Mixture mechanical, crystallization

Add 20 g. of /)-bromoaniline to 20 ml. of water in a 250 ml. beaker, and warm the mixture until the amine melts. Now add 23 ml. of concentrated hydrochloric acid and without delay stir the mixture mechanically in an ice-water bath, so that a paste of fine /> bromo-aniline hydrochloride crystals separates. Maintain the temperature of the stirred mixture at about 5° whilst slowly adding from a dropping-funnel a solution of 8 5 g. of sodium nitrite in 20 ml. of water con tinue the stirring for 20 minutes after the complete addition of the nitrite. 

Crystallization methods are widely used for the separation, or resolution, of enantiomer pairs. Enantiomer mixtures may essentially crystallize in two different ways. In around 8 per cent of cases, each enantiomer crystallizes separately, giving rise to a mechanical mixture of crystals of the two forms, known as a conglomerate. Conglomerates may usually be separated by physical methods... 

MASSECUITE A mechanical mixture of crystals in mother liquor (molasses or heavy syrup), produced in a vacuum pan. The suspension of sugar crystals in mother liquor produced during the early stages of crystallization. 

Dissolve 0-01 g. equivalent of the amino acid in 20 ml. of N sodium hydroxide solution and add a solution of 2 g. of p-toluenesulphonyl chloride in 25 ml. of ether shake the mixture mechanically or stir vigorously for 3-4 hours. Separate the ether layer acidify the aqueous layer to Congo red with dilute hydrochloric acid. The derivative usually crystaUises out rapidly or will do so on standing in ice. Filter off the crystals and recrystallise from 4 5 ml. of 60 per cent, alcohol. 

It is advisable to scratch the flask with a glass rod after three or four hours. This usually initiates precipitation of the addition product in a finely divided form. If this is not done, the addition product may crystallize slowly on the sides of the flask in a cake which is removed only with the greatest difficulty. It is also advantageous to stir the mixture mechanically for several hours after crystallization begins in order to pre tent caking. 

In 1969 Calvin [64] proposed a scheme for autocatalytic symmetry breaking, which he called stereospecific autocatalysis . Calvin s mechanism has been validated experimentally in the context of the total spontaneous resolution during the crystallization of racemic mixtures. During crystallization, crystals of one enantiomer may spontaneously separate, leaving the other enantiomer in solution. If the possibility of the equilibration of the enantiomers in solution exists and if the enantiomer in solution can convert rapidly to the enantiomer that is crystallizing before crystallization is complete, then the entire racemate may deposit as a single enantiomer. At least half a dozen examples of Calvin s stereospecific autocatalysis involving such... 

The rhodium-entrapped cage compound which is formed using a stoichiometric amount of [RhCl(CO)2]2 is a notable paradigm of the rhodium-catalyzed [2-I-2-I-1] al-kyne-alkyne and CO coupling [35]. Heating 57 in acetone at 50 °C for 8 h or irradiation by a tungsten or mercury lamp provided the cage compound in 50% yield based on NMR spectroscopy. However, due to mechanical losses it was isolated in only 16% yield from the reaction mixture, by crystallization as the hexafluorophosphate salt 58 (Eq. 13). 

Conversion to /V,/V-diphenylpicrylhydrazyl. Dissolve 3.95 g (0.01 mol) of the above hydrazine in 60 ml of dry dichloromethane, add 50 g of lead dioxide and 4 g of anhydrous sodium sulphate and shake the mixture mechanically for 1 hour. Filter, and concentrate the deep-violet filtrate on a rotary evaporator. Dilute the residual solution with two volumes of ether, and allow the product to crystallise. Filter off the large black-violet crystals of diphenylpicrylhydrazyl and wash them with ether the yield is 3.5 g (89%), m.p. 137-138°C. The product may be recrystallised from a mixture of dichloromethane and ether. 

After leaving the reaction mixture rest for a few days, a mixture of crystals were precipitated. After their mechanic separation, the existence of nitrito-N (846) and nitrito-O (847) coordinated isomers was proved by x-ray diffraction. 

Taraban E.A., Krivomchko O.P., Buyanov R.A. et al. Dehydration reactions and solid-state interactions in thermal treatment of mechanical mixtures of crystal hydroxides Al" with hydroxides Zn" Zhum. Neorgan. Khim. 1988 33 1824-28. 

A brief description of the type of "racemic" compounds is necessary for the reader to better understand the principles behind the application of crystallization methods to the separation of enantiomers. Three fundamental types of crystalline racemates exist. In the first, the crystalline racemate is a conglomerate, which exists as a mechanical mixture of crystals of two pure enantiomers. The second, which is the most common, consists of the two enantiomers in equal proportions in a well-defined arrangement within the crystal lattice this is termed racemic compound. The third possibility occurs with the formation of a solid solution between the two enantiomers that coexist in an unordered manner in the crystal. This kind of racemate is called a pseu-... 

