Menthol (peppermint camphor), C10H20O, is found in the peppermint oils of which the laevogyrate menthol constitutes the principal constituent. The oil of Hyptis suaveolens also consists in large part of menthol, but the direction of its optical rotation has not yet been ascertained. Menthol separates from peppermint oil upon cooling. If the menthol content be large the separation takes place even at ordinary temperature. In peppermint oil, menthol is likewise contained as acetate and isovalerate, also as ester of an acid C8H1202. Artificially menthol is obtained from menthone and pulegone.1) In the presence of an excess of hydrogen, menthone yields menthol only; if, however, solvents are used which themselves do not generate hydrogen with the sodium, menthopinacone is also formed. Both /- and d-menthone yield according to both methods, also with change of temperature, a strongly laevogyrate mixture from which /-menthol (m. p. 43,5°) also a slightly dextrogyrate isomenthol (m. p. 78 to 81°; [a]D + 2°) can be isolated.2)
1) Beckmann and Pleissner, Liebig's Annalen 262 (1891), 30, 32. 2) Beckmann, Journ. f. prakt. Chem. II. 55 (1897), 19, 30.
Menthol crystallizes in colorless needles or prisms which belong to the hexagonal system. It is characterized by its strong, peppermint-like odor and cooling taste.
Of physical constants the following are recorded:
M. p. 42°; b. p. 211,5° (736 mm.).1)
M. p. 42,3°; b. p. 212,5° (corr. 742 mm.); d 20o/4o 0,890 for solid and d 44.6o/4o0,8810 for liquified menthol; [a]D46o
--49,86° likewise for molten menthol.2)
B. p. 215,5° (758 mm.); aD31o - 43°45' for menthol in supercooled condition.3)
M. p. 43°; [a]D20° - 49,35° (in 20 p. c. alcoholic solution), - 50,59° (in 10 p. c. alcoholic solution):4) nC43o 1,4479.5)
According to recent observations, the melting point of perfectly pure menthol lies between 43,5 and 44,5°.
Menthol is a saturated secondary alcohol. When dehydrated with potassium acid sulphate, zinc chloride, etc., it is converted mainly into the hydrocarbon C10H18, A 3-p-menthene. The compounds obtained by replacing the hydroxy group by halogen are liquid and little characteristic. With hydrogen iodide and phosphorus it is reduced to hexahydrocymene, C10H20o 6) With chromic acid mixture it is oxidized to the corresponding ketone C10H18O, menthone.7) With potassium permanganate the same oxidation products result that are obtained from the oxidation of menthone, viz., ketomenthylic acid and B-methyladipic acid melting at 88 to 89°. As a result of these oxidation experiments the above-mentioned formula has been assigned to menthol. This formula has been strengthened by the conversion of menthone into 3-chlorcymene effected by Junger and Klages.8) That menthol could be converted into cymene by heating it with anhydrous copper sulphate to 250 to 280° had previously been demonstrated by Bruhl.1)
1) Arth, Annal. de Chim. et Phys. VI. 7 (1886), 438.
2) Long, Chem. Zentralbl. 1892, II. 525.
3) Power and Kleber, Pharm. Rundschau (New York) 12 (1894), 162; Arch, der Pharm. 232 (1894), 647, 653.
4) Beckmann, Liebig's Annalen 250 (1889), 327; Journ. f. prakt. Chem. II. 55 (1897), 15.
5) Bruhl, Berl. Berichte 21 (1888), 457, table.
6) Berkenheim, Berl. Berichte 25 (1892), 688.
7) Beckmann, Liebig's Annalen 250 (1889), 325.
8) Berl. Berichte 29 (1896), 314.
Of menthol esters a considerable number has been prepared. The above mentioned acetate, also the Isovalerate are described in the chapter on "Esters". For other esters special reference should be had to the paper by Tschugaeff-) who has given special attention to their optical rotation.
According to Montefl,8) chloral yields two liquid compounds with menthol, viz., chloralmonomenthol and chloraldimenthol. They result upon melting together the calculated amounts of the components.
Owing to the peculiar physical properties of menthol, its identificaton ought to afford but few difficulties. If necessary its phenylurethane obtained by the action of phenylIsocyanate on menthol may be utilized. This derivative, which was first prepared by Leuckart, melts at 111 to 112° and turns the plane of polarized light in the same direction as does menthol. It can be decomposed by sodium ethylate, but the menthol is thereby inactivated.4)
Other derivatives by means of which menthol can readily be characterized, are menthylbenzoate, resulting upon heating menthol with benzoic acid anhydride, which is difficultly volatile with water vapor and which melts at 54,5°;5) the dimenthyl ester of oxalic acid, melting at 67 to 68°; the dimethyl ester of succinic acid, melting at 62°; the monomenthyl ester of phthalic acid, melting at 110° and the dimenthyl ester of the same acid melting at 133°.
A mixture of menthol and menthone can be separated by converting the latter into its oxime and shaking out the oxime from the ethereal solution by means of dilute sulphuric acid.6)
1) Berl. Berichte 24 (1891), 3374.
2) Berl. Berichte 31 (1898), 364.
3) l'Union Pharm. Ace. to Zeitschr. d. allg. osterr. Apoth. Ver. 46 (1908), 272.
4) Beckmann, Journ. f. prakt. Chem. II. 55 (1897), 29.
5) Beckmann, Liebig's Annalen 262 (1891), 31; Journ. f. prakt. Chem. II. 55 (1897), 16.
6) Beckmann, Journ. f. prakt. Chem. II. 55 (1897), 17.