Terpenes

From the above-named liquids a number of terpenes have been isolated. Among the better known of these the following may be mentioned.

Pinene with a boiling-point of 160° C. It exists in two forms, distinguished by their rotatory power on polarized light: dextro-pinene is the chief constituent of German and American oil of turpentine. The other form of pinene, lævo-pinene, occurs in large proportion in French oil of turpentine; it absorbs oxygen less readily than dextro-pinene, and therefore resinifies more slowly. In consequence, it is more suitable for artistic uses, such as the thinning of paints, or as an ingredient of varnishes,

Phellandrene. Boiling-point 171° to 172°. This terpene has been separated from eucalyptus oil, that is, from the oil obtained by the distillation with water of the leaves of one of the numerous species of eucalyptus, E. amygdalina. It is one of the most alterable of all terpenes, and the oils containing it should be avoided.

Limonene. This terpene, like most of the others, occurs in two forms or varieties, having opposite actions on polarized light. It is sometimes called citrene. It boils at 177°. It is best prepared from orange-peel oil, which yields over 90 percent of dextro-limonene when distilled from caustic potash. When pure it is less alterable than dextro-pinene and phellandrene, though it resinifies after a time.

Sylvestrene. Boiling-point 176°. When pure it has the smell of bergamot, but generally presents the odour of fir-wood. It is dextro-rotatory, and forms the chief constituent of Russian and Swedish oil of turpentine, and of some of the German oils. On exposure to air it oxidizes readily, and leaves a sticky resin. It has a particularly strong odour.

Dipentene, which boils at about 177° C, is optically inactive, and may be made by heating some of the other terpenes to 250° - 270° for some hours, or by mixing dextro- and lævo-limonene together. The only natural product in which dipentene is known to occur is the volatile oil which accompanies common camphor. Its odour resembles that of citron oil: it resinifies to about the same extent as limonene.

From the preceding descriptions it may be gathered that of all the above terpenes, limonene and dipentene are the least alterable. They have, however, somewhat high boiling-points, and evaporate more slowly than most of the other terpenes.

Here it may be mentioned that the presence of water in a terpene, or a mixed essential oil, may be detected by the cloudiness which it shows when mixed with thrice its volume of benzine or of petroleum-spirit. To remove traces of water from any of the less volatile liquids we have been considering, without having recourse to distillation from caustic potash, or from quicklime, the following simple procedure may be adopted: A glass flask is three-fourths filled with the liquid, and then it is kept at a temperature of 110° to 120° C, so that the moisture present is disengaged as vapour without the terpene or essential oil itself boiling: drops of moisture will condense in the neck of the flask, and may be removed from time to time by means of a roll of blotting-paper. The mouth of the flask should be loosely plugged with carded cotton. Of course this process is applicable only to liquids which boil at temperatures considerably over 120°, like the terpenes. In connexion with the terpenes two other liquids and one solid remain to be mentioned. The liquids are 'oil of amber' and 'oil of copal.' These are obtained by strongly heating the resins in question. They are employed as efficient solvents for the harder resins.

Oil of amber may be obtained in commerce at a moderate price. Its offensive smell, partly due to compounds of sulphur, may be lessened by adding to it some white lead and solid caustic potash, and afterwards distilling it. It contains amongst other liquid constituents at least one terpene. Its boiling-point rises, as distillation proceeds, from 110° C. to 260°. Camphor is expressed by the formula C₁₀H₁₆O, and is obtained chiefly from Cinnamomum Camphora, a tree of Formosa, China and Japan. It is a tough crystalline solid of penetrating odour and pungent taste. It is soluble in all the liquids named in the present chapter. Although it boils at so high a temperature as 204° C, it readily and rapidly volatilizes at ordinary temperatures. It is used to aid the solution of some of the harder resins in the making of varnishes, but its presence in a varnish is objectionable, for it slowly escapes after the apparendrying-up of the varnish, and thus causes a deterioration of the lustre and continuity of the resinous film.