Structural Differentiation in Asymmetric Reactions 171 



spectively, will react with equal rates under any condition which 

 permits the equilibration of conformations. The chair forms, never- 

 theless, possess no simple axis greater than one. If we consider, 

 however, not the prevalent conformations but the averaged positions 

 of their nuclei, we obtain the planar form XXe, which on rotation 

 yields XX/. This representation, therefore, meets rule 4 and contains 

 a twofold simple axis. 



Example 9, Caaa{-\-b) {XXI). This case differs from example 2 

 in having an asymmetric substituent -fb instead of the symmetric b. 

 Such a molecule has no simple axis of symmetry >1. Unless rotation 

 around the central C-C bond is severely restricted, every conformation 

 (e.g., XXIa) is accompanied by two others (XXI6 and XXIe) which 



V 



+ b 



•l\ s\\ s\\ 



a' a'" a" a" a' a"' a"' a" a' 



XXIa XXIb XXIc 



are equally stable and mutually superposable. The prevailing mixture, 

 therefore, meets the superposition test jointly even if its individual 

 members do not. The three a groups therefore cannot be differentiated 

 from each other. 



Example 10, RCOOH (XXII). Tautomerism ordinarily refers to an 

 equilibrium of non-equivalent structures which is of no concern in this 

 discussion. If we consider, however, a reaction such as the prototropic 

 shift of an acid, the question of the steric equivalence of the two 

 oxygen atoms arises. A mixture of XXIIa and XXIIb will yield by 



XXII a R— C 



XXIIb R— C 



R-Cf „ XXIIc 



•0"H 



0"H 

 0' 







R-C^°, XXII d 



N 0H 



prototropy the superposable mixture XXIIc and XXIId. Hence dif- 

 ferentiation is not possible under ionizing conditions. 



An alternative and somewhat simpler way of analyzing situations 

 arising from rotational isomerism or tautomerism (examples 8-101 is 



