172 R. S. HUGHESDON, H. G. SMITH AND J. READ. 
From the inversion results, if the above explanation is 
correct, it follows that the optical rotatory effect of carbon 
atom No. 1 is less than that of No. 4, and hence the sense 
of the optical rotation is determined by the spacial distri- 
bution of the groups about No. 4. In other words, the 
optical effects of the two asymmetric carbon atoms are 
opposed in the active menthones and conjoined in the active 
isomenthones. 
Inverted l-menthone appears to consist of a mixture of 
l-menthone and d-isomenthone. By oxidising l-isomenthol 
Beckmann (J. pr. Chem., 1897, 55, 18) obtained isomenthones 
having values of |«], from+30° to+35’, whilst a preparation 
made by an indirect method from l-menthone oxime had 
the remarkably high value, |«], +93°2° (Ber., 1909, 42, 846). 
Furthermore, Barrowcliff (loc. cit.), by means of the reac- 
tion with semicarbazide, isolated an isomenthone having 
[a], +47°1° from the essential oil of American pennyroyal 
(Hedeoma pulegioides), and estimated the value for the 
optically pure ketone at about [a], +97°. 
It has already been shown (Smith and Penfold, this 
Journal, 1920, 54, 40) that the eucalyptus ketone, piperitone, 
can be hydrogenated in the presence of finely divided nickel © 
at 175 —180°, without the keto-group undergoing alteration, 
In the course of a recent series of investigations, as yet 
unpublished, in which by the application of a method 
independent of oxidative breakdown we have been able to 
confirm the structural identity of dl-piperitone with /\?- 
p-menthen-3-one (Formula II), it became necessary to 
prepare comparatively large quantities of menthone from 
dl-piperitone. Among the successful methods employed 
was that of direct hydrogenation by the catalytic method 
of Paal, in the presence of colloidal palladium. The men- 
thone obtained in this way was optically inactive and 
consisted mainly, or perhaps wholly, of isomenthone. It 
