Carvacrol (isopropyl-o-cresol), C10H14O, occurs principally in labiate oils. Thus far its presence has been demonstrated in camphor oil, schinus oil, the oils of Monarda punctata (?), M. fistulosa, M. citriodora, Satureja hortensis, S. m on tan a, Origanum vulgare, in the origanum oils from Trieste, Smyrna, Cyprus, and Syria, in the oils of thyme and wild thyme, and in the oil of Thymus capitatus (?).
Artificially carvacrol can be obtained from its isomeride carvone by treating this ketone with potassa, sulphuric acid, or phosphoric acid; from camphor by heating it with iodine; from cymene sulphonic acid by fusion with potassa; also from a number of terpene derivatives, thus from thujone, carvenone, carvotanacetone, dihydrocarvone, nitrosopinene, ketoterpin, and terpinene erythrite.
Freshly distilled, carvacrol is a colorless, viscid oil which becomes yellowish in the course of time, and which congeals in the cold. Its alcoholic solution is colored green by ferric chloride. The following constants are recorded:
M. p. +0,5 to +1°; b. p. 236 to 237° (corr.); dl5o 0,981; nD 1,525.1)
B. p. 119° (16 mm.); d20o0,9782; nDl,5228.2)
1) Jahns, Berl. Berichte 15 (1882), 817.
2) Semmler, Berl. Berichte 25 (1892), 3353.
For carvacrol from origanum oil Gildemeister found:1) M. p. +0,5°; b. p. 235,5 to 236,2° (742 mm.); d15o 0,980; d20o0,976; nD20o1,52338; and for carvacrol prepared from carvone:
M. p. +0,5°; b. p. 236 to 236,5° (742 mm.); d15 0,983; d20o 0,979; nD20o1,52295.
In connection with products made on a large scale, Schimmel and Co. observed: d15o about 0,98; aD +0°; nD20o 1,523 to 1,524; soluble in 3 vols, of 70 p. c. alcohol.
Like thymol, carvacrol can be extracted from oils with aqueous alkali and determined quantitatively (see the chapter "Examination of volatile oils" under Phenol determination): It should be remembered, however, that carvacrol can be completely extracted from its alkaline solution by shaking with ether.2) It can likewise be distilled from strongly alkaline solutions with steam, a property which, so far as phenols are concerned, is shared only by thymol.3)
Prolonged moderate heating with caustic potassa converts carvacrol into isohydroxycuminic acid, (CH3)2CHC6H3(OH)C02H (4-metho ethyl-phenol-2-methyl acid-1),4) which is readily volatile with water vapors and which melts at 93°. Chromic acid mixture oxidizes it to thymoquinone, which crystallizes in yellow plates that melt at 45,5°.5) When reduced according to Sabatier and Senderens a mixture of two isomeric alcohols, a- and B-carvacromenthol, is obtained. The latter yields /-tetrahydro-carvone upon oxidation.6)
For the identification of carvacrol, the phenylurethane,7) m. p. 140°, and nitroso carvacrol are well suited. The latter is best prepared according to the directions of Klages.8) To a solution of carvacrol in four times its volume of alcoholic hydrochloric acid, saturated at 0°, and kept cool with ice, a concentrated solution of sodium nitrite is added drop by drop. Within a few minutes, the entire liquid congeals to a thick mass of nitroso compound. This is first washed with water and then recrystallized from dilute alcohol. It crystallizes in yellow needles that melt at 153°. The benzoyl compound of nitroso carvacrol melts at 110°.
1) Arch, der Pharm. 233 (1895), 188.
2) Jahns, Berl. Berichte 15 (1882), 817.
3) Klages, ibidem 32 (1899), 1517.
4) Jacobsen, Berl. Berichte 11 (1878), 573, 1061.
5) Reychler, Bull. Soc. chim. III. 7 (1892), 34; Chem. Zentralbl. 1892,1. 380. 6) Brunei, Compt. rend. 141 (1905), 1245; 145 (1907), 1427.
7) Goldschmidt, Berl. Berichte 26 (1893), 2086, footnote; comp. also Gildemeister, Arch, der Pharm. 233 (1895), 188.
8) Berl. Berichte 32 (1899), 1518.