STEREOISOMERISM AND OPTICAL ACTIVITY 235 



The Difference between Saturated Open-Chain- and 



Ring-Compounds 



So far the two terms " asymmetry" and "enantiomorphism " 

 have been used as being coextensive in their denotation. This 

 is quite true, if we define an asymmetric carbon as one that is 

 united to four structurally different radicals, and call it " pseudo- 

 asymmetric" if any of the radicals are structurally similar, but 

 differ only in configuration, but it is true only in the case of open- 

 chain compounds. 



In open-chain-compounds of all types (except one, for which 

 see p. 241) the following three relations hold good : 



(1) The presence of an asymmetric carbon makes the whole 

 structure both asymmetric and enantiomorphous ; and con- 

 versely all asymmetric and enantiomorphous structures contain 

 at least one asymmetric carbon. 



(2) The presence of a pseudo-asymmetric carbon does not 

 make the structure asymmetric or enantiomorphous : e.g. the 

 inactive indivisible tri-hydroxyglutaric acids. 



(3) And further, a meso-pair of asymmetric carbons makes the 

 whole structure symmetric and also, of course, identical with 

 its mirror-image : e.g. meso-tartaric acid, mucic and allo-mucic 

 acids. 



But these relations do not always hold good in alicyclic or 

 saturated 



Ring- Compo u nds 



In these, the presence of an asymmetric carbon does indeed 

 make it asymmetric and enantiomorphous ; but the converse is 

 not always true, asymmetry and enantiomorphism being often 

 effected by one or more pseudo-asymmetric carbons. 



Let us, for example, consider the case of 



Inosites 



These have the constitutional formula C 6 H 6 (OH) 6 . A 

 constitutional formula of this type admits in all of nine con- 

 figurations, shown below. Three isomers only are known so 

 far ; one is of the inactive indivisible type, the other two being 

 optical antipodes. 



