Recrystallization seed crystals

S7 The above CS2 solution from which Si 2 and most of the Sg has been crystallized at —78 °C is used for the preparation of S7, Sis and 820- Stirring of this solution at - 78 °C after addition of some glass powder (or seed crystals of S7) for about 2 hours results in the precipitation of finely powdered sulfur which is isolated by removing the solution by means of an immersion filter frit. The residue is extracted three times with small amounts of toluene (leaving an organe residue) from which S7 crystallizes on cooling to —78 °C. On recrystallization from CS2 pure S7 is obtained in 0.7% yield m.p. 39 °C... 

To a solution of 3.70 g 3,6-dimethoxy-4-formylbenzonorbomane in 20 g nitromethane, there was added 1.3 g anhydrous ammonium acetate and the mixture was heated on the steam bath for 45 min. The excess reagent/solvent was removed under vacuum, and the residue was dissolved in 20 mL boiling MeOH. A speck of seed crystal started a heavy crystallization of orange crystals which were removed by filtration and washed with MeOH. After drying, the product 3,6-dimethoxy-4-(2-nitrovinyl)benzonorbomane was yellow, weighed 3.47 g, and had a mp of 88-89 °C. Recrystallization of an analytical sample from MeOH did not improve this mp. Anal. (C15H 7N04) C,H. 

In the absence of seeding, crystallization may take several weeks. It is preferable to separate a small sample of the precursor thiomenthol from its stereoisomers by HPLC (3% ethyl acetate in hexane as eluant) and prepare a small amount of pure oxathiane from this material. Alternatively, a small amount of the product may be purified by GLC on a 5% FFAP column. The melting point of pure material is 37-38°C. The checkers, who did not have seeding crystals, found that the early crops of crystals melted when the flask was allowed to warm to ambient temperature. Consequently the cold supernatant liquid was withdrawn from the crystals with a Pasteur pipette while the flask was maintained at ca. 0°C (ice-water bath). The crystals were subsequently recrystallized several times in the same flask without filtration. By this technique, white crystals melting at 32-35°C were obtained this material is spectrally pure and suitable for asymmetric synthesis. The supernatant liquid was also concentrated, as the submitters described, to obtain additional crops using this technique. 

A solution of 4-chloro-N-methyl-N-phenyl-2-(2-pyrrolyl)butanamide in toluene was added dropwise at 85°C over 40 min to 1 hour to a stirred suspension of ALIQUAT 336 (phase transfer catalyst, 2 mol % with respect to pyrrolylbutanamide) and granular sodium hydroxide (3 equivalents) in toluene (50 mL). After the addition was complete, the suspension was stirred under a nitrogen atmosphere at a temperature of 85°C for 30 min, then cooled to 35°C. Cooled water (200 mL) was rapidly added to the mixture and stirred for 15 min at 25°C. The solution was rinsed with water and the layers were separated. The organic layer was washed with water, then distilled under atmospheric pressure to recover the toluene and water. The resultant solution was cooled to 50°C and allowed to crystallize after the addition of hexane and a seed crystal. The suspension was cooled to 5°C and stirred for 15 minutes. The resultant precipitate was filtered, washed with 100 mL of hexane, and dried under vacuum at 25°C to yield approximately 38 g (63%) N-methyl-N-phenyl-2,3-dihydro-lH-pyrrolizine-l-carboxamide. This solid was recrystallized from toluene to yield colorless crystals of N-methyl-N-phenyl-2,3-dihydro-lH-pyrrolizine-l-carboxamide, melting point 112-112.5°C. 

In some cases recrystallization of super saturated solutions can be initiated with a seed crystal. A seed crystal is simply a small crystal of the product, ft is added to the super saturated solution, and the dissolved product begins to grow on the seed crystal. The seed crystal induces recrystallization by giving the dissolved product a surface from which to grow on. The recrystallization of the product stops when equilibrium of the solution is reached. 

The syrupy 33 was dissolved in a mixed solvent of diethyl ether and ethyl acetate (lO.T), and the solution was kept at -70 °C for about 5 h. In this case, no seed crystals of optically pure (8aI )-(-)-33 were needed. Crystals deposited were collected by filtration to yield enantiomerically enriched 33 [a]o -95.8° (c 1.10, benzene). The crystals obtained were further enantiomerically purified by two additional recrystallizations, to give optically pure (8a/ )-(-)-33 mp 50.5-51.0 °C, [a]o -98.96° (c 1.039, benzene). We carried out the preparation of optically pure (8a/f)-(-)-33 with this procedure three times, and all gave satisfactory results. 

A rod of the material is held vertical and heated to cause melting at one end. By moving the heat source along the rod, melting and recrystallization occur as the source is moved. By placing a seed crystal at one end the whole rod can be converted into a single crystal. 

Differential inclusion complexation in solution can be controlled by addition of seed crystals during the recrystallization process. It was also found that the control by seed crystals can be accomplished in the solid state (see below). This seeding experiment was carried out for inclusion complexation between 12 and ethyl ether. 

When 12 (0.2 g) was recrystallized from 3 ml and 5 ml ethyl ether, the 3 1 82 and the 1 2 inclusion complex 83 were obtained, respectively. This is the first discovery that differential inclusion complexation is controlled by a concentration of solution. When 12 (0.2 g) was recrystallized from 4 ml of ethyl ether, 82 or 83 was formed, depending on slightly different conditions. In this case, however, control of the differential formation of 82 or 83 can be accomplished completely by addition of their seed crystals. X-ray crystal structures of 82 and 83 are very different, as shown in Figs. 2.2.9 and 2.2.10, respectively. Although both complexes are labile and lose ethyl ether easily, 82 is slightly more stable than 83, probably because the ether molecule which is surrounded by three host molecules in 82 is only released with difficulty (Fig. 2.2.9) but the ether molecule which is accommodated in the channel of 83 can easily be released through the channel (Fig. 2.2.10) [47]. 

For operating the secondary growth, the seeded support is hydrothermally treated in a diluted synthesis mixture (with or without template) to grow the seed crystals, resulting in a compact zeolite film. A simple steaming of the seed layer can also partially consolidate the zeolite membrane by local dissolution and recrystallization processes in the seed layer. The films are finally dried or calcined if an SDA was used. 

Promotion of crystallization can be achieved, e.g., by inoculation with an appropriate enantiomorphic seed crystal, by the presence of further chiral compounds or by use of solvents which already contain a guest enantiomer in small excess. The latter method allows to win the other enantiomer which is now enriched in the filtrate of recrystallization by repetition of the process... 

Since the above mentioned experimental evidences suggest primarily that the major causes of the purity drop may come from the seed crystals added to the racemic solutions, some additional experiments were conducted to examine the effects of the quality of seed crystals (Takiyama,H. et al, TUAT, unpublished data). First, the rate of crystallization, i.e. the cooling rate of the solution, and the holding time at the final temperature after the crystallization were experimentally examined. In another series of experiments in order to eliminate any possible undesired enantiomer particles from the seed crystals, seeds are recrystallized for a number of times. 

---