Crystallization examples

The formation of acyl halide-Lewis acid complexes have been observed by several methods. For example, both 1 1 and 1 2 complexes of acetyl chloride, with AICI3 can be observed by NMR spectroscopy. The existence of acylium ions has been demonstrated by X-ray diffraction studies on crystalline salts. For example, crystal structure determinations have been reported for /i-methylphenylacylium and acetylium ions as SbFg salts. There is also a good deal of evidence from NMR measurements which demonstrates that acylium ions can exist in nonnucleophilic solvents. " The positive charge on acylium ions is delocalized onto the oxygen atom. This delocalization is demonstrated in particular by the short O—C bond lengths in acylium ions, which imply a major contribution from the structure having a triple bond ... 

From studies in solution and in the sohd state, a number of structures both sheet-like [14] and helical [19, 21, 24—36] have been proposed over the years for these polymers. Poly(/9-alanine) 3 for example crystallizes as extended chains [14]... 

Example Crystallization of 5 OEt Photoirradiation Chemical yield of dimer/% e.e. %... 

Though molecules l and 7 are closely connected in structure, they have totally different host properties, i.e. 1 readily forms inclusion compounds with a wide variety of guests (see Sect. 3.2.2) while 7 does not. For example, crystals of the pure host could be obtained from dimethylformamide, a solvent which is tightly held by /. Reasons for the different behavior of 1 and 7 have already been mentioned when the crystal structure of the free host 7 was discussed (Sect. 4.1). However, the ability of 7 to form a crystalline associate is increased, if a solvent with the property of a base is present, e.g. pyridine and substituted derivatives of pyridine (see Table 14)80). 

The distilling flask, collection flask, and column are continuously evacuated with a high-vacuum system. When the bulk of the pentane and residual hexane have distilled away, the temperature of a silicone oil bath surrounding the distillation flask is raised from room temperature to 125° over about a one-hour period. When the distillation rate diminishes appreciably, the temperature is slowly raised to 150° and maintained there until no more liquid is obtained. The distillation flask is cooled to room temperature before air is admitted to the system. The distilled product weighs 86.5 g. (89% yield checkers report 89 %t) and melts at 32 to 32.5°. In this state of purity, (bromomethyl)-carborane is suitable for most uses, but it may be further purified by crystallization from pentane or methanol. For example, crystals obtained by chilling a solution of 86.5 g. of the car-... 

The systems with several types of interfaces with different surface tensions [Pg.262]

An early application was to the pathway for conformational isomerization of molecules Ar3Z, with three aromatic rings on the same centre (Mislow, 1976). Typically the system is pyramidal (tetrahedral overall where there is a fourth substituent on Z), and the rings are close enough in space that they cannot rotate independently about the Z-Ar bond. Triphenylphosphine oxide, to take a specific example, crystallizes in a propeller conformation [4 Z = P=OJ which is chiral, with all three benzene rings rotated in the same sense from the relevant C-P-O plane. A study (Bye et al., 1982) of deformations from this geometry for more than 1000 related structures in various environments allowed a detailed description of the pathway for... 

In other cases all of the products allowed by the arrangement in the crystal are indeed formed. Sorbic acid, 81, for example, crystallizes in a structure in which there are both close head-to-tail and close head-to-head molecular juxtapositions. All three possible photodimers, 82 to 84 are formed (130). 

In simple experiments, particulate silica-supported CSPs having various cin-chonan carbamate selectors immobilized to the surface were employed in an enantioselective liquid-solid batch extraction process for the enantioselective enrichment of the weak binding enantiomer of amino acid derivatives in the liquid phase (methanol-0.1M ammonium acetate buffer pH 6) and the stronger binding enantiomer in the solid phase [64]. For example, when a CSP with the 6>-9-(tcrt-butylcarbamoyl)-6 -neopentoxy-cinchonidine selector was employed at an about 10-fold molar excess as related to the DNB-Leu selectand which was dissolved as a racemate in the liquid phase specified earlier, an enantiomeric excess of 89% could be measured in the supernatant after a single extraction step (i.e., a single equilibration step). This corresponds to an enantioselectivity factor of 17.7 (a-value in HPLC amounted to 31.7). Such a batch extraction method could serve as enrichment technique in hybrid processes such as in combination with, for example, crystallization. In the presented study, it was however used for screening of the enantiomer separation power of a series of CSPs. 

Organic solids have received much attention in the last 10 to 15 years especially because of possible technological applications. Typically important aspects of these solids are superconductivity (of quasi one-dimensional materials), photoconducting properties in relation to commercial photocopying processes and photochemical transformations in the solid state. In organic solids formed by nonpolar molecules, cohesion in the solid state is mainly due to van der Waals forces. Because of the relatively weak nature of the cohesive forces, organic crystals as a class are soft and low melting. Nonpolar aliphatic hydrocarbons tend to crystallize in approximately close-packed structures because of the nondirectional character of van der Waals forces. Methane above 22 K, for example, crystallizes in a cubic close-packed structure where the molecules exhibit considerable rotation. The intermolecular C—C distance is 4.1 A, similar to the van der Waals bonds present in krypton (3.82 A) and xenon (4.0 A). Such close-packed structures are not found in molecular crystals of polar molecules. 

