Re-crystallization

Classical methods for separation and purifica tion include fractional distillation of liquids and re crystallization of solids and these two methods are routinely included in the early portions of laboratory courses in organic chemistry Because they are capa ble of being adapted to work on a large scale frac tional distillation and recrystallization are the preferred methods for purifying organic substances in the pharmaceutical and chemical industries... 

Phase III in Fig. 5 is marked by a cooling of the bond line, which causes the bulk viscosity of the adhesive to rise. During phase IV, another sharp increase in viscosity is observed. This is caused by the re-crystallization of the polymer... 

To a solution of 180 parts of -benzyl N-benzyloxycarbonyl-L-aspartvI-L-phenylalanine methyl ester in 3,000 parts by volume of 75% acetic acid is added 18 parts of palladium black metal catalyst, and the resulting mixture is shaken with hydrogen at atmospheric pressure and room temperature for about 12 hours. The catalyst is removed by filtration, and the solvent is distilled under reduced pressure to afford a solid residue, which is purified by re-crystallization from aqueous ethanol to yield L-aspartyl-L-phenylalanine methyl ester. It displays a double melting point at about 190°C and 245°-247°C. 

Step C Preparation of 2,3-Dich/oro-4-f2-Methylenebutyryl)Phenoxyacetic Acid - The Mannich compound obtained as described above is treated with aqueous sodium bicarbonate to form 2,3-dichloro-4-(2-methYlenebutvrYl)phenoxyacetic acid, MP 115° to 118°C. Two re-crystallizations from a mixture of benzene and cyclohexane give white solid material melting at 118.5° to 120.5°C. 

A solution of 23.7 grams of 2-bromoacetamido-2 -fluorobenzophenone in tetrahydrofuran (100 cc) was added to liquid ammonia (approximately 500 cc) and allowed to evaporate overnight. The residue was treated with water (1 liter) and the crystals filtered off and refluxed in toluene (100 cc) for 30 minutes. The mixture was treated with decolorizing carbon (Norite) and filtered over Hyflo. The solution was concentrated to a small volume (25 cc) cooled, diluted with 20 cc of ether and allowed to stand. The product was re-crystallized from acetone/hexane to give 5-(2-fluorophenyl)3H-1,4-benzodiazepin-2(1 H)-one as white needles melting at 180° to 181°C. 

As described in U.S. Patent 2,507,631, 80 g of pulverized sodium amide are gradually added, while stirring and cooling, to a solution of 117 g of phenyl-acetonitrile and 113 g of 2-chloropyridine in 400 cc of absolute toluene. The mixture is then slowly heated to 110° to 120°C and maintained at this temperature for 1 hour. Water is added thereto after cooling, the toluene solution is shaken with dilute hydrochloric acid and the hydrochloric acid extracts are made alkaline with concentrated caustic soda solution. A solid mass is separated thereby which is taken up in acetic ester and distilled, a-phenyl-a-pvridyl-(2)-acetonitrile passing over at 150° to 155°C under 0.5 mm pressure. When re-crystallized from ethyl acetate it melts at 88° to 89°C, the yield amounting to 135 g. 

Preparation of 2-Chloro-4-Amino-6,7-Dlmethoxyquinazoline To BOO ml of a solution of anhydrous ammonia in tetrahydrofuran at room temperature is added 30 g of 2,4-dichloro-6,7-dimethoxyquina2oline [F.H.S. Curd et a, J. Chem. Soc., p 1759 (194B)]. The mixture is stirred for 44 hours. The precipitate (29 g, MP 267° to 26B°C) is filtered and re-crystallized from methanol to yield 19 g of 2-chloro-4-amino-6,7-dimethoxyquina2oline,... 

After cooling crystals separated from the remaining liquid are collected by filtration and re-crystallized from aqueous ethanol, whereby 2,5 parts by weight of 3-(2, 4, 5 -triethoxybenz-oyl]-propionic acid are obtained as colorless needles, melting point 150°C to 151 °C. 

Balke [55] obtained crystal hydrate of lithium hexafluorotantalate, LiTaF6 H20, by evaporating a solution prepared by the dissolution of Ta205 and Li2C03 in HF. Crystal hydrate of sodium heptafluorotantalate, Na2TaF7-H20, was prepared in the same way [56], while re-crystallization of Na2TaF7-H20 from water yielded sodium octafluorotantalate, Na3TaF8 [29]. 

Crystals of Rb7TaF7 were prepared from relatively diluted solutions of HF, while re-crystallization of rubidium heptafluorotantalate, Rb7TaF7, using a 33% HF solution resulted in the precipitation of rubidium hexafluorotantalate, RbTaFe [56]. Under the same conditions, niobium-containing solutions yielded rubidium oxyfluoroniobate, Rb2NbOF5 [29]. 

RbsNbaOFig is precipitated and re-crystallized from 20-30% HF solutions (Fig. 6). Lower concentrations of HF yield Rb5Nb3C)3F14, while RbNbF6 precipitates at concentrations over 35%. 