Enantiomers are stereoisomers, which are mirror images of eath another. An equimolecular mixture of two enantiomers is called a racemate. Crystalline race-mates occur in three different types. The first is termed a conglomerate, that is, a mechanical mixture of crystals of pure enantiomers that is formed from two solid phases. The most common type is the racemic compound, which consists only of one crystalline phase in which the two enantiomers are present in equal quantities. The third type is the pseudoracemate in which a solid solution of the two enantiomers is present. 

A solid mixture is any mixture of crystals of different substances, regardless of its solid-state structure. It may be a simple mechanical mixture of component crystals, a mixed crystal, a molecular complex, or an ionic salt. In this chapter, a simple mechanical mixture as well as all uncharacterized two-component crystals are grouped under solid mixtures. 

For example, each batch of crystals from cocrystallization can be examined under a polarizing microscope to determine whether the crystals have homogeneous morphologies or whether mixtures of crystals are present. Solution NMR results and elemental analyses can give the stoichiometry of components. Melting points of complexes are usually sharper than those of mixtures of components. A clearer distinction can be made from measurements of PXD, solid-state C NMR, IR and UV, and DSC spectra. Because the spectral patterns for a simple mechanical mixture are composites of those for each component, observation of new peaks in these spectra indicates the formation of a molecular complex. When a molecular complex is available as a single crystal of sufficient size and quality, its crystal structure and miolecular conformation can be unequivocally determined by single-crystal X-ray diffraction [4-11,13],... 

A conglomerate is an equimolecular, mechanical mixture of crystals of the two enantiomers. Thus, one single enantiomer is present in the unit cell. The principal space groups (up to 90%) in which conglomerates are found are P2i2i2i, P2i, C2 and PL... 

The separation of a racemic mixture of enantiomers, or racemate, is a process known as resolution. The first resolution of a pair of enantiomers was achieved by Pasteur in 1848 who observed the racemic conglomerate of a sodium ammonium salt of tartaric acid (4, Fig. 4.1). This is an equimolar mechanical mixture of crystals where each crystal only contains a single enantiomer. The enantiomorphous crystals could be observed under magnification and separated manually by hand. Examples of this type are not very common and the separation is labour intensive. However, the principle has been used in the development of classical resolution. 

In the context of the crystalline solid state, a mechanical mixture (or physical mixture) is a mixture of crystals or crystallites of different phases (i.e., constituting different crystal structures) that are separated by grain boundaries. 

Using a simple but informative scheme (Figure 9), Bishop and Scudder outlined four possible outcomes of allowing a 1 1 solution of a racemic mixture to crystallize. Scenario iv results in a mechanical mixture of enan-tiomorphous racemic crystals in which the enantiomers are not related to each other by crystallographic symmetry. Therefore, even though each crystal contains an equal mixture of enantiomers (i.e., dissolution of one crystal does not produce an optically active solution), the molecules must necessarily pack in accordance with one of the 65 Sohncke space groups. Bishop and Scudder called the mechanical mixture of enantiomorphous racemic crystals a false conglomerate, but in a subsequent article, they pointed out that the term kryptoracemate (i.e., hidden racemate. 

PP thus strength and heat resistance depend on the PE content [63,64]. Blends of PE and PP were immiscible in either the amorphous or the crystalline phase [65]. The two polymers tend to form mixtures of crystal structures, and each affects the crystallization of the other [66-70]. Studies on mechanical properties gave mixed results. Improvements in modulus, ultimate tensile strength, and heat deflection temperature [71] suggested good binding in the amorphous interphase [72] but use of HDPE to improve low temperature impact strength and environmental stress rack resistance required a compatibilizer such as 5% of ethylene-propylene rubber [42]. 

In some instances, the separation thus afforded leads immediately to a pure substance because one of the phases at equilibrium contains only one constituent. For example, the equilibrium vapor in contact with a liquid aqueous salt solution contains no salt regardless of the concentration of the liquid. Similarly the equilibrium solid in contact with such a liquid salt solution is either pure water or pure salt, depending upon which side of the eutectic composition the liquid happens to be. Starting with the liquid solution, one can then obtain a complete separation by boiling off the water. Alternatively, pure salt or pure water can be produced by partly freezing the solution or, in principle at least, both can be obtained pure by complete solidification followed by mechanical separation of the eutectic mixture of crystals. In cases like these, when the two phases are first formed, they are immediately at their final equilibrium compositions and the establishment of equilibrium is not a time-dependent process. Such separations, with one exception, are not normally considered to be among the mass-transfer operations. 

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