Simple Recrystallization

Simple Recrystallization.—In the simplest procedure of recrystallization, the salt is dissolved in some appropriate hot solvent, and the solution is then allowed to cool to room temperature. All the salt in excess of the weight soluble at the lower temperature crystallizes and may be filtered from the mother liquor. Insoluble impurities remain undissolved and may be removed by filtering the hot solution before crystallization begins soluble impurities remain in the solution from which the crystals separate. It is evident that this simplest process applies to the purification of salts that have a decided difference of solubility at different temperatures to cases in which the soluble impurity is not too abundant, or too sparingly soluble and to cases in which 

Before proceeding with the recrystallization of a given salt, the student should look up its solubility (expressed 

In general, it is not desirable to obtain large crystals, since no two crops will look alike. The best preparation will consist of small crystals not larger than a grain of wheat, as uniform in size as possible, not too much matted together, but not necessarily of perfect form. 

References on Solubility Comev and Hahn, A Dictionary of Chemical Solubilities (Inorganic) Seidell, Solubilities of Inorganic and Organic Compounds Landolt-Bornstein, Physikalisch-Chemische Tabellen International Critical Tables. 

Notes of Filtration.—From the outset, the student should cultivate proper habits in regard to filtration. 

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A related palladium(O)-catalyzed epimerization of y-aziridinyl-a,P-enoates 244 was also reported by Ibuka, Ohno, Fujii, and coworkers (Scheme 2.60) [43]. Treatment of either isomer of 244 with a catalytic amount of Pd(PPh3)4 in THF yielded an equilibrated mixture in which the isomer 246 with the desired configuration predominated (246 other isomers = 85 15 to 94 6). In most cases the isomer 246 could be easily separated from the diastereomeric mixture by a simple recrystallization, and the organocopper-mediated ring-opening reaction of 246 directly afforded L,L-type (E)-alkene dipeptide isosteres 243. 

Subsequent to the development of the (salen)Cr-catalyzed desymmetrization of meso-epoxides with azide (Scheme 7.3), Jacobsen discovered that the analogous (salen)Co(n) complex 6 promoted the enantioselective addition of benzoic acids to meso-epoxides to afford valuable monoprotected C2-symmetric diols (Scheme 7.15) [26], Under the reaction conditions, complex 6 served as a precatalyst for the (salen) Co(iii)-OBz complex, which was fonned in situ by aerobic oxidation. While the enantioselectivity was moderate for certain substrates, the high crystallinity of the products allowed access to enantiopure materials by simple recrystallization. 

Pure diastereomers are easily obtained by simple recrystallization from heptane, dichlorometh-ane/heptane or methanol/water in yields of 75-95%. 

Trihalopyrimidines initially give mixtures of 2- and 4-monoaminated products, although in the case of benzimidazole 125 and 2,4,6-trichloropyrimidine 126, a simple recrystallization gave clean 2-substituted product 127 in 53% yield <2000CPB1778>. 

Thus, the N,N-dibenzyl-protected aminonitrile 55 was prepared via Swern oxidation of N,N-dibenzylaminoethanol 54 followed by treatment with the enantio-pure amine auxiliary (S,S)-53 and HCN, resulting in the formation of a 3 2 epimeric mixture of the aminonitriles 55 in 55% yield, from which the single dia-stereomers could be isolated by chromatography. After lithiation with LDA, addition to the requisite (E)-a, P-unsaturated esters and hydrolysis of the aminonitrile moiety with silver nitrate, the desired a-amino keto esters R)-S6 were obtained with yields of 65-81% and enantiomeric excesses ee of 78-98%, which could be improved to ee > 98% by a simple recrystallization. Since the amino ketone functionality can be cleaved oxidatively, the 5-amino-4-oxo-esters 56 could be transformed to the corresponding succinic half-esters 57 with hydrogen peroxide in methanol in good to excellent yields (68-90%) (Scheme 1.1.15). 

As a preliminary experiment to prove that simple recrystallization will not remove ferrous sulfate from copper sulfate, proceed as follows ... 

The [Ni(PMe3)4]BPlu complex has been unexpectedly obtained through a reductive elimination reaction, by the simple recrystallization in THF of the five-coordinate nickel(II) complex [Ni(Me)(PMe3)4JBPh4.3 ... 

