THE STEREOISOMERIC FORMS OF MENTHONE. 17t 
optically active menthones are then represented by the 
symbols Dd, Ll; Ld, and DI. The first two of these optical 
isomers are wholly uncompensated substances, possessing 
the maximum rotatory power for the structure concerned, 
whilst the last two may be regarded as partially internally 
compensated compounds of lower rotatory power. Further- 
more, these four optically active forms may combine in 
pairs to give rise to two fully racemic menthones, repre- 
sented by the symbols Dd,Ll and Ld, Dl. 
Menthone occurs in essential oils in dextro- and levo- 
gyrate forms which are generally supposed to correspond 
with each other and with the l-menthone obtained from 
natural l-menthol by oxidation with chromic acid (Beck-. 
mann, Annalen, 1889, 250, 325). Since, however, the 
menthones do not combine with sodium bisulphite, and 
since, further, they are liable to undergo “ inversion’? when 
regenerated from derivatives, such as the oximes and semi=. 
carbazones, the stereochemical characterisation of natural 
or synthetic forms of this ketone is a problem of consider- 
able difficulty. 
l-Menthone obtained by the oxidation of natural l-menthol 
has a maximum optical rotatory power of [«], —28°46°, and 
by treatment with cold 90 per cent. sulphuric acid a maxi- 
mum inversion value of[«], +28°14° is exhibited (Beckmann, 
loc. cit., 335; Ber., 1909, 42, 846). The inversion, which 
is conditioned by alkali as well as by acid, has been 
attributed to enolisation and consequent loss of asymmetry 
of carbon atom No. 4: it appears, therefore, that l-menthone 
Should be assigned the configuration Ld (compare Barrow- 
cliff, Trans. Chem. Soc., 1907, 91, 875; also Gardner, Perkin 
aud Watson, Trans. Chem. Soc., 1910,97, 1756). d-Menthone 
would thus be DI, and the configurations Dd and Ll would 
correspond, respectively, with the dextro- and leevo-rotatory 
forms of isomenthone, that is, with d- and l-isomenthone.. 