A few examples in which aromatic cation radicals have been isolated as crystalline salts, actually consist of mixed valence units. For example, crystal structure analysis showed that the naphthalene radical cation (NAP) + forms a mixed valence dimer (NAPy4 in which the two components are arranged face to face in n-stacks with an interplanar separation significantly closer than van der Waals contacts. Such an intermolecular organization arises from the sponta-... 

The crystallization tendency of polycarbonate is enhanced by the action of solvents. For example, crystallization may be accomplished by slow evaporation of solvent from cast film (14, 17) or by treatment with swelling agents such as ethyl acetate or acetone. 

It is certain that the proton does not exist by itself in water. The simplest formula found in some crystalline salts is H30+. For example, crystals of perchloric acid monohydrate contain pyramidal hydronium (also called hydroxonium) ions ... 

Short peptides three to five residues in length that contain one or two APhe residues adopt a fJ-bend 45"48 and 310-helical folding. For example, crystal structure studies of a three-residue peptide containing two APhe residues, Ac-APhe-Ala-APhe-NHMe, 49 and a five-residue peptide with one APhe residue, Boc-Leu-Phe-Ala-APhe-Leu-OMe, 50 show the presence of two consecutive (3-bends, corresponding to an incipient 310-helix in both peptides. Such didehydropeptides exist in both screw senses and the spacing between APhe residues does not seem to affect the type of helix formed. 51,52 ... 

Which of the following pairs of compounds can be separated by physical methods (for example, crystallization or distillation) ... 

With a sequence alignment and secondary structure, one can reasonably assess the length requirement for each helix, as this is an important parameter in creating an initial library of variants. For example, crystals of a k... 

Many processes involve criteria other than solids suspension, for example, crystallization, precipitation, and many types of leaching and chemical reactions. In crystallization, the shear rate around the impeller and other mixing variables can affect the rate of nucleation, and can affect the ultimate particle size. In some cases, the shear rate can be such that it can break down forces within the solid particle and can affect the ultimate particle size and shape. There are some very fragile precipitate crystals that are very much affected by the mixer variables. 

Crystals interact with molecules of the environment via the surfaces that delineate them. Consequently, several of their properties, such as their morphology, structure and symmetry of solid-solutions and their etch-pit patterns formed upon partial dissolution, depend on an interplay between the surface structures of the crystal faces and the composition of the solution. For example, crystallization of a racemate undergoing spontaneous resolution in the... 

In general, because the value of a crystal property depends on the direction of measurement, the crystal is described as anisotropic with respect to that property. There are exceptions for example, crystals having cubic symmetry are optically isotropic although they are anisotropic with respect to elasticity. For these reasons, a description of the physical behaviour of a material has to be based on a knowledge of crystal structure. Full descriptions of crystal systems are available in many texts and here we shall note only those aspects of particular... 

Be this as it may, lithium attempts to bind to several bonding partners the structural consequences for the enolates of a ketone, an ester, and an amide are shown in Figure 13.2 In contrast to the usual notation, these enolates are not monomers at all The heteroatom that carries the negative charge in the enolate resonance form is an excellent bonding partner such that several of these heteroatoms are connected to every lithium atom. Lithium enolates often result in tetramers if they are crystallized in the absence of other lithium salts and in the absence of other suitable neutral donors. The lithium enolate of fert-butyl methyl ketone, for example, crystallizes from THF in the form shown in Figure 13.3. 

Unlike the racemic pair ( + )A( + )B and ( —)A( —)B, the diastereoisomeric pair (4-)A( + )B and (+ )A( — )B can also be separated by simple processes (which do not involve chiral agents or influences)—for example, crystallization or chromatography. This is the basis for Pasteur s resolution of ( )-tartaric acid and for countless other similar resolutions. (For a recent account of strategies in optical resolutions, see [30].)... 

These enzymes utilize L-glutamate and the 4-pro-S position of the coenzyme [38,39]. Crystals have been obtained in many cases (summarized in [95]). Those from mammalian liver were not suitable for structure determination. For example, crystals of the rat liver enzyme had a large unit cell, and were unstable in the X-ray beam [96], The NADP-dependent enzyme from Neurospora crassa is smaller than the mammalian enzyme, and less complicated with respect to allosteric and aggregation effects. Nevertheless, carefully grown crystals did not provide X-ray diffraction patterns favourable for structure determination (A.C.T. North, personal communication). At present, best hopes reside in crystals of the yellowfin tuna liver enzyme [97]. 

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