Optical activity in ketones. The rotatory dispersion and circular dichroism of m-methyl cyclohexanone and of pulegone in their ketonic absorption bands

Abstract

Recent investigations of the rotatory dispersions of aldehydes and ketones led to the conclusion that the optical activity of these compounds is due almost entirely to the electrons of the carbonyl group and not directly to the asymmetric carbon atoms. A particularly striking example of this was found in the aldehydic sugar derivative, μ-arabinose pentaacetate, by Hudson, Wolfrom and Lowry (1933), who showed that when the calculated partial rotation contributed by the absorption band between 3000 and 2600 A was subtracted from the observed rotations for the whole molecule, a negligible residual rotation was left at every wave-length at which measurements were made between 6700 and 2300 A. In general, however, absorption frequencies lying in the Schumann region appear to make a small contribution; but the analysis has always been complicated in the cases so far examined by the fact that the molecules contain either two asymmetric carbon atoms, e. g. in camphor (Kuhn and Gore 1931), menthone and carvomenthone (Lowry and Lishmund 1935a), or as many as four in the open-chain sugar derivatives (Hudson and others 1933; Baldwin, Wolfrom and Lowry 1935). The absence of any large direct contribution from the asymmetric atoms might therefore be explained in two ways, namely, either that their partial rotations are actually very small, or, as may well be the case in the sugars (cf. Lowry 1935, p. 274), that, though large, they are of opposing signs and tend to annul one another at wavelengths remote from the region of absorption. It was thus desirable to examine an optically active ketone whose molecule contains only a single asymmetric carbon atom, with no other disturbing factor, and m-methyl cyclohexanone (I) was selected for this purpose.