16 LECTURE II. 



had ingeniously brought forward in order to explain the optical difference 

 between dextro-tartaric and laevo-tartaric acids, to the individual carbon 

 atom. This. atom is in combination with four different masses. Every 

 asymmetric carbon atom in a compound causes the possibility of two 

 optical isomers, one rotating the plane of polarized light to the right, and 

 the other to the left. We can illustrate this best, according to van 't Hoff, 

 by imagining the valences or affinities of the carbon atom extending to- 

 wards the apexes of a tetrahedron in the center of which the carbon atom 

 itself is placed. 



Fig. 1. 



(Ri, R 2 , RS, R4, are the four different masses with which the carbon 

 atom is combined.) 



The above drawing represents this kind of isomerism. The two forms 

 are in the same relation to one another as an object and its reflected 

 image, or as a right and left glove; i.e., they cannot be superposed one upon 

 the other, so that the corresponding parts will all coincide. There are, 

 then, three possible modifications in the case of every carbon compound 

 containing an asymmetric carbon atom, namely, two optically active forms, 

 and one which is inactive, being composed of an equal number of molecules 

 of each of the other two forms. In the last case the two asymmetric 

 carbons, although both active, have an equal and exactly opposite effect 

 upon polarized light, so that they neutralize one another. 



If these assumptions are correct, then if there are two or more asymmet- 

 ric carbon atoms in the molecule, the number of possible optical isomers 

 must increase regularly and amounts to 2 n where n is the number of asym- 

 metric carbon atoms. This theory has been confirmed empirically 

 to a most remarkable degree, and, indeed, in no part of chemistry has 

 the work of Le Bel and van 't Hoff been so strongly supported as in the 

 development of carbohydrate chemistry according to this point of view 

 by Emil Fischer. 



In advance, it may be mentioned, for example, that there are several 

 different sugars having the empirical formula CeH^Oe. Of these we need 

 mention only d-glucose, mannose, and galactose. Now all of these sugars 

 contain, as shown by the following general structural formula, no less than 

 four asymmetric carbon atoms: 



