Cuminal C10H12O, Aromadcndral C10H14O. Cryptal C10H16O. and Another.

One or more members of this very interesting group of aromatic aldehydes occur in the oils distilled from the members of the typical "Box" group (E. hemiphloia, etc.) and chemically related "Mallees" (E. polybractea, &c), as well as from other closely associated Eucalypts.*

These aldehydes do not seem to occur in the oils of the earlier members of the genus (the pinene yielding group), nor in those of the more recent species, particularly those in which phellandrene is the more pronounced terpene. In these latter species the characteristic constituent is the ketone piperitone.

It thus appears that they are common to a large proportion of the oils of Eucalyptus species occupying the middle portion in the evolutionary sequence of the genus (groups IV and V), and they are so shown grouped together in the evolutionary table elaborated in this work.

It may also be stated as a general rule that cymene is present in either larger or smaller amount in the oils of species containing these aldehydes, and we have isolated this hydrocarbon from many of the oils of this class, and proved its identity chemically.

When the members of this aldehydic group are pronounced, at least two and sometimes three or them occur together, particularly at certain periods of the year; at other times the cuminal is practically absent, and it is probably due to this peculiarity that the conclusions of previous investigators have not been altogether in agreement. The presence of cuminal in the oils of certain Eucalypts was noticed many years ago.

Some members of the "Box" group yield an oil in which the terpene phellandrene can be detected at certain periods of the year, although absent at other times (E. hemiphloia for instance), and this has often been observed with species which are on the border line, as it were, of phellandrene production. One peculiarity which has been noticed is that when phellandrene can be readily detected in these oils, cuminal is present in greatest amount; and it was fortunate that in the work originally carried out on these alhehydes from the oil of E. hemiphloia, for the first edition of this work, the material was distilled at the time of the year when phellandrene could not be detected, and cuminal absent or only present in very small amount, consequently the aldehyde regenerated from the crystallised sulphite compound was at that time almost entirely aromadendral.

This diminution in the amount of cuminal, and the deficiency in phellandrene, is probably responsible to a certain extent for the formation of the dihydro- and tetrahydrocuminals, which can then be isolated from these oils.

Wallach has shown [Ann. 340 (1905) 12] that the oxidation of /3-phellandrene produces a glycol, which on treating with dilute sulphuric acid gives dihydro- and tetrahydrocuminaldehydes. Molecular re-arrangement of a somewhat similar character may perhaps take place naturally under climatic conditions and the seasons' changes.

* In the first edition of this work, the optically active aldehydes peculiar to the oils of certain Eucalypts, were considered to be a single substance and named aromadendral. This name is retained in this edition to denote the occurrence of one or more members of this group of aldehydes,

The aldehyde cryptal had, up to quite recently, escaped us, because it does not form a solid compound with sodium bisulphite, and also because the liquid combination is not decomposed by sodium carbonate, so that in previous investigations it had been thrown away. As this aldehyde had eluded us for twenty years, it was decided to name it cryptal. Now that it has been found it is perhaps remarkable that it can be separated and isolated more satisfactorily than either cuminal or aromadendral, and consequently can be prepared in a state of purity.

Cuminal readily forms a solid compound when shaken with a 30 per cent, solution of sodium bisulphite, and aromadendral mostly combines in the solid form also, but cryptal only forms the liquid combination. The bisulphite compound with both cuminal and aromadendral is decomposed by sodium carbonate, but that with cryptal is not so changed, and a solution of sodium hydrate is required to set the aldehyde free. A further means of purification is that cryptal combines quantitatively with a 35 per cent. solution of neutral sodium sulphite in the cold, and in this way can be separated from the other aldehyde, which also forms a liquid bisulphite compound, and which agrees somewhat closely with the tetra-hydrocuminal isolated by Schimmel & Co. from water fennel oil, and named by them phellandral. (Die Aetherischen Oele, 2nd Edition, p. 449.)

When thus prepared the analytical figures for cryptal were in agreement with the C10Hl6O molecule, and the molecular refraction worked out almost theoretically, a result not so far obtained with the other aldehydes of this group occurring in Eucalyptus oils.

Cuminal does not contain an asymmetric carbon atom, consequently it is optically inactive. Aromadendral evidently exists in both optical rotatory modifications, although, so far, the dextro-rotatory form has not been isolated. Cryptal also shows irregular optical rotations when isolated from the oils of different species, so that probably both forms of this aldehyde also occur.

This irregularity in optical rotations is indicated from the results obtained with the oils of species in which these aldehydes are pronounced, and the crude oils and fractions of the following species show this clearly.

The oil of Eucalyptus hemiphloia, New South Wales, distilled in September, gave the following results: -

Corresponding results have also been obtained with the oils of other Eucalypts.

The crude oil of E. cneorifolia, one of the "Mallees" growing in South Australia, appears to be always lævo-rotatory when prepared by ordinary methods of distillation. This optical activity is also due to the presence of members of this aldehyclic group.