The Role of Dissymmetry 77 



The experimental evidence concerning the spontaneous resolution of race- 

 mates and photochemical asymmetric synthesis will be considered below. The 

 part played by asymmetric catalysis and asymmetric adsorption in producing 

 the dissymmetry of living material will be considered separately. 



SELECTIVE SPONTANEOUS CRYSTALLIZATION 

 OF ANTIPODES 



The self-propagating separation of enantiomorphs, commonly called spon- 

 taneous crystallization, may occur under various conditions: from optically 

 active solvents; by seeding with a crystal of one or other antipode of the same 

 compound or one isomorphous with it; or by spontaneous crystallization. 



These cases of selective crystallization are due to differences in the nature of 

 racemates. The spontaneous crystallization of true racemic compounds contain- 

 ing equal amounts of (+)- and (— )-enantiomorphs in the crystal lattice of the 

 molecules is a comparatively rare phenomenon [4]. 



Crystallization under the influence of isomorphic crystals, or those of one 

 antipode of the crystallizing racemate, is possible when the racemate crystaUizes 

 as a conglomerate in which each of the antipodes crystallizes in its own lattice. 

 True spontaneous crystallization occurs when mixed crystals are formed, the 

 antipodes entering into a common lattice in any proportions. In specially favour- 

 able cases the crystalline deposit contains a great excess of one antipode [5]. 



1. Crystallization in an optically active solvent. When crystallizing sodium or 

 ammonium tartrate from a solution of D-glucose, Kipping & Pope [6] usually 

 obtained an excess of one enantiomorph in the deposit. McKenzie [7] separated 

 out (+)-ammonium tartrate by crystaUizing ammonium tartrate from a solution 

 of (— )-ammonium malate. CrystaUization from a solution of (-t-)-ammonium 

 malate led to the separation of the (— )-tartrate [8]. 



This case is of great interest as being the first example of the isolation of one 

 antipode of a racemic acid by the action of another optically active acid instead 

 of by a base as ordinarily used. 



2. Crystallization initiated by a crystal of one antipode or by an isomorphous 

 crystal. A considerable amount of work has been devoted to the study of the 

 selective crystallization of optical antipodes, or crystalline enantiomorphs, by 

 insertion, into a supersaturated solution, of a crystal of one antipode of the same 

 substance, or some other crystal isomorphous with it. 



On the basis of Pasteur's work Gernez [9] proposed a method of isolation of 

 one antipode under the influence of crystals of the (+)- or (— )-antipode [10]. 

 Kipping & Pope [11] were incUned to ascribe such separation to the action of 

 seeds of crystalhzation reaching the solutions in the dust of the air of the labora- 

 tory. In fact, when a compound crystalhzes as a conglomerate of (+)- and (— )- 

 crystals, crystallization with free access of air leads to an equal probability of the 

 separation of either (+)- or (— )-isomer. 



This is confirmed by the observations of Anderson & Hill [12] on the crystal- 

 lization of synthetic atropine sulphate. Read [13] also inclines towards this 

 view. 



