Recrystallization, preferential crystallization

Experimental-2] Example of resolution by preferential crystallization To a few 100 mL Erlenmyer Flask was added 0.6-0.8g of ( )-2 and 30 mL of water, and the mixture was heated up to give a clear solution. The solution was then gradually cooled and inoculated with a needle crystal obtained from Experimental-1, and allowed to crystallize. When about 15-30 mg of crystals were deposited, they were filtered off and dried. Yield and optical rotation were measured. Crystals with the same chirality were combined and recrystallized from water (10 20 parts (v/w) versus crystals) to afford enantiomerically pure salt 2 mp 104-105°C, [a]D25 + and - 24.5°C (c 1, EtOH). 

Phenomena of purity drop in racemic resolution by preferential crystallization were surveyed and factors relevant to spontaneous crystallization and their significance were quantitatively summarized. It was empirically concluded that washing of seed crystals would be the best way to prevent such purity drop, although it could not completely suppress the occurrence. With additional experiments on the preparation method of seed crystals, adequate procedures were found. However, recrystallization under these conditions could only prolong the start of purity drop but could not prevent it. A combined process to produce optically pure enantiomers consisting of preferential crystallization, racemization reaction and solvent evaporation steps was proposed. 

The reaction of Li2PH in DME leads preferentially to compounds 9 and 10, which appear as a white powder, from which compound 9 can be obtained as cubic crystals by recrystallization with pentane or toluene. The presence of Me2Si(PH2)Cl (1), Me2Si(PH2)2 (2), as well as 3,4,5,6, 7, and 8 can be demonstrated in the liquid products of the reaction (34). 

Topochemical Polymerization The chiral crystalline environment of a monomer itself can be a source of asymmetric induction in solid-state polymerization [69-72], Prochiral monomers such as 37 give enantiomorphic crystals, one of which can be preferentially formed by recrystallization with a trace amount of optically active compounds. Photoir-... 

Figure 4. Enantiomeric resolution of a racemic mixed crystal using Preferential Enrichment a case of an enrichment of the R enantiomer in solution after the first recrystallization of the racemic sample. Actually, the probability for either the R or the S enantiomer to be enriched in solution after recrystallization of an exactly racemic sample was 50% (see ref 9a).

These are all unusual. Thus, by collecting the enantiomerically enriched mother liquors with the same handedness, very efficient separation of the two enantiomers (>96% ee) has been easily achieved. Therefore, to probe if Preferential Enrichment occurs or not for a given compound, one only have to repeat recrystallization of the racemic sample several times at 25, 0, or -20°C and measure the ee value of the supernatant solution after each crystallization. 

Finely divided calcium for laboratory purposes can also be prepared by recrystallizing bulk calcium from a sodium solution. The bulk calcium dissolves in sodium at elevated temperatures and, upon cooling, crystallizes out in small dendritic platelets that become trapped in a sodium matrix when the sodium solidifies. As in the previous synthesis, the sodium matrix may be removed by preferential reaction with a hydrous lower alcohol. 

Another thermal separation unit often used for the laboratory scale purification of ionic liquids is recrystallization [125]. It is an attractive option for those ionic liquids that can form solids with a high degree of crystallinity. Crystals of ionic liquids are expected to be pure because each molecule or ion must fit perfectly into the lattice as it leaves the solution. Impurities preferentially remain in solution as they do not fit as well in the lattice. The level of purity of the crystal product finally depends on the extent to which the impurities are incorporated into the lattice or how much solvent is entrapped within the crystal formed. 

The title compound (107) [42] has been found to form inclusion complexes with a wide variety of solvent molecules [43]. In the complexation, racemates or conglomerates of 107 were formed, depending on the choice of solvent. In the latter case, the inclusion crystals consisting of one enantiomer of 107 were formed preferentially and the enantiomeric separation of 107 could be performed. For example, recrystallization of rac-107 from the solvents shown in Table 3.3-10 gave a 1 1 complex of the rac-107 with the solvent as yellow crystals. On the other hand, recrystal-lization of the rac-107 from the solvents shown in Table 3.3-11 gave a 1 1 complex... 

The recrystallization of S-layer proteins at phosphoethanolamine monolayers on aqueous subphases has been also studied on a mesoscopic scale by dual label fluorescence microscopy andFourier transform infrared spectroscopy (FTIR) [110]. It has been shown that the phase state of the lipid exerts a marked influence on the protein crystallization. When the surface monolayer is in the phase separated state between fluid and crystalline phase, the S-layer protein is preferentially adsorbed at the boundary line between the two coexisting phases and crystallization proceeded underneath the crystalline phase. Crystal growth is much slower under the fluid lipid and the entire interface is overgrown only after prolonged protein incubation. In turn, as indicated by characteristic frequency shifts of the methylene stretch vibrations on the lipids, protein crystallization affects the order of the alkane chains and drives the fluid lipid into a state of higher order. However, the protein does not interpenetrate the lipid monolayer as confirmed by x-ray reflectivity studies [105-107]. 

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