"A volatile oil distilled from the leaves and flowering tops of Thymus vulgaris, Linné (Nat. Ord.—Labiatae). It should be kept in well-stoppered bottles, in a cool place protected from the light"—(U. S. P.).
Preparation.—This oil is chiefly obtained in southern France, where the plant grows in immense quantity. Spain also produces oil of thyme, although this is of somewhat different physical and chemical characters. The crude product is known as red oil of thyme, and is usually sold under the name of oil of origanum (see Oleum Origani). The rectified oil is known as white oil of thyme. In southern France these two grades are known respectively as huile rouge de thyme and huile blanche de thyme. In this connection, Gildemeister and Hoffmann (Die Aetherischen Oele, 1899, p. 817) point out that rectified oil of thyme, unless rectified under especial precautions, soon acquires the red color of the crude oil again. The permanently pale French oil is stated to be produced by distilling the crude oil with an excess of oil of turpentine; hence such oils contain at most 5 per cent of phenols, as against 20 to 25 per cent, sometimes 42 per cent in normal oils. The yield of oil of thyme varies from 0.3 to 0.4 per cent (fresh herb, German) to 1.7 per cent (dry, German), and 0.9 per cent (fresh, French) to 2.5 and 2.6 per cent (dry, French).
Description.—"A yellowish or yellowish-red liquid, having a strong odor of thyme, and an aromatic, pungent, afterward cooling taste. It becomes darker and thicker by age and exposure to the air. Specific gravity, 0.900 to 0.930 at 15° C. (59° F.). It does not fulminate with iodine. The oil is soluble in half its volume of alcohol, forming a clear solution which is neutral or only very slightly acid to litmus paper. The oil is also soluble, in all proportions, in carbon disulphide, and in glacial acetic acid. With a drop of ferric chloride T.S. the oil yields a greenish-brown color, which changes to reddish"—(U.S. P.). French and German oil of thyme is soluble in 1 or 2 volumes of alcohol of 80 per cent, but 15 to 30 volumes of 70 per cent are required for complete solution, while the Spanish oil, probably from another botanical source, forms a clear solution with the latter solvent.
Chemical Composition and Tests.—The characteristic constituents of oil of thyme are the solid thymol and the liquid carvacrol, two isomeric phenols of the formula C10H14O (see Thymol). As stated before, the total amount of phenols in oil of thyme (French and German oils) is 20 to 25 per cent, seldom as high as 42 per cent; thymol is usually the only phenol present; sometimes it is either entirely or to a small extent, replaced by carvacrol. The Spanish oil contains from 50 to 70 per cent of carvacrol exclusively (compare Oleum Origani). The correct formula for thymol was established by Lallemand (1853). He also found oil of thyme to contain the hydrocarbons cymol (C10H14), and a small quantity of l-thymene, which was identified by Schimmel & Co. (1894) as l-pinene. The latter also established the presence of borneol and linalool in the higher fractions of oil of thyme; these results were confirmed by Labbé (1898).
The presence of oil of turpentine in oil of thyme may be recognized by the specific gravity being lower than 0.900, or by the diminished solubility in alcohol, and the deficiency in the phenol contents of the oil. A convenient method for determining the amount of phenols in oil of thyme consists in shaking a given volume of the oil (e. g., 10 Cc.) in a burette with an equal volume of a 5 per cent caustic soda solution, allowing to stand from 12 to 24 hours, and measuring the volume of the remaining non-phenols (compare Oil of Cassia). By drawing off the aqueous phenolate, and rendering acid with sulphuric acid, the regenerated phenols will solidify upon standing when consisting of thymol, but will remain fluid when they consist of carvacrol. An iodometric method for the estimation of phenols in oil of thyme was devised by E. Kremers and O. Schreiner (see Pharm. Review, 1896, p. 221). The U. S. P. directs the following test: "If 1 Cc. of the oil be shaken with 10 Cc. of hot water, and, after cooling, the liquid be passed through a wet filter, the filtrate should not assume, with a drop of ferric chloride T.S., a bluish or violet color (absence of carbolic acid)"—(U. S. P.)
Related Oils.—OLEUM SERPYLLI, Oil of wild thyme. The essential oil distilled from Thymus Serpyllum, Linné. A colorless or golden-yellow, laevogyrate oil of the specific gravity 0.905 to 0.930. Odor slightly like thyme, but more like melissa. Constituents, chiefly cymol (C10H14), with 1 per cent of thymol, carvacrol, and probably other phenols.
OIL OF THYMUS CAPITATUS.—Has a pronounced thyme-like odor; specific gravity, 0.901; constituents, thymol (6 per cent), cymol, pinene, dipentene, bornyl-acetate, and possibly carvacrol (Schimmel & Co.'s Report, October, 1889). Produced in Spain.
OIL OF THYMUS CAMPHORATUS.—Specific gravity, 0.904. Constituent, carvacrol (Power, Essential Oils).
King's American Dispensatory, 1898, was written by Harvey Wickes Felter, M.D., and John Uri Lloyd, Phr. M., Ph. D.