary monamines the three atoms of hydrogen are replaced by three atoms of the same or different radicals; their formula therefore is $\backslash text\{RR\text{'}R\text{'}\text{'}N\}$, when R, R', R'' may or may not differ from one another. Trimethyl-amine ($\backslash text\{CH\}\_3\backslash text\{N\}$), and methyl-ethyl-phenyl-amine ($\backslash text\{CH\}\_3)(\backslash text\{C\}\_2\backslash text\{H\}\_5)(\backslash text\{C\}\_6\backslash text\{H\}\_5)\backslash text\{N\}$, afford examples of the radicals being all the same, and of their being all different.

(2) Although all attempts at forming in the laboratory those alkaloids that naturally exist in plants, such as morphia, quinia, and strychnia, have hitherto failed, a large number of organic bases have been prepared by artificial means, such as: a. By the destructive distillation of organic bodies containing nitrogen. Thus, in the preparation of coal-gas, four at least of these compounds are obtained—viz., aniline, picoline, leukol (or quinoline), and pyridine. b. By the distillation of certain nitrogenous compounds with caustic potash. In this way aniline is obtained from indigo. c. By the combination of ammonia with the aldehydes and with certain volatile oils which possess the properties of aldehydes. Thus acetic aldehyde yields dimethyl-amine, and oil of mustard yields thyosinamine. d. By the substitution (by the action of strong nitric acid) of one molecule of nitrous acid, $\backslash text\{NO\}\_2$, for one atom of hydrogen in certain hydrocarbons. e. By the processes of fermentation and putrefaction. Thus wheaten flour yields by putrefaction trimethyl-amine, ethyl-amine, and amyl-amine.