STEREOISOMERISM AND OPTICAL ACTIVITY 237 



But when we come to No. 8, we see at once that here the 

 molecule is not only asymmetric, but also non-superposable on 

 its mirror-image, which is No. 9. Evidently these two con- 

 figurations represent the two optical isomers. 



Here we have asymmetry, enantiomorphism, and optical 

 activity, without the presence of an asymmetric carbon. 



Let us consider another important case among the alicyclic 

 compounds, viz. that of molecules having the so-called 



Indirect Plane of Symmetry 



Ladenburg 1 was the first to draw attention to what he 

 thought to be the exceptional character of a configuration like 

 this : 



a / x y x b 



C 



It contains two truly asymmetric carbons forming a meso-pair; 

 but the structure as a whole possesses no plane of symmetry, 

 although it is identical with its mirror-image. Here again the 

 behaviour of a meso-pair is different from what it is in open- 

 chain-compounds. 



Several examples of this type are known : e.g. the keto-form 

 of trans-succinylo-succinic acid, and trans-3, 6-dimethyl-i,4- 

 cyclo-hexadiene-i,4-dicarboxylic acid ; and they are all inactive. 



There are some substances of this class known which contain 

 two meso-pairs : e.g. 1, 3-dimethyl-cyclobutane-2,4-dicarboxylic 

 acid. 



COOH 



Here again the structure is asymmetric. 



These examples clearly show the difference in behaviour 

 between open-chain- and ring- compounds. The cause of this 



1 Ber. 28, 1995, 3104 (1895). 