The fact that tantalum and niobium complexes form in fluoride solutions not only supplements fundamental data on the coordination chemistry of fluoride compounds, but also has a broad practical importance. This type of solution is widely used in the technology of tantalum and niobium compounds in raw material digestion, liquid-liquid extraction, precipitation and re-pulping of hydroxides, and in the crystallization and re-crystallization of K-salts and other complex fluoride compounds. 

The precipitated precursor can be dissolved and re-crystallized from fluorine-free solutions. This provides excellent conditions for deep purification of the material and reduction of problematic impurities such as titanium, fluorine, etc. Peroxometalates decompose at relatively low temperatures forming tantalum or niobium oxides containing small amount of absorbed water. The absorbed water separation is achieved by further thermal treatment - drying and calcination - of the product ... 

Additional purification of the product and improvement of particle size and shape can be achieved by re-ciystallization. The process consists of sequential dissolutions of potassium heptafluorotantalate in appropriate solutions at increased temperatures, filtration of the solution to separate possible insoluble parts of the product and cooling of the filtrated solution at a certain rate. The precipitated crystals are filtrated, washed and dried to obtain the final product. Re-crystallization can be performed both after filtration of the preliminary precipitated salt or after drying if the quality of the product is not sufficient. HF solutions of low concentrations are usually used for re-ciystallization. In general, even water can be used as a solvent if the process is performed fast enough. Nevertheless, practical experience suggested the use of a 30—40% HF solution within the temperature interval of 80-25°C, and a cooling rate of about 8-10°C per hour. The above conditions enable to achieve an acceptable process yield and good performance of the product. 

Re-crystallization also leads to a significant reduction in the level of carbon and oxygen impurities however, the reduction of cationic impurities is usually not efficient. To prevent co-precipitation of complex fluorotantalates of polyvalent metals, some suitable complexonates are added to the solution. 

An ice-cold solution of a dia/atricydoheptene 1 (0.5-1.5 mmol) in McCN (300 mL) was irradiated with a 30-W low-pressure Hg lamp for 10-15 min. The solvent was removed under reduced pressure and the residue was chromatographed (silica gel. Et20/hexane 1 1) to give the yellow product, which was re-crystallized (i-Pr20). 

Kinetics of Ammonia Synthesis on Re Crystal Surfaces. The kinetics of the ammonia synthesis reaction on Re were studied on the Re(ll50) surface which is composed of atoms having seven-fold coordination. The rate of production at 70 K and 20 atm. ... 

The lamella thickening depends sensitively on the initial lamella thickness as well as on the annealing temperature. We first considered the thickening of very thin lamella of about 18-bonds thick this thickness nearly corresponds to that of the lamella crystallized at 0 K (Fig. 7). The temperature of annealing Ta was taken between 20 K and 150 K a quick jump to a higher temperature resulted in partial melting and re-crystallization and a continuous thickening process could not be observed. 

The color of the a-ketoglutaric acid is darker if the pot temperature goes much above 90° during the evaporation and re-crystallization. 

The product is of satisfactory purity for use in the 4,6,8-tri-methylazulene preparation without further purification. Re-crystallization of a small sample of the 2,4,6-trimethylpyryKurri perchlorate from a seven-fold amount of hot water, containing a few drops of perchloric add and some carbon blade, gives colorless crystals, m.p. 245-247° dec. However, the recrystallization of larger amounts in this way presents some hazard and is not recommended. Concentration of filtrates should be avoided (see Note 4). 

The mother liquors from initial crystallizations and from recrystallizations of LSD can be concentrated by evaporation under reduced pressure to produce additional crops of crystals. The second and third crops of crystals are usually dirty enough to require re-crystallization. After three crops, the mother liquors usually become very syrup-like. They then contain mostly iso-LSD (as the tartrate... 

X-ray studies carried out by Gallezot et al. (46) on a 53 percent EDTA-dealuminated Y zeolite, have shown that the aluminum extraction does not leave any vacancies in the framework after calcination at 400°C in flowing, dry oxygen and nitrogen (46). It was suggested that a local re-crystall-ization of the framework occurs even in the absence of steam. The silica necessary for the process presumably originates in the destroyed surface layers of the crystallite and diffuses into its interior. 

The formation of prepolymer can also be achieved by transesterification of dimethyl terephthalate (DMT) with EG, releasing the by-product methanol. High-purity DMT is easily obtained by distillation and in the early years of PET production, all processes were based on this feedstock. During the late 1960s, highly purified TPA was produced for the first time on an industrial scale by re-crystallization. Since then, more and more processes have shifted to TPA as the feedstock and today more than 70 % of global PET production is based on TPA. The TPA-based PET production saves approximately 8 % of total capital investment and 15% of feedstock cost (Figure 2.1). 

Table 5.38. Mn, Tc, Re crystal structures, lattice parameters of their allotropes and calculated densities.

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