Fractional crystallization, defined

The dependence of the solubility of a solid on temperature varies considerably, as shown in Figure 9.9. For example, the solubility of KNO3, for example, increases sharply with temperature, while the solubilities of NaBr and NaCl change very little. This wide variation makes it possible to obtain pure substances from mixtures through the process of fractional crystallization, defined as the separation of a mixture of substances into pure components on the basis of their differing solubilities. 

Dry fractionation is defined as crystallization from a melt without dilution with solvent (O Brien, 1998 Anderson, 1996 Bailey, 1950). This process is the simplest and least expensive process for separating high and low-MP fatty derivatives (Illingworth, 2002 Kellens and Hendrix, 2000 O Brien, 1998 Anderson, 1996 Krishnamurthy and Kellens, 1996). It is also the most commonly practiced form of fat fractionation technologies currently in use (Illingworth, 2002 O Brien, 1998). 

Preparation. A number of methods have been reported for both the racemic and asymmetric preparations of l-amino-2,3-dihydro-lH-inden-2-ol (1), most commonly starting from inexpensive and readily available indene. These methods have been described in detail in recent reviews. The valuable properties of 1 as both a component of a medicinally active compound and as a chirality control element, derive primarily from its rigid and well-defined stereochemical structure. As a result, the compound is most desirable in enantiomerically pure form. One of the most efficient asymmetric syntheses of 1, which may be employed for the synthesis of either enantiomer of the target molecule, involves an asymmetric epoxidation (89% yield, 88% ee) of indene to give epoxide 2 using the well-established Jacobsen catalyst. This is followed by a Ritter reaction using oleum in acetonitrile resulting in conversion to the oxazoline (3) which is subsequently hydrolysed to the amino alcohol. Fractional crystallization with a homochiral diacid permits purification to >99% ee (eq 1). ... 

Most attention will be paid here to the well-defined series (PNCU) and related halides. The relative proportions of the individual chloride polymers in the soluble fraction vary with reaction conditions, but are, typically, trimer 40-45%, tetramer 15-20%, pentamer 15-20%, hexamer 1-3%, heptamer and upwards 25% (average). They may be separated by a combination of fractional crystallization and fractional distillation in vacuo. This method was first used by Stokes (89), who isolated the trimer to hexamer as crystalline solids, and the heptamer as an oil. The undistillable residue obtained after removal of the heptamer consists of a mixture of polymers, approximately (PNCI2) 10-11 This has similar properties to the pure compounds of lower molecular weight, and is probably composed of molecules with similar structures. 

Optical isomerism is concerned with chirality, and some important terms relating to chiral complexes are defined in Box 19.2. The simplest case of optical isomerism among fi -block complexes involves a metal ion surrounded by three didentate ligands, for example [Cr(acac)3] or [Co(en)3] (Figures 3.16b and 19.12). These are examples of tris-chelate complexes. Pairs of enantiomers such as A-and A-[Cr(acac)3] or A- and A-[Co(en)3]Cl3 differ only in their action on polarized hght. However, for ionic complexes such as [Co(en)3], there is the opportunity to form salts with a chiral counter-ion A. These salts now contain two different types of chirality the A- or A-chirality at the metal centre and the (-I-) or (—) chirality of the anion. Four combinations are possible of which the pair (A-(- -) and A-(—) is enantiomeric as is the pair A-(—) and A-(- -). However, with a given anion chirality, the pair of salts A-(—) and A-(—) are diastereomers (see Box 19.2) and may differ in the packing of the ions in the solid state, and separation by fractional crystallization is often possible. 

A variety of 3-D superlattices formed from metal nanopartides has been reported [1-8]. For instance, gold nanopartides, when stabilized by alkylthiolates RS (R = n-Ci2H2s) and rather narrowly distributed in size, could be obtained by the repeated fractional crystallization of a mixture of 1.5-3.5 nm particles from unpolar solvents. The success of fractionation was controlled by mass spectroscopy [6j. Based on these data, definite numbers of atoms could be assigned to the particles of distinct fractions (e.g., 140, 225, 314, and 459), all of which consisted of fee-structured octahedra. Solutions of these remarkably well-defined particles can be used to... 

This reaction was first reported by Marckwald in 1904. It is the synthesis of chiral L-valeric acid (a-methyl propanoic acid) from the pyrolysis of brucine salt of racemic o -methyl-o -ethylmalonic acid. Therefore, it is generally known as the Marckwald asymmetric synthesis. Occasionally, it is also referred to as the Marckwald method. In this reaction, the brucine salts of racemic a-methyl-a-ethylmalonic acid essentially exist as a pair of diastereomers that are separated by fractional crystallization the one with lower solubility is isolated. Upon pyrolysis of such crystalline salt at 170°C, the corresponding brucine salt of L-valeric acid forms upon decarboxylation, resulting in a 10% e.e. In addition, Marckwald defined the asymmetric synthesis as reactions that produce optically active molecules from symmetrically constituted compounds with the use of optically active materials and exclusion of any analytical processes, such as resolution. However, this work was challenged as not being a trae asymmetric synthesis because the procedure was similar to that of Pasteur. In fact, the If actional crystallization of the diastereomers is a resolution process. This process is used as base for many other preparations of chiral molecules, such as tartaric acid and under its influence, the kinetic resolution and tme asymmetric synthesis have been developed in modem organic synthesis. The asymmetric synthesis has been redefined by Morrison and Mosher as the reaction by which an achiral unit of the substrate is converted into a chiral unit in such a manner that the two resulting stereoisomers are produced in unequal amounts. ... 

Fig. 13.9 The stratigraphic variations of the major rockforming minerals of the Vanda hypersthene sill define a central mafic zone, an overlying felsic zone, and a pegmatoid zone near the top of the sill. The pegmatoid zone partly overlaps the felsic zone and includes a thin layer at about 270 m above the base where the pegmatoid assemblage reaches 44.5%. The stratigraphic variation of the abundances of the rock-forming minerals is evidence for the internal differentiation of the Vanda sill by fractional crystallization (Data from Gunn 1966)...

A direct measurement of surface tension is sometimes possible from the work of cleaving a crystal. Mica, in particular, has such a well-defined cleavage plane that it can be split into large sheets of fractional millimeter thickness. Orowan... 

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