Pulegone, a ketone C10H16O, has thus far been found in labiate oils only. In these it frequently occurs in large amounts, occasionally together with menthol and menthone. European pennyroyal oil contains about 80 p. c. of this ketone. Diptam origanum oil likewise consists largely of pulegone. Its presence in the following oils has also been determined, viz., in the oils of Hedeoma pulegioides, Pycnanthemum lanceolatum, Bystro-pogon origanifolius and Calamintha Nepeta. In all of these oils it occurs in the dextrogyrate form. Its presence has further been ascertained in Canadian mint oil, in Japanese peppermint oil, and in marjoran oil. In these cases, however, references to the rotation are wanting.
1) Arch, der Pharm. 221 (1883), 285.
2) Claus and Fahrion, Journ. f. prakt. Chem. II. 39 (1889), 365; Report of Schimmel & Co. April 1898, 47, footnote.
In as much as European pennyroyal oil consists chiefly of this ketone, pulegone can be obtained relatively pure by fractional distillation of this oil. In a purer state it can be separated from its compound with sodium acid sulphite or with neutral sodium sulphite. To cause the pulegone to combine with acid sulphite, the oil diluted with 1/4 its volume of alcohol is subjected to prolonged shaking with the aqueous sulphite solution.1) Its capacity to react with normal sulphite, also makes possible the quantitative estimation of pulegone. (See the chapter "The examination of volatile oils" under determination of ketones.) Furthermore, the semicarbazone, which can be hydrolyzed with acids, can be used for the purification of pulegone.
In a round-about way, pulegone has been obtained synthetically from citronellal.2) Another ketone, which is not identical with natural pulegone, Wallach3) obtained by condensation of 1,3-methylcyc/ohexanone with acetone.
Pulegone is a colorless liquid which turns slightly yellow upon prolonged standing. It possesses a peppermint-like, sweetish odor reminding of menthone.
For pulegone purified only by means of distillation, Beckmann and Pleissner4) found the following constants:
B. p. 130 to 131° (60 mm.); d20o0,9323; [a]D + 22,89°; nD 1,47018.
Barbier5) records the following:
B. p. 222 to 223°; d23o 0,9293; [a]D + 25°15'.
For pulegone regenerated from the bisulphite addition product, Baeyer and Henrich6) observed the following constants:
B. p. 100 to 101° (15 mm.); [a]D + 22,94°; and Wallach:7)
B. p. 221 to 222°; d 0,936; nD 1,4846.
1) Baeyer, Berl. Berichte 28 (1895), 652.
2) Tiemann and Schmidt, Berl. Berichte 29 (1896), 913; 30 (1897), 22.
3) Berl. Berichte 29 (1896), 1597, 2955; Liebig's Annalen 300 (1898), 267. 4) Liebig's Annalen 262 (1891), 3, 4, 20. 5) Compt. rend. 114 (1892), 126.
6) Berl. Berichte 28 (1895), 653.
7) Ibidem 1965.
In connection with technical products prepared in the laboratory of Schimmel & Co., the following constants were observed: d15o 0,939 to 0,941; aD + 20 to +23°; nD20o1,484 to 1,488; soluble in 4,5 vol. of 60 p. c. alcohol and in 1,5 vol. of 70 p. c. alcohol.
For a pure pulegone regenerated from its sulphite compound the following constants were observed:
