The optically inactive form of limonene, the dipentene, has been found frequently in nature, though not as commonly as limonene. It must also be born in mind that the dipentene identified may have been formed from pinene or limonene during the process of fractionating the oil. Dipentene has been found in the following oils: In the oils from the turpentines of different species of Pinus, in pine tar oils, in the needle oil from Picea excelsa, in Swedish turpentine oil, in Siberian pine needle oil, in the needle oil from Callitris glauca, in the oils of palmarosa, lemongrass, citronella, gingergrass, cardamom, pepper, cubeb, nutmeg, boldo leaves, kuromoji, campher, cinnamon root, massoy-bark, apopin, bergamot, limette leaves, neroli, wartara, buchu, frankincense, myrrh, myrtle, ajowan, coriander, cumin, and fennel, also in the oils of Satureja Thymbra, Thymus capitatus, in American pennyroyal oil, in kessoroot oil and in goldenrod oil.
1) Berl. Berichte 27 (1894), 448.
2) Pharm. Rundschau (New York) 12 (1894), 160; Arch, der Pharm. 232 (1894), 646.
3) Chem. Ztg. 22 (1898), 827.
Aside from its formation from equal parts of d- and /-limonene, it is obtained synthetically by polymerization of the unsaturated aliphatic hydrocarbon isoprene, C5H8; also, together with terpinene, upon dehydration of the aliphatic alcohols linalool and geraniol. Furthermore, it results upon isomerization of other hydrocarbons C10H16, e.g. pinene, limonene, phellandrene; also by inducing the proper changes in related oxygenated compounds, such as cineol, terpineol, and terpinhydrate. Dipentene, together with cineol and limonene, was also obtained by the action of nitrous acid on fenchylamine.
A relatively pure dipentene can be obtained by the dry distillation of caoutchouc. After the removal of the isoprene, fraction 172 to 178° is carefully and repeatedly fractionated over sodium. The presence of any cineol, which has a like boiling point, can be ascertained by a corresponding increase in the specific gravity. A less pure preparation can be obtained from dipentene dihydrochloride by splitting off hydrogen chloride with aniline or sodium acetate in glacial acetic acid;1) also by the dehydration of crystalline terpineol by means of potassium acid sulphate.2)
Physically, dipentene differs from limonene only by its optical inactivity. The boiling point of pure dipentene appears to be the same as that of limonene. However, when in research for dipentene, it is necessary to take into consideration fractions that boil somewhat higher than the corresponding limonene fractions.1) Specific gravity and index of refraction agree fully with those determined for limonene. For dipentene from caoutchouc the following data were ascertained in the laboratory of Schimmel *&Co.:
1) Wallach, Liebig's Annalen 239 (1887), 3; 245 (1888), 196. Comp. also Tilden and Williamson, Journ. chem. Soc. 63 (1893), 294.
2) Wallach, Liebig's Annalen 275 (1893), 109.
B. p. 175 to 176°; d20o 0,844; nD20o 1,47194.
Dipentene is relatively stable. When heated it is not converted into isomeric hydrocarbons C10H16, but polymerizes. However, when heated with alcoholic sulphuric acid, it is changed to terpinene. Its derivatives are inactive and can be obtained from dipentene as well as by the union of equivalent amounts of the corresponding derivatives of limonene. The dipentene derivatives reveal minor differences from the corresponding limonene derivatives, notably in their melting points. Toward hydrohalogen, bromine and nitrosylchloride dipentene behaves like the active limonenes. The solid addition products that result upon the addition of two molecules of hydrohalogen, exist in two modifications, viz., the cis- and trans-forms.2) Of these the one with the lower melting point and the greatest solubility is designated the cis-form. The higher melting trans-form is the more stable and always results when the reaction mixture is allowed to become warm. In the cold, both forms usually result simultaneously. In as much as the trans-form results in most cases, the following data pertain to this modification. The dichlor-, dibrom- and diiod-hydrates of dipentene result when hydrohalogen is passed to saturation into an ethereal or glacial acetic acid solution of either limonene or dipentene. Upon evaporation of the solvent, or upon the addition of water thereto the compounds separate in the form of an oil which soon crystallizes. Since terpinene yields analogous compounds, and since mixtures frequently cause very appreciable melting point depressions, these compounds may prevent each other from crystallizing. However, a separation of the dihydrohalogen addition products of dipentene and terpinene may be obtained by taking into consideration their difference in stability toward dilute alkali.1)
1) Wallach, Liebig's Annalen 286 (1895), 138; Berl. Berichte 40 (1907), 600. 2) Baeyer, Berl. Berichte 26 (1893), 2861.
Dipentene dichlorhydrate melts at 50° and can be crystallized from its alcoholic solution by the careful addition of water; the dibromhydrate forms rhombic, shiny plates that melt at 64°; the diiodhydrate crystallizes in various forms and melts between 77 to 81°. From all of these, dipentene can be regenerated upon the withdrawal of hydrohalogen. When shaken with dilute alkali, they yield a-terpineol and terpinhydrate.
As in the case of limonene, so the nitrosochloride of dipentene exists in two physically isomeric forms (a and B), which are more soluble than their active components. When treated with alcoholic potassa, they lose hydrogen chloride and yield inactive carvoxime m. p. 93°. When treated with organic bases, the ni-trosochlorides yield two nitrolamines in each case; of these the nitrolamine bases melt at 154 and 152° respectively, the nitrol aniline bases at 125 and 149°, and the particularly characteristic a-nitrolbenzylamine base at 110°. A dipentene nitrosate, also a nitrosate of the dipentene monochlorhydrate are known.2)
For the identification of dipentene the preparation of its tetrabromide3) is commonly resorted to. It is prepared, like the limonene tetrabromide, from dipentene, or when concentrated solutions of equal parts of d- and /-limonene tetrabromide are mixed.4) The crystals differ from those of limonene tetrabromide by their appearance, the dipentene tetrabromide revealing peculiar striations in the vertical zone. They are also very fragile. Dipentene tetrabromide also differs from the limonene tetra-bromides by its lesser solubility and its higher melting point, viz., \2A to 125°. For its identification the above-mentioned nitrosochloride and the corresponding nitrolbenzylamine base can be used, also the carvoxime obtained from the nitrosochloride and finally the dichlorhydrate described above.
In the case of mixtures of limonene and dipentene, which may not occur infrequently, it should be remembered that, as a rule, the dipentene compounds separate first.
1) Wallach, Liebig's Annalen 360 (1906), 160.
2) Wallach, Liebig's Annalen 246 (1888), 258.
3) Wallach, Liebig's Annalen 227 (1885), 278.
4) Wallach, ibidem 246 (1888), 226.