Citronellol 129

(The rhodinol of Barbier?)

Citronellol, C10H20O, was first obtained by Dodge3) by the reduction of citronellal from citronella oil. The citronellol thus prepared like the aldehyde from which it is obtained, is dextrogyrate. In volatile oils citronellol has been found repeatedly. Both optical modifications occur in the geranium oils, /-citronellol in oil of rose, and d-citronellol in citronella oil from Java and in the oil of Barosma pulchellum. Citronellol has also been found in savin oil. Probably its esters as well as the free alcohol occur in volatile oils.

1) v. Soden and Treff, Berl. Berichte 39 (1906), 907.

2) v. Soden and Treff, Chem. Ztg. 27 (1903), 897; comp. Report of Schimmel & Co. April 1904, 109.

3) Americ. chem. Journ. 11 (1889), 456.

As has been demonstrated, the alcohols described as "rho-dinol"1) and "reuniol"'-) revealed themselves as mixtures of citronellol and geraniol. The "rosed" of Markownikoff and Reformatsky,3) that constitutes the bulk of oil of rose, has likewise been found to consist of a mixture of citronellol and geraniol. However, as a result of more recent investigations, Barbier and his colaborers, more particularly Bouveault, insist that the lasvo-gyrate alcohol C10H20O which occurs in pelargonium oil and oil of rose differs from the ordinary {d-citronellol more than by being merely its lsevogyrate modification. Bouveault,4) therefore, insists on the retention of the name "rhodinol". Whether this is justified remains to be seen. The work thus far done on this subject by Hesse,5) Wallach and Naschold,6) Erdmann and Huth,7) Bertram and Gildemeister,8) Tiemann and Schmidt9) has not yet settled the question. This much, however, is demonstrated that rhodinol consists at least in part of citronellol.

According to Bouveault10) formula I should be assigned to citronellol, formula II to rhodinol. He is lead to this assumption by the different behavior of the corresponding aldehydes (comp. under citronellal).

Of the citronellol, obtained by the reduction of citronellal, Schimmel & Co.11) prepared the pyrouvic ester and the semi-carbazone and thus established the identity of the latter derivative with that obtained by Bouveault from "rhodinol". The respective melting points are 110 to 111° and 112°.

1) Eckart, Arch. der Pharm. 229 (1891), 355; Berl. Berichte 24 (1891), 4205; Barbier and Bouveault, Compt. rend. 117 (1893), 177, 1092; 118 (1894), 1154; 119 (1894), 281, 334; 122 (1896), 530, 673.

2) Hesse, Journ. f. prakt. Chem. II. 50 (1894), 472.

3) Ibidem II. 48 (1893), 293; Berl. Berichte 23 (1890), 3191; 27 (1894), Ref. 625.

4) Bull. Soc. chim. III. 23 (1900), 458.

5) Journ. f. prakt. Chem. II. 50 (1894), 472.

6) Nachr. K. Ges. Wiss. Gottingen 1896, Session of February 8.; Chem. Zentralbl. 1896, I. 809.

7) Journ. f. prakt. Chem. II. 53 (1896), 42.

8) Ibidem II. 49 (1894), 185; comp. also Report of Schimmel & Co. October 1894, 29; April 1895, 44.

9) Berl. Berichte 29 (1896), 903; 30 (1897), 33.

10) Loc. cit. and Compt. rend. 138 (1904), 1699; comp. also Barbier and Bouveault, Compt. rend. 122 (1896), 737.

11) Report of Schimmel & Co. October 1904, 119.

Recent publications by Harries and Himmelmann1) would seem to indicate that citronellol also consists of a mixture of two isomers, to which two separate formulas should be assigned. This conception may lead to an explanation of the differences in the investigations of citronellol found by Bouveault on the one hand and by Tiemann and Schmidt and Schimmel & Co. on the other hand.

Geraniol and citronellol occur frequently together. In as much as they cannot be separated by fractional distillation either as such or in the form of their esters, and since the calcium chloride method does not admit of a quantitative separation, it was difficult to obtain pure citronellol. In this Wallach2) first succeeded, who observed that when geraniol is heated with water in an autoclave to 250° it is completely decomposed with the formation of hydrocarbons, whereas citronellol remains unchanged. A method of separation suggested by Tiemann and Schmidt3) depends on the action of phosphorus trichloride on the alcohols in ethereal solution. Geraniol is thereby converted partly into hydrocarbons, partly into geranyl chloride; citronellol, however, into a chlorinated acid ester of phosphorous acid which is soluble in alkalies and can thus be separated from the other compounds. The crude citronellol regenerated by the saponification of the ester is purified by distillation with water vapor. A separation of the two can also be affected by heating the mixture of alcohols with phthalic acid anhydride to 200°. Whereas the geraniol is thus destroyed, the citronellol is converted into the acid phthalic acid ester, the sodium salt of which is soluble in water and which can be saponified by means of alcoholic potassa. According to Walbaum and Stephan4) the mixture can be heated with strong formic acid, whereby the geraniol is decomposed and the citronellol converted into the formate. According to Barbier and Bouveault5) the geraniol can also be destroyed by heating with benzoyl chloride to from 140 to 160°.

1) Berl. Berichte 41 (1908), 2187.

2) Nachr. K. Ges. Wiss. Gottingen 1896, Session of February, 8.

3) Berl. Berichte 29 (1896), 921.

4) Berl. Berichte 33 (1900), 2307.

5) Compt. rend. 122 (1896), 530.

Artificially citronellol can be obtained from citronellal, its corresponding aldehyde, by means of reduction with sodium amalgam and glacial acetic acid.1) Other methods of formation, starting with geranic acid, have been suggested by Tiemann2) and by Bouveault and Gourmand.3) The products thus obtained are optically inactive. Furthermore, by the cleavage of menthon-oxime and change in the resulting aliphatic compound, Wallach4) obtained an alcohol C10H20O which very much resembles citronellol, but is not identical with it.