Till] DIVISION OF TIIK RACKMIC COMPOUNDS 177 

 2. Bimolecular, optically inactive, appearing only as double molecules, 

 one of which belongs to the dextro- and the other to thelevo- modification 

 of the compound. Type: 2D-2L = 0. The inactive double molecule 

 can be split up into two optically active components, 2D and 2L. 

 Example : racemic acid (Fr. acide racemique, Ger. Traubensdure). 



The first of these two modifications, viz., the monomolecular, indivisible kind, 

 is termed the anti- combination ; the second, or bimolecular modification, being 

 known as the para- form, or (from the best-known example) racemic modification. 



On the other hand, it is found, both theoretically and in practice, that bodies 

 containing but a single asymmetric carbon atom can appear in only one inactive 

 form, namely, the divisible, bimolecular para- form. 



139. The Division of the Racemic Compounds. 



The correctness of the assumption that the racemic modifications of 

 stereoisomeric bodies are actually double compounds, consisting of an equal 

 number of dextro-rotatory and levo-rotatory molecules, can be demonstrated 

 in two ways. The first is by synthesis. If, for example, equal quantities 

 of equally strong (nearly saturated) solutions of dextro- and levo-tartaric 

 acid be mixed together, combination of the two, attended with the evolution 

 of heat, occurs, and the mixture solidifies in consequence of the crystallisa- 

 tion of a racemic acid which can be proved to be identical with natural racemic 

 acid. Again, if equal quantities of dextro- lactate and levo-lactate of zinc be 

 dissolved in water and mixed, the optically inactive zinc salt of " fermentation 

 lactic acid" possessing all the properties of this substance will crystallise 

 out. This synthetic construction of the inactive double molecule from the two 

 optically active components is, however, of merely theoretical interest, but is 

 without any practical value, no new and hitherto unknown compounds being 

 thereby obtained. 



As a rule, it is the inactive para- form with which we have to deal, and 

 this has to be split up into its constituent molecules of the dextro- and 

 levo- modifications. This division can be effected in several ways. In some 

 cases the inactive substance decomposes spontaneously on crystallising out 

 from solution, and the components which differ in their crystalline habit 

 can be separated by hand (selecting the crystals according to their structure). 

 The earliest example of this was given by PASTEUR (XI.). If a solution of 

 sodium-ammonium racemate (0 4 H 4 (NH 4 )Na0 6 + 4 aq.) be allowed to evaporate 

 slowly, hemihedral crystals are obtained. Pasteur found that the hemihedral 

 surfaces did not occupy the same position in all the crystals, but that the 

 one class of crystals formed, as it were, the reflected image of the other, one 

 being dextro-heinihedral but levo-rotatory, and the other levo-hemihedral 

 but dextro-rotatory. On separating the two kinds, and preparing the acid 

 from each, he obtained dextro-tartaric acid in the one case and levo-tartaric 

 acid in the other, but in no instance was the inactive racemic acid produced. 



The most usual method of division consists in allowing the inactive sub- 

 stance to unite with an optically active one. If the substance to be split up 

 is an acid, then an active alkaloid (e.g. morphine, quinine, strychnine, &c.) is 

 used ; if a base, it is combined with dextro-tartaric acid. Experience teaches 

 that the resulting compounds have very different degrees of solubility, on 

 which account the two optically active modifications can be separated without 

 much difficulty. Thus, it was shown by T. PURDIE and J. W. WALKER (I.) in 

 1892, that by combining the optically inactive " fermentation lactic acid " with 



