Thymol (isopropyl-m-cresol), C10H14O, is frequently accompanied by carvacrol. In addition to p-cymene and terpenes, it constitutes the larger part of ajowan oil. It occurs also in the oils of Ocimum viride, Monarda punctata, Satureja Thymbra, Origanum floribundum, thyme, Thymus capitatus, wild thyme, Mosla japonica and probably also in Canadian mint oil and in culina oil. Occasionally it separates from the oils at ordinary temperature.
As a source for thymol on a large scale, ajowan oil is mostly used.
Artificially it can be prepared in a variety of ways. Of interest is its production from dibrommenthone by splitting off hydrogen bromide by means of quinoline.1)
Thymol consists of colorless, transparent, monoclinic or hexagonal crystals, the odor of which reminds of thyme. They melt at 50,5 to 51,5° and boil at 232° (752 mm., the entire mercury thread in the vapor). Thymol crystals sink in water, hence have a density greater than 1. Liquid thymol, however, floats on the surface of water. It can be cooled considerably below its congealing point without solidifying. In connection with liquid thymol, Perkin1) has observed the following densities: d4o0,9872; d15o0,9790; d20o0,9757; d25o0,9723; d50o 0,9624.
1) Beckmann and Eickelberg, Berl. Berichte 29 (1896), 420.
As for the index of refraction (nD24.4o) Nasini and Bernheimer2) record 1,51893. Schimmel & Co. found: d15o 0,9760 and nD20o 1,52269 (supercooled).
Thymol is but sparingly soluble in water (1:1200) and in glycerin (1:1000), more so in paraffin oil (about 1:20), readily in alcohol, ether, chloroform, benzene, glacial acetic acid, volatile and fatty oils. In distinction to carvacrol, the alcoholic solution is not colored by ferric chloride. However, the aqueous solution of thymol sulphonic acid, C6H2(S03H)(CH3)(C3H7)(OH) resulting upon the solution of thymol in concentrated sulphuric acid produces a violet color with ferric chloride.
As a phenol, thymol forms water soluble "salts" with the alkalies. Hence it can be shaken out from its ethereal solution by means of 5 to 10 p. c, aqueous solutions of alkali. This property is utilized for the quantitative estimation of thymol. (Comp. the chapter on "The estimation of volatile oils" under Phenol determinations.)
Of thymol derivatives the following may here be mentioned: the phenylurethane, m. p. 107°, and the hitroso compound, m. p. 160 to 162°3) which results from the action of nitrous acid upon thymol. Upon oxidation it can be converted into thymoquinone.
Like its derivatives, thymol possesses decided disinfecting properties. These are, however, destroyed in the presence of oxidizing ferments in the presence of air.4)
Ethers from neither thymol nor carvacrol have been found in volatile oils. Guillaumin5) has prepared and described isomerides of thymol, but as yet it is not known whether or not these occur in volatile oils.
1) Journ. chem. Soc. 69 (1896), 1183.
2) Gazz. chim. ital. 15 (1885), 59; Jahresber. d. Chem. 1885, 314.
3) For the method of preparation comp. Klages, Bed. Berichte 32 (1899), 1518.
4) Cousin and Herissey, Journ. de Pharm. et Chim. VI. 26 (1907), 487.
5) Etude chimique et pharmacologique des thymols synthetiques derive's des acides cresotiniques. lnaug. Dissert, Paris 1909; Report of Schimmel & Co. April 1909, 136.