Preparation from natural sources The alkaloids

As mentioned above, the first pure pharmaceuticals isolated from natural sources were the alkaloids. Though the methods adopted for their extraction from plants vary in detail, all are based on three general characteristics of these compounds. First, most alkaloids are only slightly soluble in water but readily soluble in certain organic solvents such as benzene, chloroform, ether, and light petroleum. Second, alkaloids combine with acids to form salts that are usually freely soluble in water but only slightly so in organic solvents. Third, alkaloids are liberated from their salts by alkalies.

Application of these general principles can be seen in the following generalized outline of extraction methods. The crude drug, ground to a suitable state of subdivision, is mixed with water, a water-soluble alkali such as lime, and some organic solvent that does not mix with water. The mixture separates into two layers: one contains water, lime, and impurities and is discarded; the other contains the alkaloids dissolved in the organic solvent. Fresh water and dilute acids are now added to the mixture. Again there are two layers, but the acid has caused the alkaloids to pass from the organic layer into the aqueous layer. The aqueous layer is now separated, and the alkaloidal salt or salts may be crystallized out by cooling or concentrating the solution. The process described above is used to obtain quinine sulfate from cinchona bark.

Glycosides

Glycosides such as digoxin are another important group of drugs obtained from plants. Generally, the glycoside is extracted from the crude drug with alcoholic extract that causes the impurities to fall to the bottom. After being poured through a filter, the alcoholic extract is concentrated, and the glycosides may crystallize out. Often, however, concentration of the extract is not sufficient to cause crystallization, and more complicated procedures must be employed.

Volatile, or essential, oils

Volatile, or essential, oils, also obtained from plants, may be extracted by distillation, steam distillation, expression, or by extraction with fats or organic solvents. In steam distillation, the most common method, the crude drug - either fresh or dried - is used in powder form. Water is mixed with the powder, serving the double purpose of preventing decomposition of plant material by excessive heat and of facilitating volatilization of the essential oil. The powder-water mixture is usually placed in a basket through which the steam penetrates. The distillate consist of a water-oil mixture; the oil forms a separate layer, which is run off. Anise, cinnamon, clove, coriander, fennel, and peppermint oils are all obtained by this method, as is the pharmaceutical substance camphor from camphor wood.

Fixed, or fatty, oils cannot be obtained by distillation, but only by expression or extraction. Castor, olive, and sesame oil, for example, are all obtained by expression. Cod-liver and halibut-liver oils, rich sources of vitamins A and D, are both extracted by passing steam into tanks containing the livers suspended in water, until the temperature reaches 70-80 ºC (158-176 ºF). The tissues disintegrate, and the oils float to the surface and are skimmed or centrifuged off.