ALKALOID, in chemistry, a term originally applied to any organic base, i.e. a nitrogenous substance which forms salts with acids; now, however, it is usual to restrict the term to bases of vegetable origin and characterized by remarkable toxicological effects. Such bases occur almost exclusively in the dicotyledons, generally in combination with malic, citric, tartaric or similar plant-acids. They may be extracted by exhausting the plant-tissues with a dilute acid, and precipitating the bases with potash, soda, lime or magnesia. The separation of the mixed bases so obtained is effected by repeated fractional crystallization, or by taking advantage of certain properties of the constituents.
A chemical classification of alkaloids is difficult on account of their complex constitution. I. A. Wyschnegradsky, and afterwards W. Königs, expressed the opinion that the alkaloids were derivatives of pyridine or quinoline. This view has been fairly well supported by later discoveries; but, in addition to pyridine and quinoline nuclei, alkaloids derived from isoquinoline are known. The purely chemical literature on the alkaloids is especially voluminous; and from the assiduity with which the constitutions of these substances have been and are still being attacked, we may conclude that their synthesis is but a question of time. Piperine, conine, atropine, belladonine, cocaine, hyoscyamine and nicotine have been already synthesized; the constitution of several others requires confirmation, while there remain many important alkaloids—quinine, morphine, strychnine, &c.—whose constitution remains unknown.
The following classification is simple and convenient; the list of alkaloids makes no pretence at being exhaustive.
- (1) Pyridine group. Piperine; conine, trigonelline; arecaidine; guvacine; pilocarpine; cytisine; nicotine; sparteine.
- (2) Tropine group. Alkaloids characterized by containing the tropine (q.v.) nucleus. Atropine; cocaine; hygrine; ecgonine; pelletierine.
- (3) Quinoline group. The alkaloids of the quina-barks: quinine, &c.; the strychnos bases: strychnine, brucine; and the veratrum alkaloids: veratrine, cevadine, &c.
- (4) Isoquinoline group. The opium alkaloids: morphine, codeine, thebaine, papaverine, narcotine, narceine, &c.; and the complicated substances hydrastine and berberine.
In addition to the above series there are a considerable number of compounds derived from purin which are by some writers classed with the alkaloids. These are treated in the article Purin. There are also reasons for including such compounds as muscarine, choline, neurine and betaine in this group.
The greater number of these substances are of considerable medicinal value; this aspect is treated generally in the article Pharmacology. Reference should also be made to the articles on the individual alkaloids for further details as to their medicinal and chemical properties.
The chemistry of the alkaloids is treated in detail by Amé Pictet in his La Constitution chimique des alcaloides végétaux (Paris, 1897); enlarged and translated by H. C. Biddle with the title The Vegetable Alkaloids (New York, 1904); and by J. W. Bruhl, E. Hjelt, and O. Aschan: Die Pflanzen-Alkaloide (1900). A pamphlet, Die Alkaloidchemie in den Jahren 1900–1904, by Julius Schmidt, may also be consulted.