164 



Essays in Biochemistry 



are mirror images of each other and hence not superimposable. There- 

 fore any attempt to place a" into the position of a' results in an 

 arrangement different from the original (e.g., XHIa and XIII£>). The 



+ b -b- 



e c f 



a'-^«C^^~ a' 



I 



I 



I 



I 

 e C f 



a' 



T 



d 

 XHIa 



-b +b 



f c e 



I 

 I 



I 

 I 



I 

 I 

 I 



f\ c e 



d 

 Xlllb 



two a groups therefore can react at different rates with an asymmetric 

 reagent. This result cannot be foreseen by applying rule 1 since both 

 a groups lie in the plane of symmetry. It is predicted, however, by 

 rule 2, since there is no axis greater than one. The definition given 

 for the meso carbon atom did not refer specifically to such structures 

 but could quite logically be interpreted to include them. If we do this 

 we should realize, however, that we make a choice different from that 

 usually made in defining an asymmetric carbon atom, since the struc- 

 ture Ca( + b)(— b)c gives rise to two stereoisomers which are not 

 optical antipodes. 



Example 5, Caa{ + b) ( + 6) (XIV). If the group +b is defined as 

 in the preceding example, the compound is not superposable on its 

 mirror image. In optically active compounds differential reactivity 

 of identical substituents even in reaction with symmetrical reagents 

 occurs with such frequency that this fact hardly requires comment. 

 Example 5 shows, however, that this is not invariably the case and that 

 even optically active compounds may possess substituents that cannot 

 be differentiated from each other by any process. Rotation of structure 

 XlVa through 180° around an axis which passes through the central 

 carbon atom and is perpendicular to the plane of the paper yields XIV6. 

 As both arrangements are indistinguishable, the two a groups cannot 

 be differentiated from each other nor can the b groups. [This conclu- 

 sion of course should not be construed to mean that the reactivity of 

 the a groups in this compound, in reaction with an asymmetric reagent, 



