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The second aliphatic terpene aldehyde with ten carbon atoms that is found in volatile oils is the citronellal, C10H18O. It occurs occasionally as companion to citral, of which it is the dihydro-derivative. Differing from citral, citronellal is optically active. However, with but one exception, it has been found in the dextrogyrate modification only. It is probable that whenever citronellal is found with a low angle of rotation it is a mixture of both active modifications.

1) Tiemann, Berl. Berichte 33 (1900), 880.

2) K. Wedemeyer, Ober Kondensationen mittelst aromatischer Basen usw. Inaug. Dissert Heidelberg 1897. p. 24.

3) Berl. Berichte 31 (1898), 822.

(/-Citronellal has been found in citronella oil, in the oils of Barosma pulchellum, Eucalyptus maculata var. citriodora, E. dealbata, and of balm; /-citronellal in "Java lemon olie". With certainty the presence of citronellal has been ascertained in the oil from the bark of Tetranthera polyantha var. citrata, but its angle of rotation has not been recorded. Its presence in oil of lemon and in mandarin oil is still doubtful.

In as much as citronellal can be separated by means of its crystalline bisulphite compound, its isolation from oils containing a large percentage of aldehyde, such as citronella oil and oil of Eucalyptus maculata, offers no difficulties. Since citronellal is very susceptible towards acids as well as toward alkalies, alkali carbonate is used to decompose the bisulphite compound. Artificially citronellal has been obtained upon oxidation of the primary alcohol citronellol C19H20O; however, the yield is even smaller than when geraniol is oxidized to citral. In this manner the /-citronellol from rose oil is converted into /-citronellal, which has been designated rhodinal by Bouveault.

Upon the ring cleavage of menthoxime to aliphatic compounds Wallach obtained an aldehyde C10H18O which he designated "Menthocitronellal", and which closely resembles the natural citronellal, but is not identical with the latter.1)

According to Tiemann and Schmidt2) citronellal boils at 205 to 208° under atmospheric pressure and at 103 to 105° under a pressure of 25 mm.; d17.5o 0,8538; nD 1,4481; mOl. refr. 48,29, computed for C10H18O/= 47,92.

For a citronellal regenerated from a recrystallized sodium acid sulphite double compound, Tiemann3) records the following constants:

B.p. 203 to 204°; 89 to 91° (14 mm.); d17.50,8554; nD1,4461; mol. refr. 48,00.

1) Liebig's Annalen 278 (1894), 317; 296 (1897), 131. 2) Bed. Berichte 29 (1896), 905. 3) Berl. Berichte 32 (1899), 818.

As angle of rotation, Kremers1) recorded [a]D + 8,18°. Later [a]D+ 12°30' was observed for a preparation purified by means of its acid sulphite derivative.-)

In the laboratory of Schimmel & Co. the following constants were observed in connection with products obtained on a commercial scale:

B. p. 205 to 208° (ord. pressure); 12 to 73° (4,5 mm.); d16o0,855 to 0,860; aD + 10 to +11°; nD20o1,444 to 1,449; soluble in 5 to 6 vols, of 70 p. c. alcohol.

Citronellal isolated from "Java lemon olie" revealed the following properties:

B. p. 205 to 208°; d15o0,8567; aD - 3°; nD20o1,44791.3)

Citronellal is an unsaturated aldehyde with one double bond. When reduced in alcoholic solution, that is kept slightly acid by the addition of acetic acid, with sodium amalgam it is converted into the primary alcohol citronellol, C10H20O.4) Like citral, citronellal is very susceptible toward alkalies and acids. Whereas citral is broken up into acetaldehyde and methyl-heptenone when treated with alkali, citronellal resinifies. In contact with acids citral loses the elements of a molecule of water and yields cymene; whereas citronellal is converted into an isomeric compound, the isopulegol5) C10H18O (comp. p. 386). This alcohol, which is likewise isomeric with pulegol, is obtained upon reduction of pulegone, and can be oxidized to the ketone Isopulegone, C10H16O, which can be inverted to natural pulegone. According to Harries and Roeder,6) the semicarbazones of iso-pulegone melt at 173° (a-modification) and 183° (/i-modification). This ring-formation of citronellal to Isopulegone takes place very readily, so that commercial citronellal, at least when purified through the bisulphite compound, always contains isopulegol.7)

1) Americ. chem. Journ. 14 (1892), 203.

2) Tiemann and Schmidt, loc. cit.

3) Report of Schimmel & Co. April 1903, 22.

4) Tiemann and Schmidt, Berl. Berichte 29 (1896), 906.

5) Tiemann and Schmidt, Berl. Berichte 29 (1896), 913; 30 (1897), 22. 6) Berl. Berichte 32 (1899), 3367.

7) Tiemann, Berl. Berichte 32 (1899), 825; Labbe, Bull. Soc. chim. HI. 21 (1899), 1023.

The change to isopulegol can also be utilized for the quantitative determination of citronellal. For this purpose compare the chapter "The examination of volatile oils".

Toward sodium acid sulphite, citronellal behaves similar to citral. In addition to the crystalline normal double compound with one molecule of NaHS03, in which the bisulphite has been added to the aldehyde group, it also yields hydrosulphonic acid derivatives with one or two molecules of NaHS03. In these latter compounds the sodium acid sulphite molecule is added to the double linkage between carbon and carbon. However, if two molecules are added, one must necessarily add to the aldehyde group, there being but one double linkage between carbon and carbon.1) It is from the first-mentioned compound only that citronellal can be regenerated, whereas it cannot be regenerated from the hydrosulphonic acid derivatives by either sodium carbonate or alkali hydrate. With neutral sulphite, citronellal likewise reacts with the formation of a non-decomposable hydro-sulphonic acid derivative. However, the reaction then only takes place when a strong current of carbon dioxide is passed into the mixture or if a sufficient amount of some other acid is added. This behavior of citronellal can be utilized for its separation from citral, which combines with neutral sulphite without addition of acid. Care must be taken, however, to neutralize the sodium hydroxide as it is liberated.