Several review articles containing discussions of enantioselective syntheses with ester-cleaving enzymes have appeared recently (as of 1987).9 13 Of the many examples, the ones in which meso-substrates are employed are most attractive since the theoretical yield is 100%. In many applications of PLE the enantiomeric excess (% ee) of the product depends crucially upon the source of the enzyme. This effect has not been noticed in the enantioselective saponifications of nitrodiol diacetates, either because the reaction is insensitive to it, or because this complication is overcome by the great crystallization tendency of the products. The only problem we observed when the reaction was carried out with commercial crude PLE-powder (but not with the self made one) was the production of a certain amount of diol which could not be removed by simple recrystallization. In this case, filtration over a short silica gel column with methylene chloride as eluent gives after recrystallization pure monoacetate. 

Due to the sulfonamide group, the products are usually solids and the enantiomeric excesses can be increased by simple recrystallization. The difficulties in removing the sulfonyl group represent the drawback of this procedure. 

Since lithium salts obtainable by purchase, even of so-called c.p. or reagent quality, frequently contain impurities totaling about 1 per cent, it is desirable to have a rapid method for obtaining such salts in a reasonably pure state. The following simple procedure for the purification of c.p. or reagent grade lithium carbonate provides such a method, since the resulting pure carbonate may be readily converted by treatment with the proper pure acid into practically any lithium salt desired. The procedure is based upon the fact that lithium carbonate, in contrast to the salts that contaminate it, is much less soluble in hot than in cold water. J In other words, simple recrystallization is employed, but the process is carried out in the reverse direction. 

The complexes of dimethyltin dihalides were readily studied since they may be obtained easily, owing to the high acceptor ability of the starting halides, and may be purified by simple recrystallization or sublimation. 

If instead of a mild Lewis base such as THF a strong Lewis base such as pyridine is used in the reaction between copper acetate and dtbp-H, one obtains yet another type of one-dimensional polymeric phosphate (Chart 16).36c Recrystallization of this water-bridged polymer in a DMSO/ THF/CH3OH mixture results in the transformation of the polymer to a more stable tetranuclear copper cluster [Cu4-(u3-OH)2(dtbp)e(py)2] in about 60% yield (Figure 3). This observation demonstrates that even simple recrystallization can bring about an interesting structural transformation at room temperature in such materials. 

FIGURE 3. Conversion of a 1D polymer to a cage structure by simple recrystallization. 

This is an ancient alchemical formula for exalting our matter. In herbal work it is applied to the salts we collected and from the standpoint of chemistry it is a simple recrystallization where our salts are dissolved into water, filtered and again evaporated to dryness. From the alchemical viewpoint we are passing our matter through the life, death and rebirth cycle. 

The inverted filtration device is convenient for simple recrystallizations as well as repeated extractions. It is of particular advantage for the manipulation of volatile, inflammable solvents and of lachrymatory solutions. 

S)-4-(l-Methylethyl)-5,5-diphenyl-2-oxazolidinone (3), whose preparation is described here, has several advantages over Evans original auxiliaries i) Derivatives of 3 are more likely to crystallize. In many cases the separation and purification of diastereoisomers can be achieved by simple recrystallization rather than by expensive and time-consuming chromatography, ii) Acylation of 3 can be carried out at 0°C (instead of-78°C for 4 and 5) by deprotonation with BuLi, followed by treatment with an activated carboxylic acid derivative, iii) Lithium enolates of N-acyl derivatives of 3 can be obtained directly by treatment with BuLi at -78°C, in comparison to 4 and 5 when the more expensive... 

The attainment of high levels of enantiopurity is not always possible by enzymatic or diaste-reomeric resolutions or by asymmetric syntheses alone. It is however frequently possible to prepare a pure enantiomer from a partially resolved sample by simple recrystallization. For this process to proceed successfully it is necessary that the initial enantiopurity of the mixture is greater than that of the eutectic point in the phase diagram. By utilization of the phase diagram, the optimal quantity of solvent required can be calculated. It is also possible to calculate the maximum expected yield. 

Surprisingly simple Friedel-Crafts reactions introduce ferrocene units into amphiphiles. For example, 11-bromo-undecanoyl chloride can be added directly to ferrocene in dichloromethane with aluminium chloride as a catalyst. The corresponding (11-bromo-undecanoyl) ferrocene can be isolated via simple recrystallization, and the ketone removed by amalgamated zinc (does not remove the bromide). The bromide can finally be substituted by alcohols and amines to yield ethers and ammonium salts (Scheme 2.13). 

A brief investigation of the polymorphic form of each candidate salt shonld be nndertaken and as far as possible, the existence of polymorphism shonld be conhrmed or rnled ont. Polymorphs can be prodnced by simple recrystallization from a range of solvents, or solvent mixtnres, of different polarities and dielectric constants and their existence most easily conhrmed by a combination of hot stage microscopy (HSM), DSC or infrared or Raman spectroscopy and by powder X-ray techniqnes." ... 

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