STEREOISOMERISM AND OPTICAL ACTIVITY 233 



carbon to the middle points of the straight lines joining a — a or 

 b— b. 



In the third case, it is the line joining the central carbon to 

 the middle point of the straight line joining a — a. When the 

 ray passes along that direction, the two other groups b and c 

 can in no sense be said to describe a spiral round it. 



Thus it is clear, that substances of these types will not be 

 optically active according to this new hypothesis; and none 

 such are known. 



The same considerations apply in cases where there are more 

 than one asymmetric carbons. 



Of these, we need only consider the apparently anomalous 

 case of trihydroxy-glutaric acid. 



COOH . CHOH . CHOH . CHOH . COOH 



When in this formula the two side carbons are of opposite sign, 

 they neutralise each other's optical effect, but make the central 

 carbon asymmetric ; but the difference in the nature of the two 

 groups is not of a kind calculated to have any effect on the 

 twisting of the bonds ; as far as that is concerned, the substance 

 is of the type Ca 2 , b, c ; the structure as a whole does possess a 

 plane of symmetry, and thus shows no optical activity. But the 

 isomerism manifests itself in different chemical and physical 

 properties; it thus suggests an analogy with the cis-trans- 

 isomerism in the alicyclic compounds. 



A carbon, like the central carbon here, which is united with 

 four radicals, which are not all different structurally, but only so 

 configurationally, is called a " pseudo-asymmetric " carbon. 



Le Bel's Views 



The view of the spatial distribution of the four valencies of 

 carbon, put forth above, comes very near to that of Le Bel. Le 

 Bel's ideas appear to the writer to be more sound ; but they 

 were not further developed because they were more complicated 

 than the rigid ideas of Van't Hoff. Although Van't Hoff 

 originally made no definite statement as to the nature of his 

 tetrahedron, all the further developments of the tetrahedron 

 hypothesis have been based on the tacit assumption that it is 

 regular. All this is very clearly shown in the case of 



