Crystallization procedures

The visible crystals that develop during a crystallization procedure are built up as a result of growth either on nuclei of the material itself or surfaces of foreign material serving the same purpose. Neglecting for the moment the matter of impurities, nucleation theory provides an explanation for certain qualitative observations in the case of solutions. 

Sulfates and nitrates are known and in all cases they decompose to the oxides on heating. Double sulfates of the type M (S04)3.3Na2S04.12H20 can be prepared, and La (unlike Sc and Y) forms a double nitrate, La(N03)3,2NH4N03.4H20, which is of the type once used extensively in fractional crystallization procedures for separating individual lanthanides. 

The yield of this crystallization procedure is 95.7%. The above process provides the7j modification of 2,6-dihydroxymethylpyrldine-bis(N-methylcarbamate), which can be tabletted directly. The substance melts at 134°C to 1 36°C,its purity is99S% (determined by UV spectrophotometry). 

The enantiomeric excess which is achieved for a given ally carbamate is independent of the carbonyl compound used it reflects the skill of the operator in the crystallization procedure. The high degree of reagent-controlled chirality transfer is also obvious from the reaction with either enantiomer of 2-benzyloxypropanal103a 107a. 

In the fourth section the influence of comonomeric units on the polymorphic behavior of some polymers is described. In particular it is shown that comonomeric units can produce changes in the solid-solid transition temperatures as well as variations of the crystalline forms, which can be obtained for a given crystallization procedure. In the same section, some recent studies, showing that the polymorphic behavior of some polymers can be altered in particular miscible blends, are also reviewed. 

To obtain a uniform product from lot to lot, strict adherence to the procedures developed for a particular crystallization must be followed, including control of pH, rates of addition, solvent concentrations and purity, temperature, and mixing rates. Each crystallization procedure has to be designed to ensure sterility... 

Fig. 4 Crystallization procedure indicated by T against time t. ECSCs or FCCs once melted are kept at Tmax for 5 min and then isothermally crystallized at a Tc...

During crystallization procedures, undesirable oiling-out (i.e. liquid-liquid demixing) may occur. A UV-vis study of such systems with highly dispersed vesicles, or crystals, in transmission mode will capture... 

The e.s.r. spectra obtained for several oxovanadium (iv) complexes, including [VO(acac)2], orientated in nematic glasses, compare favourably with those obtained by dilute single-crystal procedures.X-Band e.s.r. spectra of magnetically dilute glasses at 77 K show that EtOH and py form weak complexes with [VO(P-dik)] molecules [P-dik = acac, bzac, tropolone, 3-bromotropolone, (35), or (36)], and that vanadium(iv) in solutions of these... 

Evaporation of the xylene filtrate and trituration of the residue with methylene chloride yields ca. 11 g of solid which consists mainly of cyclic hexamer and cyclic tetramer. The methylene chloride contains inter alia, cyclic pentamer, cyclic heptamer, and bishomo compound, and these can be obtained as pure samples in low yield by fractional crystallization procedures. The compositon of the reaction mixtures can be qualitatively established by TLC by means of the following values in 9 1 petroleum... 

The submitter obtained the product in 89% yield (one crop) by using this crystallization procedure. 

In one run (5.0-mmol scale), the checkers obtained a crude product that contained ca. 40% of recovered 1. Attempts to purify the aldol product from this mixture by using the described crystallization procedure was unsuccessful. Accordingly, the crude product (2.36 g) was purified by flash chromatography on 170 g of silica gel using 1000 ml of a 9 1 1 mixture of hexane, ethyl acetate and methylene chloride. This provided 0.72 g of starting ester 1, 1.39 g of pure aldol 2, and 0.259 g of mixed fractions containing 2 and the minor aldol diastereoisomer (4 1 by H NMR analysis). 

The (S)-amine 23 was synthesized after 1 day and the enzyme was recovered from the reaction by filtration, then washed and reused. The residual amine was extracted from the MTBE solution with dilute phosphoric acid (pH 2.9-4.4). The aqueous extract was concentrated to give a phosphate salt of the (S)-sec-butylamine with 99.7%. This was followed by a crystallization procedure in ethanol to give 99.8% and 50% yield. 

Synthesis of MCM-41 with Additives. The hydrothermal crystallization procedure as described earlier [10] was modified by adding additional salts like tetraalkylammonium (TAA+) bromide or alkali bromides to the synthesis gel [11]. Sodium silicate solution ( 14% NaOH, 27% Si02) was used as the silicon source. Cetyltrimethylammonium (CTA) bromide was used as the surfactant (Cl6). Other surfactants like octadecylltrimethylammonium (ODA) bromide (C,8), myristyltrimethylammonium (MTA) bromide (C,4) were also used to get MCM-41 structures with different pore diameter. Different tetralkylammonium or alkali halide salts were dissolved in little water and added to the gel before addition of the silica source. The final gel mixture was stirred for 2 h at room temperature and then transferred into polypropylene bottles and statically heated at 100°C for 4 days under autogeneous pressure. The final solid material obtained was washed with plenty of water, dried and calcined (heating rate l°C/min) at 560°C for 6 h. 

