39* 



brought about largely by the influence of the acid from the esters, under the 

 conditions stated, because the pure ketone undergoes little alteration when 

 distilled directly, but if geranyl-acetate be first added the alteration in rotation 

 is considerable. 



It was due to this tendency to racemisation that in the original paper, the 

 ketone was shown to be inactive, as it had been first obtained in the crude 

 condition by direct distillation of the original oil. If, however, the first 

 separation be carried out under greatly reduced pressure, the laevo-rotatory 

 form can be separated and prepared in a pure condition by the aid of sodium 

 bisulphite. This compound of piperitone is very soluble in aqueous solution, but 

 after the lapse of several days the saturated liquid forms a crystalline mass from 

 which pure piperitone can be recovered, although there is often a considerable 

 reduction in rotation. Piperitone can also be removed by the aid of the neutral 

 sodium sulphite. 



Piperitone has considerable prospective economic value, in that it forms 

 thymol by treatment with ferric chloride ; inactive menthone by reduction, 

 when a nickel calalyst is employed ; and inactive menthol by the reduction of 

 the menthone with sodium in an aqueous ether solution. (These reactions were 

 first shown by H. G. Smith and A. R. Penfold in a paper read before the Royal 

 Society of New South Wales, June, 1920.) 



As piperitone can be obtained in very large quantity, the production of 

 one or more of these substances from this ketone, on a manufacturing scale, 

 seems most promising, and should be profitable when the best conditions for 

 commercial purposes shall have been worked out. 



Piperitone being an unsaturated ketone with one double bond, it follows, 

 from the above, that it must be a menthenone with the carbonyl group in the 

 3 position. This is proved by the formation of thymol and menthone. When 

 piperitone is oxidised in the cold with potassium permanganate in alkaline 

 solution, iso-butyric acid is one of the products of oxidation, and it does not 

 seem possible to entirely prevent the formation of this acid at any stage of the 

 process, even when the temperature is kept down by the addition of ice, and the 

 theoretical amount of permanganate employed. From the ready formation of 

 iso-butyric acid it might be supposed that the double bond was in the 4 position, 

 but so far the corresponding /3-methylghitaric acid has not been prepared. 



Wallach and Meister [Ann. 362 (1908) 261] have, however, shown that 

 4 4-menthenone 3 boiled at 212-213 C., but Auwers [Ber. 41 (1908) 1801, 

 and 42 (1909) 2408] doubts the correctness of this statement, although Wallach 

 (Chem. Lentr., 1912, n, 922-923) confirmed his original determination. The 

 question whether piperitone is A 4-menthenOne 3, or 4 i-menthenone 3, 

 remains at present unsettled, and the formation of menthone and menthol does 

 not assist in deciding the position of the double bond. 



Piperitone has a peppermint odour and taste, resembling pulegone very 

 closely in these respects. It is colourless when pure, but becomes yellowish with 

 age. It reduces an alkaline solution of silver nitrate and reproduces the red 

 colour with Schiff's reagent. 



PREPARATION OF LAEVO-ROTATORY PIPERITONE. 



200 c.c. of an ordinary commercial oil of E. dives were subjected to dis- 

 tillation under reduced pressure, and after the removal of the lower boiling 

 terpenes, two fractions were obtained boiling at n millimetres pressure : 



52 c.c. at 105-109 C. ; ketone content = 76 per cent. 



32 c.c. at 110-112 C. ; ,, = 92 



