Both optical modifications of menthone, C10H18O, are found in nature. d-Menthone has been found in the oil of Barosma pulchellum and in American pennyroyal oil from Hedeoma pulegioides; /-menthone in Reunion geranium oil, buchu-leaf oil, peppermint oil and American pennyroyal oil. Menthone has also been found in European pennyroyal oil from Mentha pulegium, in the oil of Bystropogon origanifolius and in the oil from cassie blossoms (?), but the direction of rotation has not been recorded in these instances.
1) Wallach and Schrader, Liebig's Annalen 279 (1894), 389; 286 (1895), 127. 2) Wallach, Liebig's Annalen 275 (1893), 117; 279 (1894), 381. 3) Berl. Berichte 28 (1895), I960. ') Ibidem 32 (1899), 3372, footnote.
Inasmuch as menthone does not combine with acid sulphites, and since it cannot be obtained pure by fractional distillation, it must be isolated either by means of its oxime or by means of its semicarbazone which are prepared from the menthone fractions. It should be noted, however, that when these compounds are hydrolyzed, mostly with the aid of dilute sulphuric acid, the angle of rotation is altered.
Pure /-menthone with normal optical activity is readily obtained from menthol by oxidation with the chromic acid mixture recommended by Beckmann.1)
Synthetically, menthone has been obtained by several methods.
Pure menthone is a mobile, colorless liquid with a pepper-mint-like odor and a slightly cooling, bitter taste. For the ketone obtained by the Oxidation of menthol, Beckmann2) found the following constants:
B. p. 207°; d20o 0,8960; [a]D20o- 28,18°; nD12o 1,4525; and Binz3) the following ones: d24o 0,8934; [a]D24o-27,67°.
According to Wallach,4) menthone regenerated from its semi-carbazone (m. p. 184°) possesses the following properties:
B. p. 208°; d 0,894; nD 1,4496.
In the laboratory of Schimmel & Co. the following were observed: d15o0,894 to 0,899; aD - 20°27' to - 26°10'; nD20o1,450 to 1,451; soluble in 3 vols, of 70 p. c. alcohol. dl5o0,8971; aD - 26°10'; [a]D - 29,17°.
If /-menthone is treated at a low temperature with concentrated sulphuric acid it is changed to its dextrogyrate isomer.5) In this manner Beckmann obtained what he regarded as d-menthone of the same angle of rotation ([a]d+28,1°) as the /-menthone ([a]D - 28,5°) from which he started. It was shown, however, that the former was not the optical antipode of the latter, but a mixture of /-menthone with the much more strongly active d-Isomenthone. A highly dextrogyrate (d-isomenthone he obtained in the following manoer: Menthoxime was reduced, the resulting menthylamine treated with nitrous acid and the menthol thus obtained oxidized. This isomenthone had a specific angle of rotation of + 93,2 V)
1) Beckmann, Liebig's Annalen 250 (1889), 325.
2) Ibidem 327.
3) Zeitschr. f. physik. Chem. 12 (1893), 727.
4) Berl. Berichte 28 (1895), 1963.
5) Beckmann, loc. cit. 334.
Beckmann is of the opinion that the change in the angle of rotation brought about by acids is due to intermediary enolising. This interpretation is favored by the observation made by Mannich and Hancu,2) namely that /-menthone ([a]D - 22,4°) when subjected to prolonged heating with acetic acid anhydride to 240° yields the ester of l-methyl-4-methoethyl cyclohexene-(2 or 3)-ol-3 (b. p. 98° at 11 mm.; aD + 6,65°) which upon saponification yields a dextrogyrate menthone (aD+1,540 in a 20 mm. tube).
In general, the change in the angle of rotation brought about by acids and alkalies is noteworthy.
When reduced in alcoholic solution with sodium, it is converted into the corresponding secondary alcohol, the /-menthol, C10H20O. As a by-product small amounts of the slightly dextrogyrate Isomenthol result. When indifferent solvents are used some menthopinacone,3) m. p. 94°, also is formed.
In dilute alcoholic solution, /-menthone condenses readily with hydroxylamine to /-menthoxime, m. p. 60 to 61o 4) The other modifications of menthone yield oximes that are either liquid or possess a higher melting point.
If the oxime is treated with dehydrating agents, it is converted into an aliphatic nitrile, C9H17CN. When further modified this yields compounds that very much resemble the corresponding members of the citronellal group.5) Upon molecular rearrangement, induced by sulphuric acid, /-menthoneoxime yields a lactam, the menthone isoxime, m. p. 119V)
1) Berl. Berichte 42 (1909), 847. 2) Berl. Berichte 41 (1908), 570.
3) Beckmann, Journ. f. prakt. Chem. II. 55 (1897), 18, 30. 4) Beckmann, Liebig's Annalen 250 (1889), 330; Wallach, Liebig's Annalen 277 (1893), 157; 278 (1894), 304.
5) Wallach, Liebig's Annalen 278 (1894), 308; 296(1897), 120; 312 (1900), 171.
Semicarbazide reacts on menthone with the formation of a semicarbazone that crystallizes in needles which melt at 184°.2) Noteworthy is also the thiosemicarbazone, m. p. 155 to 157° and the semioxamazone, m. p. 177°.
If menthone is oxidized with a solution of chromic acid in glacial acetic acid, there results as first oxidation-product a keto acid C10H18O3 (keto- or hydroxymenthylic acid).3) Upon further oxidation with either potassium permanganate or chromic acid mixture, this is converted into the dibasic B-methyl adipic acid (B-pimelic acid of Arth),4) hence into the same "Abbau" product obtained from pulegone and citronellal.
If isoamyl nitrite and hydrochloric acid are allowed to react on menthone in the cold, bisnitrosomenthone, (C10H17O NO)2, m. p. 112,5° and the oxime of ketomenthylic acid, m. p. 103°, /'. e. of the first oxidation product of menthone, are formed.5)
If bromine (2 mol.) is allowed to act on menthone (1 mol.) in chloroformic solution, there results a crystalline dibromemen-thone, C10H16Br2O (m. p. 79 to 80°). When acted upon by quinoline, this looses hydrogen bromide and is converted into thymol.6)
These changes, as also the conversion of menthone into 3-chlorcymene brought about by Junger and Klages,7) are in harmony with the structural formula given above, which also expresses the relation of menthone to pulegone.
For the identification of menthone, the preparation of the semicarbazone or oxime is resorted to. For further characterization the ketone can be reduced to menthol and this converted into the benzoic ester (see menthol). For the quantitative determination of menthone consult the chapter on "The examination of volatile oils", ketone determinations.
1) Beckmann and Mehrlander, Berl. Berichte 20 (1887), 1508; Wallach, Liebig's Annalen 278 (1894), 304.
2) Wallach, Berl. Berichte 28 (1895), 1963; Beckmann, Liebig's Annalen 289 (1896), 366.
3) Beckmann and Mehrlander, Liebig's Annalen 289 (1896), 368.
4) Arth, Annal. de Chim. et Phys. VI. 7 (1886), 433; Beckmann and Mehrlander, loc. cit.*378; Manasse and Rupe, Berl. Berichte 27 (1894), 1818.
5) Baeyer and Manasse, Berl. Berichte 27 (1894), 1913, 1914; see also Baeyer and Oehler, Berl. Berichte 29 (1896), 27.
6) Beckmann and Eickelberg, Berl. Berichte 29 (1896), 418. 7) Berl. Berichte 29 (1896), 315.