Although camphene is the only solid hydrocarbon C10H16 which occurs in Nature, its isolation in a crystalline state from volatile oils has been accomplished but rarely. In this form /-camphene has been isolated from the Siberian Fichtennadelol; from the first fractions of Ceylon citronella oil Schimmel & Co. likewise obtained solid camphene. In the vegetable kingdom this hydrocarbon is found in both active modifications.
1) Wallach. LiebiVs Annalen 359 (1908). 266.
As (d-camphene (the austracamphene of Berthelot), it has been found in cypress oil, in Siberian Fichtennadelol, in the oils of ginger, nutmeg, camphor, lemon, orange-petitgrain, sweet orange blossoms, of Eucalyptus Globulus, and spike.
As l-camphene (the terecamphene of Berthelot),1) it has been found in the oil from the twigs of Juniperus phcenicea and in the oils of lemon, neroli (?), valerian, kesso, Artemisia Herba-alba, and of Chrysanthemum sinense var. japonicum.
Artificially camphene can be obtained in various ways, principally by splitting off hydrohalogen from bornylchloride or bornyl bromide (from pinene or borneol). Most conveniently, however, it is obtained upon dehydration of isoborneol with zinc chloride, though in this manner frequently mixtures of different hydrocarbons are obtained.
Camphene occurs as a white crummy-crystalline mass, with a faint camphor-like odor, that tends toward sublimation. Otherwise it is much more stable toward air and light than the other terpenes. In as much as it can be obtained in solid form and since by careful precipitation from its alcoholic solution with water it can be purified from liquid impurities, it is one of the few terpenes that have been prepared with a fair degree of purity. As its constants the following have been recorded: for camphene from borneol-bornylchloride: M. p. 48 to 49°; b. p. 160 to 161°2); d48o 0,850; nc48o 1,45553); M. p. 53,5 to 54°; d 58.6o/4o 0,83808; nD58.6o 1,453144) for camphene from pinene-bornylchloride: M. p. 51 to 52°; b. p. 158,5 to 159,5°; d 54o/4o 0,84224; nD54o1,455141); for camphene from isoborneol: M. p. 50°; b. p. 159 to 160°;'2) 56° at 15 mm.
1) The term terecamphene is at times applied to any camphene prepared from pinenechlorhydrate.
2) Wallach, Liebig's Annalen 230 (1885), 234.
3) Wallach, Ibidem 245 (1888), 210.
4) Bruhl, Berl. Berichte 25 (1892), 164.
The corresponding values at 20° should be approximately d 0,870 and nD 1,470.
The angle of rotation of the camphene prepared artificially from pinene hydrochloride or bornyl chloride varies not only according to the angle of rotation of the initial material, but also according to the temperature - both height and duration - employed in the process. Thus Bouchardat and Lafont3) obtained hydrocarbons, the [a]D of which lay between -80°37' and - 30° 30' when they allowed /-pinene-bornylchloride ([a]D - 28° 30') to be acted upon by an alcoholic solution of potassium acetate at a temperature of from 150 to 170°.
In connection with a (/-camphene obtained from borneol-bornylchloride, Kachler4) observed [a]D85o + 20° (100,3 mm.).
The camphene molecule does not rearrange itself to isomeric forms as do other terpenes. While it is true that prolonged heating at a higher temperature, or treatment with dehydrating agents such as zinc chloride, phosphoric acid anhydride, or concentrated sulphuric acid changes it, the resulting decomposition products do not correspond to the formula C10H16.
As a terpene with one double bond, camphene yields addition products with halogen (m. p. of the dibromide 91 to 91,5°) and with hydrohalogen, but not with nitrosylchloride. However, compounds of camphene with the oxides of nitrogen have been obtained both directly and indirectly. Extensive investigations have been made concerning the relation between pinene hydrochloride, camphene hydrochloride, and the corresponding derivatives of borneol and isoborneol.
1) Bruhl, Berl. Berichte 25 (1892), 162.
2) Bertram and Walbaum, Journ. f. prakt. Chem. II. 49 (1894), 8. 3) Compt. rend. 104 (1887), 694; Bull. Soc. chim. II. 47 (1887), 439. 4) Liebig's Annalen 197 (1879), 97.
