This ketone, which has thus far not been found in volatile oils, may receive brief mention at least. As described on p. 446, it can be prepared by way of the oxime from pulegone. It can also be obtained by treating pulegone hydrobromide with basic lead nitrate.2) Furthermore from citronellal, by converting this into Isopulegol (seep. 416) and the subsequent oxidation of this alcohol to Isopulegone.1)
1) Berl. Berichte 25 (1892), 3515.
2) Harries and Roeder, Berl. Berichte 32 (1899), 3368.
Whereas the first two methods mentioned yield a single, active product, the change from citronellal to Isopulegone results in a mixture of active and inactive ketone, the ratio of which varies according to the conditions of the experiment.2) The active Isopulegone yields a likewise active oxime which melts at 120 to 121° (respectively at 123 to 124°. See p. 446). The inactive ketone yields an inactive oxime which melts, according to Wallach, at. 138 to 139°.3) According to Wallach, the separation of the two oximes is best accomplished by repeated recrystallization from methyl alcohol, in which the lower-melting oxime is more readily soluble. However, it is difficult to free the latter entirely from the higher-melting oxime, hence the poorly marked melting points.
The active Isopulegone yields a semicarbazone which melts at 172 to 174° and which is readily soluble in ether, the inactive ketone a semicarbazone which melts at 182 to 183° and which is difficultly soluble in ether.
For an isopulegone obtained from d-citronellal, Tiemann and Schmidt4) record the following constants:
B. p. 90° (12 mm.); d17.5o 0,9213; aD + 10°15'; nD 1,4690.
For an Isopulegone obtained from pulegone hydrobromide, Harries and Roeder5) observed the following constants:
B. p. 98 to 100° (13 mm.); d19.5o0,9192; aD - 7°8'.
For its characterization the above-mentioned derivatives can be used.
1) Tiemann and Schmidt, Berl. Berichte 30 (1897), 22; Tiemann, ibidem 32 (1899), 825.
2) Wallach, Liebig's Annalen 365 (1909), 253. Harries and Roeder assumed the existence of two stereoisomeric ketones, viz., a- and B-isopulegone. Berl. Berichte 32 (1899), 3362. The former is identical with the active, the latter with the inactive /'sopulegone.
3) Harries and Roeder as well as Semmler found 143°. Wallach (loc. cit.) explains this discrepancy by assuming that these investigators had an inactive oxime totally free from the active modification.
4) Berl. Berichte 30 (1897), 28. 5) loc. cit. 3371.