B. p. 224° (750 mm.); 93 to 94° (8 to 9 mm.); 85° (5 mm.); d15o0,9405; aD + 20°48'; nD20o 1,48796.
As an unsaturated compound pulegone combines with bromine but yields a liquid dibromide. When heated with sodium ethylate, this yields pulegenic acid, C10H16O2, from which a long series of derivatives has been obtained and which has served as the starting-point for extensive investigations. With hydrogen chloride and hydrogen bromide, pulegone yields crystalline addition products.1)
Pulegone reveals the character of a ketone. When carefully reduced in alcoholic solution with metallic sodium, pulegol,2) C10H18O, the corresponding alcohol is presumably first formed. Further addition of hydrogen reduces it to the saturated /-men-thol, C10H20O.3)
When pulegone is heated with anhydrous formic acid or with alkali or with water in an autoclave to 250° it is hydrolyzed yielding acetone and 1,3-methylcyc/ohexanone, C7H120.4)
Pulegone reacts with hydroxylamine, but the oxime of the normal ketone is not yet known. What was formerly regarded as such, is isopulegone oxime, since Wallach5) has shown that in the presence of alkali and hydroxylamine pulegone undergoes isomerization to (active) isopulegone. At the same time the pulegone undergoes partial hydrolysis as indicated above, so that the yield of oxime leaves much to be desired.
1) Beckmann and Pleissner, Liebig's Annalen 262 (1891), 21; Baeyer and Henrich, Berl. Berichte 28 (1895), 653.
2) Tiemann and Schmidt, Berl. Berichte 29 (1896), 914.
3) Beckmann and Pleissner, loc. cit. 30.
4) Wallach, Liebig's 289 (1896), 338; 365 (1909), 243. 5) Liebig's Annalen 365 (1909), 240.
Wallach1) obtained the best results by observing the following directions:
To a solution of 10 g. of pulegone in 30 ccm. of absolute alcohol a solution of 30 g. solid potassa in 20 ccm. of water are added and then as rapidly as possible a solution of 1 g. of hydroxyl-amine hydrochloride in 1 ccm. of water. Care should be taken that the temperature of the reaction mixture does not exceed 75°. Finally, the mixture is heated for 10 minutes to 85° on a water bath. When cooled, the mass is poured on ice and allowed to stand. The oxime, which usually separates in large flakes, is shaken out with ether, the ether recovered by distillation and the residue subjected to steam distillation. The oxime passes over with the steam and separates in the form of fine needles which can be collected on an asbestos filter and recrystallized from ether or ligroin, when they melt at 120 to 121°. By repeated recrystallization from methyl alcohol, Wallach succeeded in raising the melting point to 123 to 124°. The oxime is lasvogyrate, [a]D22o - 25,833°. When hydrolyzed by heating with oxalic acid, the oxime yields Isopulegone. With sulphuric acid, however, pulegone is regenerated, since the sulphuric acid isomerizes the Isopulegone back to pulegone.
In addition to this oxime two addition reaction products of pulegone with hydroxylamine are known: first, a compound C10H16ONH2OH2) which results from the addition of hydroxyl-amine to the pulegone and which melts at 155 to 157°; and secondly, a dioxime or rather hydroxamino oxime, C10H16NOH NH2OH, which melts at 118°.3)
The semicarbazone which results from the action of semi-carbazide on pulegone melts at 167,5 to 168°.4) It is difficultly soluble in ether.
According to Baeyer and Henrich,5) the identification, however, can be brought about more readily by means of the characteristic bisnitrosopulegone. For its preparation a very small amount of hydrochloric acid is added to a solution of 2 ccm. of pulegone or pulegone-containing oil in 2 ccm. of ligroin and 1 ccm. of amyl nitrite which has been cooled down in a good freezing mixture. After a short time the bisnitroso compound separates in the form of fine needles which can be purified by drying them on porous plates and washing them with petroleum ether, when they melt at 81,5°. When recrystallized they decompose. Isopulegone does not give this reaction.
1) Liebig's Annalen 365 (1909), 244.
2) Beckmann and Pleissner, Liebig's Annalen 262 (1891), 6; Wallach, ibidem 365 (1909), 246.
3) Semmler, Berl. Berichte 38 (1905), 146.
4) Wallach, Liebig's Annalen 365 (1909), 246.
5) Berl. Berichte 28 (1895), 654; comp. Baeyer and Prentice, Berl. Berichte, 29 (1896), 1078; Gage, Pharm. Review 16 (1898), 413.
When oxidized with potassium permanganate, pulegone yields acetone and active methyladipic acid, C7H10O4 melting at 84 to 85°. Because of these results, Semmler1) has assigned to pule-gone the above formula which also seems to explain the hydrolysis of this ketone into acetone and methyl hexanone.