389 



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-^-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 dis- 

 solved 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 110-112 under 12 millimetres pressure, 

 and is readily soluble in alcohol. 



The sample we prepared from the oil of E. Australiana had specific 

 gravity at 15 C. = 0-894, and refractive index at 20 C. = 1-4875. 



Citronellal, 



THIS characteristic aldehyde is also one of the olefine terpene series. It occurs 

 most abundant^ 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 C JO H I8 O 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 225 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 205-208 under 

 atmospheric pressure, and at 89 to 91 at 14 millimetres; has specific gravity 

 at 15 C. = 0-8556; and refractive index at 20 : 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. 



