1G2 Essays in Biochemistry 



in such a way that the second arrangement is indistinguishable from 

 the first, the groups a' and a" cannot be differentiated from each other 

 in any reaction. However, if such superposition is impossible, the 

 groups a' and a" can react with an asymmetric reagent at different 

 rates.* 



The validity of this rule can be demonstrated as follows: If the 

 representation of the molecule A is indistinguishable from one showing 

 a" in place of a', any product resulting from the change of a' to d 

 must also be superimposable on the product resulting from the change 

 of a" to d. This will be true regardless of the symmetry or asymmetry 

 of d. The products must be superimposable even if the reaction is 

 not confined to the a groups but involves additional changes at other 

 substituents. Similarly, any approach of the reagent E (which again 

 may be symmetric or asymmetric) towards a' or some other atom is 

 indistinguishable from the corresponding approach of E towards a" 

 or the corresponding other atom. Furthermore, if the products of the 

 direct reaction at (or near) a' and a" are indistinguishable from each 

 other, there can also be no differentiation in any successive process. 

 If the reaction should involve several products, the same argument 

 applies to each one of them. We conclude then that the a' and a" 

 groups cannot be differentiated fijom each other in any process if the 

 superposition specified is possible. If, on the other hand, such super- 

 position is not possible, mechanisms exist which permit the differentia- 

 tion of the a groups. For instance, the product resulting from the 

 combination of a' with the optically active agent E cannot be super- 

 imposed on that resulting from the combination with a''. These prod- 

 ucts cannot be antipodes if E, as is ordinarily the case, retains its 

 asymmetry during the reaction. Hence the thermodynamic properties 

 of the two products are expected to differ. If the outcome of the 

 reaction of E with A depends on other factors, analogous arguments 

 apply as were outlined above for the specific case of Caabc. Since a 

 possibility of differentiation exists whenever the a groups do not meet 

 the superposition test, the validity of the rule is considered to be fully 

 established. To illustrate its application and utility a few specific 

 examples will be discussed. 



Example 1, Caabb (X). Rotation of Xa through 180° around an 

 axis passing through the central carbon atom C and perpendicular to 

 the plane of the paper results in X6 which is indistinguishable from Xa. 

 Hence the two a groups cannot be differentiated from each other. The 



* The application of this rule to compounds which cannot be dealt with ade- 

 quately by a single representation is illustrated in examples 8-10. 



