This unsaturated aromatic ketone is more generally distributed in the oils of Eucalypts growing in the Eastern and South Eastern portion of the continent, and in Tasmania, where the members of the whole group are known vernacularly as "Peppermints."

This peppermint odour was noticed by the first white settlers in Australia in 1788, and oil was distilled by them at that time, from the leaves of Eucalyptus piperita, a tree somewhat extensively distributed around Sydney, the coastal ranges, and coastal districts of New South Wales and Victoria. It is thus worthy of notice that both the vernacular name ("Peppermint") and the scientific name (Eucalyptus piperita) were given to this tree on account of the presence of this peppermint constituent in the oil; in fact, chemical constituents were often a guiding factor in the early naming of Eucalypts.

On the mountain ranges in New South Wales and Victoria, as well as in Tasmania, the members of the "Peppermint" group of Eucalypts form a large proportion of the natural vegetation, and are distributed over hundreds of square miles of country.

It is now recognised that all the principal constituents found in Eucalyptus oils increase in amount through a range of species until a maximum is reached in one or more of them. Piperitone follows this rule, and Eucalyptus dives, a "Broad-leaved Peppermint," appears to be the species in which it reaches a maximum content. This Eucalypt is one of the most plentiful of all the members of the "Peppermint" group, and also gives a good yield of oil, averaging from 3 to 4 per cent. according to the time of year, the care taken in collecting, and on the duration of the primary distillation, and if this be somewhat extended, say from 6 to 8 hours, the oil will often contain as much as from 40 to 50 per cent. of piperitone. It is thus evident that this ketone could be produced in great quantity, and at a cheap rate. The remainder of the oil of E. dives consists largely of phellandrene, and is employed in the mining industry for flotation work, as well as for other economic purposes.

Piperitone is the only ketone found in Eucalyptus oils; it was first isolated by one of us, and the results of a preliminary chemical investigation submitted to the Royal Society of New South Wales, in October, 1900. It was named piperitone by us in the first edition of this work.

Piperitone appears to occur only in the oils of species occupying the more recent end of the genus, and is not found in the oil of any member of the groups occupying the anterior position in the evolutionary sequence of the genus.

Although piperitone is usually found occurring with phellandrene, yet, in the case of E. apiculata, this terpene was not detected; and altogether the oil of this species was of such a character as not to be readily placed in any of the well defined groups.

In the oils of most species piperitone is found associated with the corresponding lævo-rotatory secondary alcohol piperitol, and we have isolated this alcohol from the oil of E. radiata, where it occurs as a well-defined constituent.

Under natural conditions piperitone is lævo-rotatory, but readily forms the racemic modification when heated above its boiling point, in the process of separation by direct distillation of the original oil. This alteration appears to be brought about largely by the influence of the acid from the esters, under the conditions stated, because the pure ketone undergoes little alteration when distilled directly, but if geranyl-acetate be first added the alteration in rotation is considerable.

It was due to this tendency to racemisation that in the original paper, the ketone was shown to be inactive, as it had been first obtained in the crude condition by direct distillation of the original oil. If, however, the first separation be carried out under greatly reduced pressure, the lævo-rotatory form can be separated and prepared in a pure condition by the aid of sodium bisulphite. This compound of piperitone is very soluble in aqueous solution, but after the lapse of several days the saturated liquid forms a crystalline mass from which pure piperitone can be recovered, although there is often a considerable reduction in rotation. Piperitone can also be removed by the aid of the neutral sodium sulphite.

Piperitone has considerable prospective economic value, in that it forms thymol by treatment with ferric chloride; inactive menthone by reduction, when a nickel calalyst is employed; and inactive menthol by the reduction of the menthone with sodium in an aqueous ether solution. (These reactions were first shown by H. G. Smith and A. R. Penfold in a paper read before the Royal Society of New South Wales, June, 1920.)

As piperitone can be obtained in very large quantity, the production of one or more of these substances from this ketone, on a manufacturing scale, seems most promising, and should be profitable when the best conditions for commercial purposes shall have been worked out.

Piperitone being an unsaturated ketone with one double bond, it follows, from the above, that it must be a menthenone with the carbonyl group in the 3 position. This is proved by the formation of thymol and menthone. When piperitone is oxidised in the cold with potassium permanganate in alkaline solution, iso-butyric acid is one of the products of oxidation, and it does not seem possible to entirely prevent the formation of this acid at any stage of the process, even when the temperature is kept down by the addition of ice, and the theoretical amount of permanganate employed. From the ready formation of iso-butyric acid it might be supposed that the double bond was in the 4 position, but so far the corresponding /3-methylglutaric acid has not been prepared.

Wallach and Meister [Ann. 362 (1908) 261] have, however, shown that Δ - 4-menthenone - 3 boiled at 212-2130 C, but Auwers [Ber. 41 (1908) 1801, and 42 (1909) 2408] doubts the correctness of this statement, although Wallach (Chem. Lentr., 1912, 11, 922-923) confirmed his original determination. The question whether piperitone is Δ - 4-menthenOne - 3, or Δ - 1-menthenone - 3, remains at present unsettled, and the formation of menthone and menthol does not assist in deciding the position of the double bond.

Piperitone has a peppermint odour and taste, resembling pulegone very closely in these respects. It is colourless when pure, but becomes yellowish with age. It reduces an alkaline solution of silver nitrate and reproduces the red colour with Sctoffis reagent.