Stereoisomerism. 



spectively. It will be noticed from these figures that one 

 form of tartaric acid will be dextro-rotatory, another will 

 be laevo-rotatory, whereas in a third form, the configura- 

 tion of groups round one carbon atom will cause dextro 



COOH 



rotation, whilst that round the other carbon atom will 

 cause laevo rotation. In the two first-named forms, the 

 configuration round both carbon atoms is the same. Now, 

 as in tartaric acid the elements or radicles connected with 

 each carbon atom are the same, the dextro rotation due to 

 one set of groups in the third form will be equal to the 

 Isevo rotation due to the other set. An optically inactive 

 stereoisomeride of this character is actually known, and is 

 designated meso-tartaric acid, and it is to be distinguished 

 from racemic acid in that the optical inactivity in the 

 latter form is due to a mixture in equimolecular propor- 

 tions of dextro- and Isevo-acids. The racemic form can be 

 resolved, by methods given below, into the dextro- and 

 laevo-components, whereas this is not possible in the case 

 of " internally compensated " compounds such as the meso- 

 acid, where optical inactivity is due to compensation by 

 the two asymmetric halves of the same molecule. The 

 tetrahedron formulae for the tartaric acids given above 

 can be conveniently condensed into the following forms : 