The crystallization procedure employed by Pasteur for his classical resolution of ( )-tartaric acid (Section 5-1C) has been successful only in a very few cases. This procedure depends on the formation of individual crystals of each enantiomer. Thus if the crystallization of sodium ammonium tartrate is carried out below 27°, the usual racemate salt does not form a mixture of crystals of the (+) and (—) salts forms instead. The two different kinds of crystals, which are related as an object to its mirror image, can be separated manually with the aid of a microscope and subsequently may be converted to the tartaric acid enantiomers by strong acid. A variation on this method of resolution is the seeding of a saturated solution of a racemic mixture with crystals of one pure enantiomer in the hope of causing crystallization of just that one enantiomer, thereby leaving the other in solution. Unfortunately, very few practical resolutions have been achieved in this way. 

The high stability of the aluminate ion allows the production of concentrated solutions of aluminum with the virtual exclusion of the main metallic impurity, viz. iron as an oxide residue. The resultant impure aluminate solution is clarified and its temperature reduced when the reverse of the above reaction occurs with the formation of A1203,3H20 by a slow crystallization procedure. The high-purity alumina trihydrate product is calcined and then reduced electrochemically in a molten fluoride bath by the well-known Hall-Heroult process. The major problems in the Bayer process have their origin in the coordination chemistry of aluminum in alkaline solutions. The... 

Akzo has been instrumental in developing a new process for the stereospecific synthesis of trans- 1,4-cyclohexane diisocyanate [7517-76-2] (21). This process, based on the conversion of polyethylene terephthalate) [25038-59-9], circumvents the elaborate fractional crystallization procedures required for the existing -phenylenediamine [108-45-2] approaches. The synthesis starts with polyethylene terephthalate) (PET) (32) or phthalic acid, which is converted to the dimethyl ester and hydrogenated to yield the cyclohexane-based diester (33). Subsequent reaction of the ester with ammonia provides the... 

The stereochemistry of the pentakisisocyanidecobalt(I) and (II) complexes is apparently a function of crystallization procedures. To date, four isomeric structures have been identified for isocyanide complexes of co-balt(l) (121) and three for those of cobalt(II) (284). Crystal structure determinations of [Co(CNPh)5]C104 CHCl3 (285) and [Co(CNPh)5]-(CI04)2 C1CH2CH2C1 (284) have shown the coordination around the cobalt to be square pyramidal, whereas with [Co(CNC6H4Me-p)5][Co(NM A)3] (NMA = nitromalonaldehyde) a trigonal bipyramidal structure was found for the cation (286). 

In this process, the readily soluble co-existing salt is in equilibrium with the target salts, and acts as a buffer to the supersaturation of the opposite enantiomer. Thus, we can carry out the preferential crystallization procedure more easily and effectively. For instance, a-methylbenzylamine cinnamic acid salt can be efficiently resolved by adding hydrochloric acid salt of the racemic amine as a co-existing salt. 

A solution containing 442 g of N,N-dimethyl-3-phenyl-3-hydroxypropylamine in 5 L of chloroform was saturated with dry gaseous hydrogen chloride. 400 ml of thionyl chloride were then added to the chloroform solution at a rate sufficient to maintain reflux. The solution was refluxed an additional 5 h. Evaporation of the chloroform and other volatile constituents in vacuo yielded N,N-dimethyl-3-phenyl-3-chloropropylamine hydrochloride which was collected by filtration, and the filter cake washed twice with 1500 ml portions of acetone. The washed crystals weighed about 500 g and melted at 181°-183°C with decomposition. An additional 30 g of compound were obtained from the acetone wash by standard crystallization procedures. The structure of the above compound was verified by NMR and titration. 

Compound 50c was obtained in ca. 25% yield as a precipitate from the acid-catalyzed condensation of pyrogallol and isovaleraldehyde. No evidence of any hexamer was found in the solid material. To convert this material into the hexamer (50c)6, the original precipitate can be dissolved in Et20, acetone, or methanol, with a few drops of nitrobenzene or o-dinitrobenzene, followed by crystallization upon slow evaporation. The hexamer may also be obtained by thermal treatment of the initial precipitate or the initial filtrate. The product in the initial filtrate may be converted into hexamer by extraction in Et20, followed by evaporation to dryness with subsequent dissolution in methanol. The methanol solution is then heated to 120-150 °C for at least 12 h. Methanol may be removed under vacuum to yield a red-brown solid. Colorless hexameric spherical capsules are obtained from this solid utilizing the crystallization procedure described for the initial precipitate. 

Thus all four geometrical isomers of 1,4-disubstituted 1,3-dienes can be readily prepared starting from two terminal acetylene units. The Z,Z isomer is available via Cadiot-Chodkiewicz coupling (e.g. Figure 12) both the j3,J3 and 13,j3 isomers can be prepared via the procedure in Figure 17 and the JS.JL isomer can be isolated from the equilibration-crystallization procedure described above. 

A further danger in this method is that crystallization procedures can be fairly arbitrary. For example, using (—)-strychnine as resolving agent,... 

Johnson and Mead (148) obtained, using different crystallization procedures, less soluble salts containing (+) or (—) trisoxalatochromate(III). 

Recent improvements in the details of the CLP crystallization procedure and crystal treatment have led to even better diffracting crystals. At 1.4 A resolution, refinement, including anisotropic B factors, illustrates, the thermal motion anisotropy of the retinal and key neighbors (Fig. 4, see color insert manuscript in preparation). 

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