— 160 — 



With this object in view the aldehyde was first converted into its 

 hydroxamic acid (described at the time) and this acid hydrolysed by 

 boiling with dilute sulphuric acid. If the aldehyde were really mentho- 

 citronellal of formula I the acid thus obtained should be identical 

 with the menthonenic acid 1 ) prepared by Wallach: 



(CH 3 ) 2 .C:CH.CH 2 .CH 2 .CH(CH 3 ).CH 2 .C0 2 H 



The analysis of the acid prepared by the authors agreed with the 

 formula C 10 H 18 O 2 , the acid's b.p. was between 248 and 253 (760 mm.) 

 or 142 (18 mm.), while Wallach's acid had b. p. 257 to 26 1°. 



In order to determine the position of the double linking, it was 

 oxidised with permanganate of potassium and afterwards with bichro- 

 mate of potassium and sulphuric acid, /S-methylglutaric acid (m. p. 86° 

 from benzene) and (probably) /^-butyric acid being obtained as pro- 

 ducts of oxidation. This behaviour agrees with the formula: 



(CH 3 ) 2 CH.CH:CH.CH 2 .CH(CH 3 ).CH 2 .C0 2 H -> 



(CH 3 ) 2 CH.COOH COOH.CH 2 .CH(CH 3 ).CH 2 .COOH 



Iso-butyric acid ^-Methyl glutaric acid 



The constitution of the aldehyde might thus be regarded as being 

 disclosed, but (as Wallach informed the authors by letter) it must 

 be borne in mind that in certain circumstances, in the course of the 

 process of oxidation with alkaline solution of permanganate the double 

 linking may be displaced against the carboxylic group 2 ). 



In the same paper, Ciamician and Silber report on the auto-oxi- 

 dation of menthone which occurs on exposure to light and in the 

 presence of oxygen. 3 5 g. of menthone, together with water , were 

 exposed to light for 3 1 / 2 months in large bottles of a capacity of 

 4,5 litres, filled with oxygen. At the end of this period the pressure 

 in the bottles was greatly diminished, and the precipitated liquid had 

 a pronounced acid reaction. Another bottle with similar contents, 

 which had been left in the dark, showed hardly any change in pres- 

 sure at the end of the same period. The non-acid parts having been 

 separated out by sodium carbonate, the alkaline liquid, which after 

 concentration in vacuo appeared in the form of a saponaceous mass 

 (36 g from 35 g menthone), was acidulated and extracted with ether. 

 From the ethereal extract a small quantity of liquid with an acid 

 reaction could be isolated with steam (2 1. of distillate required 

 1,9 g. sodium carbonate for neutralisation), and in this formic acid 

 was detected. The principal part volatilised very sparingly with water 

 vapour and yielded 13,8 g. of an acid which, when reconstituted from 

 the silver salt, boiled between 173 and 177 (12 mm.). The analyses 

 of the silver salt and of the oxime (m. p. 99 to ioo° from water) 



*) Liebig's Annalen 296 (1897), 131. 



2 ) Berl. Berichte 34 (1901), 2984; 42 (1909), 145; Report April 1909, 138. 



