Purification and Crystallization

Some mixtures of organic substances may be separated advantageously by cooling and partial crystallization. The extent of such recovery is limited by the occurrence of eutectic behavior. Examples 16.2 and 16.8 consider such limitations. Sometimes these limitations can be circumvented by additions of other substances that change the phase equilibria or may form easily separated compounds with one of the constituents that are subsequently decomposed for recovery of its constituents. 

Thus the addition of n-pentane to mixtures of p-xylene and m-xylene permits complete separation of the xylenes which form a binary eutectic with 11.8% para. Without the n-pentane, much para is lost in the eutectic, and none of the meta is recoverable in pure form. A detailed description of this process is given by Dale (1981), who calls it extractive crystallization. Other separation processes depend on the formation of high melting molecular compounds or 

Because the handling of solids is difficult, particularly that of soft organic crystals, several crystallization processes have been developed in which sohds do not appear outside the crystallizing equipment, and the product leaves the equipment in molten form. For organic substances, crystalline form and size usually are not of great importance as for products of crystallization from aqueous solutions. If needed, the molten products can be converted into flakes or sprayed powder, or in extreme cases they can be recrystallized out of a solvent. 

The case is that of mixtures of the three isomeric nitrotoluenes for which the equilibrium diagram is shown. Point P on the diagram has the composition 0.885 para, 0.085 meta, and 0.030 ortho. The temperature at which crystals begin to form must be found experimentally or it may be calculated quite closely from the heats and temperatures of fusion by a method described for instance by Walas (Example 8.9, 1985). It cannot be found with the data shown on the diagram. In the present case, incipient freezing is at 46°C, with para coming out at point P on the diagram. As 

Philips Crystallization Process [Fig. 16.15(b)]. The purifying equipment consists of a vessel with a wall filter and a heater at the 

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Crystallization and Purification Solvent. Dimethylacetamide is useful ia the purification by crystallization of aromatic dicarboxyHc acids such as terephthahc acid [100-21-0] and/vcarboxyphenylacetic acid [501-89-3]. These acids are not soluble ia the more common solvents. DMAC and dibasic acids form crystalline complexes containing two moles of the solvent for each mole of acid (16). Microcrystalline hydrocortisone acetate [50-03-3] having low settling rate is prepared by crystallization from an aqueous DMAC solution (17). 

After identifying the optimal etherification conditions, our attention turned to isolation of 18 in diastereomerically pure form. Diastereomers 18 and 19 were not crystalline, but, fortunately, the corresponding carboxylic acid 71 was crystalline. Saponification of the crude etherification reaction mixture of 18 and 19 with NaOH in MeOH resulted in the quantitative formation of carboxylic acids 71 and 72 (17 1) (Scheme 7.22). Since the etherification reaction only proceeded to 75-80% conversion, there still remained starting alcohol 10. Unfortunately, all attempts to fractionally crystallize the desired diastereomer 71 from the crude mixture proved unfruitful. It was reasoned that crystallization and purification of 71 would be possible via an appropriate salt. A screen of a variety of amines was then undertaken. During the screening process it was discovered that when NEt3 was added... 

To a synthetic chemist the concept of a polymer is rather different. The term is often used to describe molecules formed from quite small numbers of monomers units but which resist crystallization and purification owing to the range of chain lengths present. Such materials will normally form brittle solids which are either amorphous or have a low level of crystalline order. 

Since crystallization invariably means more or less purification, even though by no more than the removal of the solvent, the ideas and processes of crystallization and purification are inextricably interwoven and it is impossible to consider crystallization... 

Use Solvent for plastics, resins, gums and electrolytes intermediate catalyst paint remover high purity solvent for crystallization and purification. 

Liquid-liquid extraction (LLX) proved to be a highly attractive separation method for many of these applications due to low consumption of energy and of reagents, operation at about ambient temperature, avoidance of solid crystallization and purification, and reduction of the formation and treatment of by-products. 

Cobalt azide Co(N3)2 shock crystallization and purification should be handled very carefully explodes on reactions with acids susceptible to react with heavy metal salts forming their azides Explodes on heating at 200°C (392°F) susceptible to... 

Hazardous Decomp. Prods. Heated to decomp., emits toxic fumes of NOx NFPA Health 2, Flammability 2, Reactivity 0 Uses Solvent for plastics (vinyl, acrylic, cellulose, polyimide processing), resins, gums, fibers, coatings, adhesives, electrolytes, pharmaceuticals selective solvent for butadiene extraction reagent intermediate catalyst paint remover high-purity solvent for crystallization and purification reaction medium for prod, of pharmaceuticals, plasticizers... 

Salt switching step to aid in crystallization and purification (sodium to calcium) (step 5)... 

Calix[10-12]arenes Lamartine and coworkers [17] have obtained the p-tert-butylcalix[10 and 12]arenes C[10] and C[12], through a cyclocondensation of linear phenolic ohgomers (a 6 + 3 procedure), under basic conditions. These macrocycles were isolated even up to half-gram scale by usual work-up (several crystallizations and purifications by column cromatography). 

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