218 



A' I' would be symbols for compounds which could be separated 

 in this way. Because only the less soluble compound can be obtained 

 perfectly pure in this way, while the other one always has some of 

 the less soluble substance adhering to it, the pure salts A /and A']' 

 can, properly speaking, only be obtained by successive combination J ) 

 of the racemoid A A' with / or /'.However, in practice the difference 

 of solubility is occasionally sufficiently great to allow a practically 

 complete fission of A A' by simply combining it with /. 



If, however, the difference of the solubilities between Af and A'f 

 is not great enough, a useful modification of the method described 

 was proposed by Pope and Peachy 2 ). If only so little of / be 

 present, as to give precisely the theoretical quantity of the less 

 soluble compound Af, while the other component A' of the racemoid 

 be bound to some optically mactive substance p, giving with it a very 

 soluble compound A'p, then, if the solution is allowed to crystal- 

 lise slowly, a certain amount of Af will every moment be with- 

 drawn from the equilibrium-mixture in the remaining solution. 

 As a consequence, this equilibrium will be displaced so, that a new 

 quantity of Af is produced, which again will be deposited; etc. 



The result is that Af is completely gained in the crystalline state, 

 while A'p remains in the mother-liquor, from which A can easily 

 be isolated afterwards. Pope and Peachy have used this method 

 e. g. to separate the racemic tetrahydro-quinaldine into its optically 

 active components, by means of the action of ammonium-bromo-cam- 

 phor-sulponate (1 mol.) on inactive tetrahydro-quinaldine-hydrochlo- 

 ride (2 mol.); etc. 



If the combinations Af and A'f are sufficiently loose, it will after- 

 wards be possible to set A and A' free by fixing / and /' to another 

 substance 5, to which they have greater affinity. In such away 

 racemic bases may be separated by means of an optically active 

 acid, or racemic acids by means of an optically active base. This 

 method .is, however, not restricted to these two classes of compounds. 



In later times Erlenmeyer 3 ) and Neuberg 4 ) have extended 

 the method to condensation-processes with optically active aldehydes, 

 nitrogen-compounds (hydrazine-deriv&tives) ; etc. We will return 

 to those investigations later. 



!) W. Marckwald, Ber. d. d. Chem. Ges., 29, 43, (1896). 



2 ) W. J. Pope and S. J. Peachy, Journ. Chem. Soc. London, 75, 1066, (1899). 



3 ) E. Erlenmeyer Jr., Ber. d. d. Chem. Ges., 36, 976, (1903). 



4 ) C. Neuberg, Ber. d. d. Chem. Ges., 36, 1192, (1903). 



