Until very recently our knowledge of the occurrence of te-pinene was very limited. Besides the dihalogen addition product, also yielded by other terpenes, the addition product of nitrous acid was the only characteristic derivative. The designation ter-pinene heretofore applied to a hydrocarbon which yielded, with nitrous acid, a nitrosite melting at 155°. The artificially prepared hydrocarbon is a mixture of two isomers, designated a- and y-terpinene by Wallach, the composition of which varies with the method of preparation.
The hydrocarbon was first found by Weber1) in the oil of the long Ceylon cardamom. With the aid of the nitrosite m. p. 155°, its presence was likewise established in the oil of Levant wormseed, in Manila elemi oil, in the oil of dill herb, and in marjoran oil. The presence of both constituents, found by Wallach in what is designated artificial terpinene, in volatile oils was first revealed by the investigations in the laboratory of Schimmel & Co.2) Thus a-terpinene was found in coriander oil, y-terpinene in ajowan oil, lemon oil, and coriander oil.
Artificially, terpinene is obtained by the action of acids upon such hydrocarbons as pinene, dipentene, and phellandrene; or upon such oxygenated compounds as linalool, geraniol, terpineol, terpinenol, terpinene terpin, terpinhydrate, dihydrocarveol, and cineol; further by splitting off hydrohalogen from the terpinene halogenides; and by special decomposition of carvylamine, car-venone, and methyl-dichlormethyl-ketodihydrobenzene. Especially adapted to the preparation of products rich in a-terpinene are the syntheses of Semmler1) and Auwers.2) Pure products, however, are not obtained; hence the constants are but approximations. For a terpinene obtained by the action of methylmagnesium iodide on sabinaketone and purified by repeated distillation over sodium, Wallach3) records the following constants:
1) Liebig's Annalen 238 (1887), 107.
2) Gildemeister and Miiller, Wallach-Festschrift, Gottingen 1909, p. 443.
B. p. 174 to 179°; d22o 0,842; nD 1,4719; and for a preparation regenerated from its hydrochloride by heating with aniline:4)
B. p. 179 to 181°; d20o 0,846; nD 1,4789.
In connection with a fraction from coriander oil consisting of a- and y-terpinene, Schimmel & Co. observed the following constants:
B. p. 177 to 178°; d15o 0,8485; aD + 0°32'; nD20o1,47650.
Terpinene bears a great resemblence to dipentene. When acted upon by the hydrohalogens, terpinene halogenhydrides result which might easily be confounded with those of dipentene. Both cis- and trans-modifications exist. However, the latter only are concerned so far as identification is meant, for the former are liquid.
Terpinene dihydrochloride melts at 51 to 52°, the dihydro-bromide at 58 to 59°, the dihydroiodide at 76°. From the corresponding isomeric dipentene compounds they can be distinguished by the decided depression of the melting point of their respective mixtures. When the hydrochloride is boiled with bases, a mixture of several hydrocarbons results. When shaken with dilute potassium hydroxide solution, terpinene terpin (m. p. 137 to 138°), terpinenol-4, and y-terpinenol result, also cis- and trans-terpin.
1) Berl. Berichte 41 (1908), 4474; 42 (1909), 522.
2) Ibidem 42 (1909), 2424.
3) Liebig's Annalen 362 (1908), 301.
4) Liebig's Annalen 350 (1906), 149.
The addition of bromine to terpinene yields liquid compounds only. Neither could a nitrosochloride be obtained. Upon addition of nitrous acid, terpinene nitrosite melting at 155° results, which yields nitrolamines when acted upon with piper-idine and nitrolamines. The nitrolpiperidine base melts at 153 to 154°, the nitrolbenzylamine base at 137°. Careful reduction of the terpinene nitrosite and its nitrolamines with zinc dust and glacial acetic acid yields carvenone. The use of more energetic reducing agents, such as sodium and alcohol, is apt to yield tetrahydrocarvone and tetrahydrocarvylamine. According to Wallach the nitrosite is a derivative of a-terpinene, which, upon oxidation with permanganate yields i-a, a'-di-hydroxy-a-methyl-a'-isopropyladipic acid melting at 188 to 189°, and the lactone of which melts at 72 to 73°. Further oxidation of this acid yields dimethylacetonylacetone, the semi-carbazide of which melts at 201 to 202°, and the dioxime at 137°.
For the identification of terpinene in volatile oils, fraction 175 to 185° is examined. Some clue may be had from the formation of terpinene dihydrochloride, m. p. 52°, by passing hydrogen chloride gas into a glacial acetic acid solution of the hydrocarbon. Thujene and sabinene, also terpinenol and terpinene terpin, which also yield the same chloride, boil at either higher or lower temperatures. Particularly suited for identification is the terpinene nitrosite, which is prepared in the following manner: To 3 ccm. of the hydrocarbon fraction to be examined, in a narrow test tube, l1/2 ccm. glacial acetic acid and 4 1/2. ccm. of water are added. To the well cooled mixture a concentrated aqueous solution of 1,5 g. of sodium nitrite is added, a small portion at a time, causing the hydrocarbon to assume a green color. When the nitrous acid has been absorbed completely, a reddish-yellow color results. In order to avoid supersaturation, the reaction mixture is inoculated with a crystal of pure nitrosite. The crystals which separate in the course of time are washed with water and petroleum ether and crystallized from alcohol (m. p. 155°).
The presence of one or the other modifications of terpinene can be established positively only by oxidation with permanganate. Wallach1) proceeds in the following manner: A mixture of 7 g. of the hydrocarbon, 33 g. of potassium permanganate, 14 g. potassium hydroxide, 400 g. ice and 400 ccm. water, contained in a copper flask, is shaken with a shaking machine for an hour. The excess of hydrocarbon is distilled off, the oxide of manganese separated by filtration, and the filtrate saturated with carbon dioxide while it is being evaporated to dryness. The residue is extracted with alcohol, the alcoholic filtrate evaporated to dryness, this residue dissolved with a small amount of hot water and the solution set aside for crystallization. The magma of cristals is separated, washed with a little cold water, dried on porous plate and re-crystallized from 15 to 20 times its weight of 25 p. c. alcohol. The melting point of this oxidation product, the erythritol mentioned above, lies at 235 to 236°, or if heated rapidly at 237 to 238°. It is sparingly soluble in ether, ligroin, ethylacetate, or chloroform; not readily in cold alcohol or water, but more readily in these solvents when hot. The mother liquid from the erythritol is shaken out with ethylacetate and then supersaturated with sulphuric acid in the cold. This acid liquid is shaken out once with ether and then exhausted by shaking with ethyl acetate. The ethyl acetate takes up an acid which crystallizes out upon proper concentration of the solvent. Recrystallized from about six times its own weight of water, a,a'-dihydroxy-a-methyl-a'-Isopropyladipic acid melts at 189° with the elimination of water.
Even in the cold, terpinene is readily attacked and destroyed by the chromic acid mixture prepared according to Beckmann's directions.2) This property can be made use of when terpinene is to be removed from mixtures containing pinene, camphene, limonene, terpinolene, cineol, and pinol since these substances are stable toward this oxidizing agent in the cold.
1) Liebig's Annalen 362 (1908), 297; comp. also Gildemeister and Miiller, Wallach-Festschrift, Gottingen 1909, p. 443.
2) Berl. Berichte 27 (1894), 815.