Fenchone is a ketone C10H16O which closely resembles camphor but is liquid at ordinary temperature. In volatile oils both optically active modifications occur. d-Fenchone has been found in fennel oil and in the oil of Lavandula Stoechas, whereas /-fenchone has been found in the oil of Arbor vitas. For the fenchone found in the oil of Thuja plicata the direction of rotation has not been recorded.
For its purification,1) the accompanying substances found in fraction 190 to 195° are removed by oxidation with either concentrated nitric acid or permanganate solution. Fenchone, being very stable toward oxidizing agents, is effected but little. Having thus been purified in a fair measure, it congeals in the cold and can be further purified by crystallization. However, small amounts of camphor, resulting from the oxidation of borneol esters, cannot be removed in this manner. Methods for the separation of these two ketones are given below. Artificially fenchone is obtained by the oxidation of fenchyl alcohol.
Pure fenchone is a water-white, somewhat oily liquid, with an intense, camphor-like odor and a bitter taste. As to its physical properties, Wallach2) records the following data:
M. p. +5 to 6°; d19o 0,9465, d23o 0,943; [a]D18o+71,97° and - 66,94° resp. (in alcoholic solution);3) nD19o 1,46306.
1) Wallach, Liebig's Annalen 263 (1891), 130.
2) Liebig's Annalen 263 (1891), 131; 272 (1893), 102.
3) The lower angle of rotation is explained by a small camphor content of the oxime examined. Wallach, Liebig's Annalen 353 (1907), 215.
For a tf-fenchone regenerated from semicarbazone, Wallach1) ascertained the following constants:
B. p. 192 to 193°; d18o 0,948; [a]D + 62,76°, resp. +68,43° (in 13,76 percent, alcoholic solution); nD18o 1,46355.
The constitution of fenchone appears to find its expression in the formula given above. This formula was first suggested by Semmler-) and has gained in probability by the more recent work of other investigators.3)
In its chemical properties, fenchone closely resembles camphor. It has been examined into in detail by Wallach. The following account takes into consideration those derivatives only that are useful for its identification.
Like camphor, fenchone does not combine with acid sulphite; neither does it react with phenylhydrazine. However, with hydroxylamine4) it yields an oxime which is best prepared according to the directions of Wallach.5) To a solution of 5 g. of fenchone in 80 ccm. of absolute alcohol a solution of 11 g. hydroxylamine hydrochloride and 6 g. of powdered potash in 11 g. of hot water is added. After standing for some time, more particularly after some alcohol has evaporated, the oxime crystallizes out. It can be purified by recrystallization from alcohol, acetic ether, or ether. The active modifications melt at 164 to 165°, the inactive modification melts at 158 to 160°. Fenchone oxime resembles camphor oxime in so far as upon dehydration it yields the nitrile of fencholenic acid, C10H15N, which acid is isomeric with campholenic acid.
Upon reduction, active fenchone yields fenchyl alcohol, C10H18O,6) which melts at 45° and which deviates the ray of polarized light in the direction opposite to that of the fenchone from which it is obtained. Upon oxidation with permanganate, fenchone yields dimethylmalonic acid (CH3)2C(COOH)2, m. p. 188 to 189V) also acetic and oxalic acids. Upon oxidation with concentrated nitric acid it also yields isocamphoronic acid and dimethyltricarballylic acid.2)
1) Liebig's Annalen 362 (1908), 195, footnote.
2) Chem. Ztg. 29 (1905), 1313; Berl. Berichte 39 (1906), 2581; 40 (1907), 439.
3) Bouveault and Levallois, Compt. rend. 146 (1908), 180; Wallach, Liebig's Annalen 369 (1909), 63.
4) Wallach, Liebig's Annalen 263 (1891), 136. 5) Liebig's Annalen 272 (1893), 104.
6) Liebig's Annalen 263 (1891), 143.
In like manner as camphor yields p-cymene when treated with phosphoric acid anhydride, so fenchone yields m-cymene. When acted on by concentrated sulphuric acid, fenchone yields 4-acetyl-l, 2-xylene.8)
By the action of sodium amide on fenchone, Semmler4) obtained a dihydrofencholenic acid amide melting at 94°.
The semicarbazone of fenchone is formed exceedingly slowly. Only recently Wallach5) has pointed out a convenient method for its preparation. 10 g. semicarbazide hydrochloride and 10 g. sodium acetate are dissolved in 20 ccm. of water. To this reagent a solution of 10 g. fenchone in 50 ccm. alcohol is added. The clear solution is set aside at room temperature for at least two weeks when the reaction product is distilled with steam. Alcohol and uncombined fenchone distil over, whereas the semicarbazone congeals in part to a compact mass, in part crystallizes from the hot water in which it is not completely insoluble. From dilute alcoholic solutions it crystallizes in thick, well-defined, shiny rhombic prisms, one centimeter long, which melt at 182 to 183°. The inactive form melts at 172 to 173° and does not possess the same crystallising capacity.
Inasmuch as the semicarbazone of camphor is formed much more readily, this difference in behavior can be utilized for the separation of camphor from fenchone.6) Another method of separation, proposed by Semmler,7) is based on the different behavior of the two ketones when boiled with sodium. Camphor reacts with the formation of sodium camphor, whereas fenchone is said to remain unchanged. Wallach's8) investigations, however, have revealed that fenchone also does not remain unchanged when boiled with sodium, but that ultimately the change is as complete as with camphor, the reaction only progresses more slowly. Hence the separation of fenchone from camphor by means of distillation over sodium is accompanied by considerable loss.
1) Wallach, ibidem 263 (1891), 134.
2) Gardner and Cockburn, Journ. chem. Soc. 73 (1898), 708.
3) Marsh, Journ. chem. Soc. 75 (1899), 1058; Wallach, Liebig's Annalen 315 (1901), 295.
4) Berl. Beriohte 39 (1906), 2578.
5) Liebig's Annalen 353 (1907), 211.
6) Wallach, Liebig's Annalen 353 (1907), 213ff.
7) Berl. Berichte 40 (1907), 4591.
8) Liebig's Annalen 369 (1909), 65.
Finally, the method of separation by means of aluminium chloride recently suggested by Leroide1) should be mentioned.