This constituent, found in some Eucalyptus oils, is one of the olefine terpene aldehydes, and because of its close relationship to geraniol is sometimes named geranial; it can be obtained artificially from geraniol by careful oxidation with the ordinary chromic acid mixture. Citral is evidently formed in the oils of some species of Eucalyptus by natural oxidation of the geraniol, and this is very well shown with those of E. Australiana, E. phellandra, etc.

In the Nerrigundah, or South Eastern part of New South Wales, the oil of E. Australiana contains geraniol in the free condition, and also as an ester, but only to a small extent. In the Burraga district of the same State, about 200 miles from the first locality, some of the geraniol, common to this species, has oxidised naturally to citral, and this feature is so characteristic that the secondary lemon odour of the oil of E. Australiana from the Burraga district is usually a sufficient test by which to indicate the locality where the oil had been distilled.

Geraniol is such a common constituent in Eucalyptus oils that it might perhaps be expected that citral would often occur, but this is not so, and only very occasionally has it been found in the oils of the Eucalypts. The species, the oil of which contains citral in greatest abundance, is E. Staigeriana, a Queensland tree. (See under that species in this work.)

Citral has a distinct lemon odour, so that its presence in a Eucalyptus oil can usually be detected, and as it boils at a high temperature (228-230° C. at atmosphere pressure) it is possible to largely concentrate it in the higher boiling portions. Citral, similarly with most aldehydes, can be isolated by agitating the oil with a solution of sodium bisulphite, and after purification of the bisulphite compound by washing with ether and alcohol, the aldehyde set free from this by decomposing with sodium carbonate. The aldehyde may also be isolated in almost quantitative return by agitating with 35 per cent. neutral sodium sulphite solution, and regenerating with sodium hydrate.

For confirmation, the regenerated citral may be converted into the citryl-/3-naphthocinchoninic acid by Doebner's reaction. This is carried out in the following manner: -

20 parts pyrotartaric acid, and 20 parts of the oil in question are dissolved in absolute alcohol; to the solution 20 parts of /3-naphthy-lamine are added and the mixture boiled under a reflex for about three hours on the water bath. The crystalline acid which separates on cooling is then purified, when it will be found to melt at 197-200° C. Citral is optically inactive; boils at no-1120 under 12 millimetres pressure, and is readily soluble in alcohol.

The sample we prepared from the oil of E. Australiana had specific gravity at 150 C. = 0.894, and refractive index at 200 C. = 1.4875.