W. KAUZMANN 19 



r 



to change in an experiment, it is not yet possible to infer very much from 

 the magnitude of this change about the nature of the underlying change 

 in conformation. All the principal modern theories of optical activity agree, 

 however, that the following rule ought to be true: if all of the groujis at- 

 tached to the asymmetric carbon atoms are free to rotate about the bonds 

 that hold them to the asymmetric carbon atoms, then a relatively small 

 optical activity should be observed. To illustrate this, consider the second- 

 ary butyl alcohol molecule, 



1' 



CpH 



<i'' Ao. '" 



\ 1/ b 2 V 



Fig. 3 



If the ethyl group, CJi-s — Ci,Ho — , can orient freely about the Cb — C* 

 bond, and if the OH group can orient freely about the C* — bond, then 

 according to the rule the optical rotation of the molecule ought to be small.^ 

 This rule is based on an interesting internal compensation effect that is 

 described elsewhere (18). The available experimental evidence indicates 

 that if there is free orientation about bonds, then values of [aJD of less 

 than 10° will be observed in typical organic molecules that do not have 

 absorption bands in the visible or near ultra-violet. Any restriction of the 

 ability of groups to orient freely about these bonds will usually produce 

 much larger optical activities. Such restrictions can be expected if the 

 asymmetric carbon atoms occur in small rings, or if the groups attached 

 to the asymmetric carbon atoms are bulky. 



In the case of restriction by bulky groups, the magnitude of the optical 

 activity almost always decreases when the temperature is raised because 

 higher temperatures lead to greater freedom of orientation. With ring 

 compounds an increase in temperature cannot cause completely free ori- 

 entation about single bonds without rupturing some of the bonds in the 

 ring, so the temperature coefficient of the magnitude of the optical ac- 

 tivity is as often positive as it is negative. Of course, it is possible to have 



^Strictly speaking, in order that a small oiitical activity be observed, it is only 

 necessary that Ca occupy with equal frecjuency the three symmetrically disposed 

 positions, 1, 2 and 3 indicated in the above sketcli, and that the hydrogen atom on 

 the hydroxyl group occupy with equal frequency the three symmetrically disposed 

 positions 1', 2' and 3'. The term, 'free orientation about a bond,' will be imderstood 

 to mean the existence of equal populations in several such symmetrical positions. 