Camphene chlorhydrate is obtained by passing gaseous hydrochloric acid into an alcoholic solution of camphene. It melts at about 155°. References1) do not agree, the melting points recorded varying from 142 to 160°.
Oxidizing agents, such as permanganate, nitric acid, chromic acid mixture, and ozone, do not act in like manner on the hydrocarbon. The oxidation with permanganate would seem to indicate that artificial camphene is not a unit, but consists of a mixture of at least three hydrocarbons. Thus the oxidation of a camphene, obtained from Isoborneol, with permanganate yields three different products: camphene glycol, C10H16(OH)2, m. p. 200°; camphene camphoric acid, C10H16O4, m. p. 135,5 to 136°; and cyclene, C10Hl6, m. p. 68°, b. p. 152,8°.2) The further oxidation of camphene glycol yields camphenylic acid, C10H16O3 (m. p. 171 to 172°) and camphenilone, C9H140 (m. p. 36 to 38°). From natural camphene an isomeric camphene-camphoric acid (m. p. 142°) appears to result which is not identical with the one mentioned above.3) Nitric acid oxidizes camphene to the tribasic camphoylic acid (Marsh and Gardner), C10H14O6 (Bredt's carboxylapocamphoric acid, m. p. 202°). In addition minor amounts of the above-mentioned ketone C9H140 result. Chromic acid mixture yields principally camphor with little camphoric acid and other products. Oxidation with ozone produces camphenilone.
However, none of these derivatives can be used to advantage for the characterization of camphene. With fairly pure fractions rich in camphene, the latter can be separated in the form of its chlorhydrate. It is better, however, to identify camphene by converting it into Isoborneol. Should larger amounts of pinene be present, the identification of camphene by this method is difficult since terpineol as well as isoborneol is formed, and since the separation of the two can be attained but imperfectly.
1) Kachler and Spitzer, Liebig's Annalen 200 (1880), 343; Jungerand Wages, Berl. Berichte29(1896), 545; Reychler, lbidem32(1899), 2302; Kondakow, Journ. f. prakt. Chem. II. 65 (1902), 201.
2) Moycho and Zienkowski, Liebig's Annalen 340 (1905), 17, 40.
3) Wallach, Liebig's Annalen 366 (1907), 84.
For the conversion of camphene into isoborneol the following method is used:1)
100 parts of the camphene fraction are heated with 250 parts of glacial acetic acid and 10 parts of 50 percent, sulphuric acid for 2 to 3 hours to 50 to 60° with frequent shaking; the mixture which at first separates into two layers finally becomes homogeneous and has a slightly reddish color. When the reaction is ended, the acetate formed is separated by water, washed repeatedly, and then saponified by heating with a solution of 50 g. of potassium hydrate in 250 g. of alcohol. After removing the alcohol, the isoborneol is precipitated as a crumbly mass by the addition of water and is purified by recrystallization from petroleum ether. The melting point of Isoborneol is about 212°; the determination must, however, be made in a sealed capillary tube on account of its great tendency to sublime. For the further characterization of the Isoborneol, its bromal compound, m. p. 71 to 72° may be used.
It should be remembered, however, that this isoborneol always contains some borneol. The phenylurethanes of both alcohols melt at 138 to 139°.
Recent observations appear to render it probable that cam-phene occurs in several modifications which differ from each other not only by their physical constants, but also by their oxidation products. As to their constitution, however, no details are as yet known. Thus Wallach2) isolated from a Siberian pine needle oil a camphene with the following properties:
M. p. 39°; b. p. 160 to 161°; d40o 0,8555; [a]D - 84,9°; nD40o 1,46207; mol. refr. 43,71; calculated for C10 H16/= 43,53.
Upon oxidation of this camphene with permanganate there resulted an acid, isomeric with camphene-camphoric acid, that melted at 142°. Its chlorhydrate melted at 51°. The same cam phene has likewise been found in citronella oil.
By acting on bornylamine with sodium nitrite, Wallach3) obtained a camphene (m. p. 50°; b. p. 160 to 161°), which was characterized by a remarkably strong optical rotation (jV|D + 103,89°).
1) Bertram and Walbaum, Journ. f. prakt. Chem. II. 49 (1894), 1. - In addition to isoborneol some borneol is formed. Aschan, Berl. Berichte 40 (1907), 4923.
2) Liebig's Annalen 357 (1907), 79.
3) Ibidem 84.