This characteristic aldehyde is also one of the olefine terpene series. It occurs most abundantly in the oil of Eucalyptus citriodora, in fact at most times of the year it is found to contain over 90 per cent. of citronellal. The oil of this species has the distinctive odour of that aldehyde, and in this respect is quite different from any other known Eucalyptus oil. Citronellal is a very useful substance for perfumery purposes, particularly as it can be reduced to the corresponding alcohol citronellol, which substance has even a more pronounced rose odour than has geraniol. Citronellal has the formula C10Hl8O and contains one double bond.

It combines with sodium bisulphite, from which it is separated by alkali carbonates, and in this way citronellal may be prepared in a pure condition; it is, however, very sensitive towards both alkalis and acids.

Its identity may be determined by the formation of the semicarbazone, melting at 84° C.; as well as by the preparation of the citronellyl-/3-naphthocin-choninic acid by Doebner's reaction. When finally crystallised from alcohol this substance melts at 2250 C.

Citronellal differs from citral in being optically active, although the product from E. citriodora usually shows but slight activity, and probably consists of a mixture of both optically active modifications. It boils at 205-2080 under atmospheric pressure, and at 89 to 910 at 14 millimetres; has specific gravity at 150 C. = 0.8556; and refractive index at 200 = 1.4481.

Citronellal in Eucalyptus oils may be quantitatively determined by absorption in a 30 per cent. commercial sodium bisulphite solution, using a flask with a narrow graduated neck; 5 c.c. of the oil may be taken, agitated with the bisulphite solution, and then heated on the water bath with frequent shaking, until absorption is complete. The unabsorbed portion is then lifted into the neck, read off, and subtracted from the amount taken, which multiplied by 20 gives the percentage of citronellal in the oil.