The occurrence of this terpene in the oils of certain Eucalypts was first announced by one of us, in a paper read before the Royal Society of New South Wales in December, 1918. It had been isolated from the oil of E. megacarpa, a West Australian species, forwarded to the Technological Museum for investigation by Mr. C. E. Lane-Poole, the Conservator of Forests for that State.

The oil of this Eucalypt consists principally of terpenes, and in addition to the terpinene, pinene and limonene were both detected, and their characteristic chemical combinations prepared.

Limonene rarely occurs in Eucalyptus oils, and it may be that terpinene will be found more frequently associated with that terpene in the oils of species growing in the northern and north-western portion of Australia, and at present uninvestigated. The formation of terpinene from terpineol through the terpene terpinolene is thus of special interest, because dipentene can also be formed from terpineol, and it is now known that this alcohol is a common constituent in the oils of many Eucalyptus species.

The pinene and limonene in the oil of E. megacarpa were both laevo-rotatory, and it is possible that dipentene was present also. The cineol content was 30 per cent., and from the aggregate of results it appears that about 10 per cent. of terpinene was present also.

Terpinene can be formed artifically by the action of alcoholic sulphuric acid on many of the terpenes and terpene derivatives, as well as from terpineol, geraniol, dihydrocarveol, and cineol. This formation is thus interesting in this connection, as besides cineol, pinene, and limonene, geraniol was also present as an ester in the oil of E. megacarpa.

Terpinene was first recognised as a sepaiate terpene by Wallach [Ann. (230) 254], but has so far been detected in only a very few essential oils, such as those of Marjoram, Dill, and Coriander. It has been suggested that in these cases its presence may have been brought about by the influence of heat during distillation, but this opinion cannot now be supported in so far as it relates to Eucalyptus oils.

Terpinene has not, so far, been regenerated from its solid derivative in a pure form, so that its physical properties cannot be definitely stated, but from the results of various observations the following may be given: - Boiling point, 171-1810 C.; specific gravity, 0.842 at 220 C, to 0.848° at 18°; refractive index, 1.4719 to 1.4789.

The two reactions which serve to distinguish terpinene from other terpenes are: (a) the crystallised nitrosite, C10HI6N2O3, melting at 155o C, which is formed when the oil is treated with nitrous acid; and (b) the comparative ease with which it is destroyed when treated in the cold with Beckmann's chromic acid mixture; this reagent under such conditions has little action upon either pinene, limonene, or cineol.

(a) The Formation Of The Terpinene Nitrosite

A portion of the oil of E. mcgacarpa boiling between 170-190° C. was mixed with an equal quantity of petroleum ether and an equal bulk of a saturated solution of sodium nitrite added, and afterwards sufficient glacial acetic acid to decompose the salt. The whole was then allowed to stand on one side. After twenty-four to thirty hours, crystals commenced to form, and increased considerably in amount after two days. The crystals were separated, pressed on porous plate to absorb the liquid products, and purified from a mixture of alcohol and chloroform. The terpinene nitrosite thus formed melted at 155° C. (corr.), and answered to the more easily applied chemical reactions for this substance. Terpinene nitrosite, which is a much more stable substance than the corresponding compound formed with phellandrene, has had the two following formulae suggested for it, but no decision as to which is correct has yet been arrived at.

(b) Treatment With Beckmann's Reagent

A portion of the fraction boiling between 170-190° was repeatedly treated in the cold with small quantities of Beckmann's chromic acid mixture, until the formation of the brown precipitate was less noticeable. The optical rotation of the fraction before treatment was 35.6° to the left, due principally to the lsvo-rotatory limonene.

After the first treatment, the optical rotation had increased to 36.4°, and after further action to 36.6°. The substance removed was evidently the inactive terpinene, the treatment apparently having little action on the activity of the limonene.

E. megacarpa, in the sequence of species, approaches more nearly the earlier members of the genus, and this is also indicated by the principal chemical constituent in the oil being pinene. The chief terpene in the oils of the more recent species of the genus is phellandrene, so that a peculiarity is noticedhere, in that terpinene is found at that end of the genus more largely represented in Western Australia, while the other closely agreeing terpene (phellandrene) is a characteristic constituent in the oils of certain Eucalypts which occur more abundantly in the eastern and south-eastern portion of the Continent. (See also the map in this connection, Plate 11.)