Solvents recrystallizing

Until one develops a feel for recrystallization, the best procedure for known compounds is to duplicate a selection in the literature. For new compounds, a literature citation of a solvent for an analogous structure is often a good beginning point. To assist in the search, Table A3.4 lists several of the common recrystallizing solvents with useful data. The dielectric constant can be taken to be a rough measure of solvent polarity. 

The caprolactam obtained must meet die specifications of permanganate number, volatile bases, hazen color, UV transmittance, solidification point, and turbidity in order to be used for repolymerization alone or in combination witii virgin CL.5 Reported CL purification methods include recrystallization, solvent extraction, and fractional distillation. One solvent extraction technique involves membrane solvent extraction. Ion exchange resins have been shown to be effective in the purification of aqueous caprolactam solutions. In one such process,... 

The crystal of 2 OPr recrystallized from EtOH/H20 solution, and the mixed crystal of the same ethyl and propyl cinnamate derivatives (2 OEt and 2 OPr), on photoirradiation for 2h at room temperature with a 500 W super-high-pressure Hg lamp, afforded the highly strained tricyclic [2.2] paracyclophane (2 OEt-2 OPr-cyclo) crystal quantitatively (Maekawa et ai, 1991b). A crystal structure analysis was carried out of a single crystal of the complex of 2 OEt-2 OPr-cyclo with HFIP (recrystallization solvent) in a 1 2 molar ratio. Fig. 13 shows the molecular structure of 2 OEt-2 OPr-cyclo viewed along the phenylene planes. The short non-bonded distances and deformation of the benzene rings, as seen in Fig. 13, are common to those of [2.2] paracyclophanes, as previously reported (Hope et ai, 1972a,b). 

A polymorph is a solid crystalline phase of a compound resulting from the possibility of at least two different crystal lattice arrangements of that compound in the solid state [42], Polymorphs of a compound are, however, identical in the liquid and vapor states. They usually melt at different temperatures but give melts of identical composition. Two polymorphs of a compound may be as different in structure and properties as crystals of two different compounds [43,44], Apparent solubility, melting point, density, hardness, crystal shape, optical and electrical properties, vapor pressure, etc. may all vary with the polymorphic form. The polymorphs that are produced depend upon factors such as storage temperature, recrystallization solvent, and rate of cooling. Table 2 suggests the importance of polymorphism in the field of pharmaceutics [45],... 

Plastic deformation, unlike elastic deformation, is not accurately predicted from atomic or molecular properties. Rather, plastic deformation is determined by the presence of crystal defects such as dislocations and grain boundaries. While it is not the purpose of this chapter to discuss this in detail, it is important to realize that dislocations and grain boundaries are influenced by things such as the rate of crystallization, particle size, the presence of impurities, and the type of recrystallization solvent used. Processes that influence these can be expected to influence the plastic deformation properties of materials, and hence the processing properties. 

The melting endotherm is followed immediately by a strong exothermic degradation. Since bromocriptine mesilate decomposes under melting, the transition temperature is strongly dependent on the heating rate. A broad but weak endotherm between 40 and 100 °C indicates the volatilization of sorbed recrystallization solvent (usually butanone-2, see section 3). 

GC cannot be applied to the analysis of bromocriptine mesilate due to its low volatility and its thermal instability. A procedure according to 29 or 30, which claims excellent identification and quantitation on the basis of well-defined peptide section pyrolysis products, has not yet been attempted. However, GC is very useful determining the residual recrystallization solvent butanone-2. 

I ve included 2532 (Benzoic acid, 2-acetamido) to show that you sometimes get a bonus. Here nd(aa) means you get needle-like crystals from acetic acid. Acetic acid (aa) is the recrystallization solvent (see Chapter 10, Recrystallization ), and you don t have to find it on your own. Thus, pa ye nd (al) means that pale yellow needles are obtained when you recrystallize the compound from ethanol. 

Add 3 ml of a solvent, stopper the tube, and shake the bejesus out of it. If all of the solid dissolves at room temperature, then your solid is soluble. Do not use this solvent as a recrystallization solvent. (You must make note of this in your notebook, though). 

If none (or very little) of the solid dissolved at room temperature, unstopper the tube and heat it (Careful—no flames ) and shake it and heat it and shake it. You may have to heat the solvent to a gentle boil (Careful Solvents with low boiling points often boil away). If it does not dissolve at all, then do not use this as a recrystallization solvent. 

If the sample dissolved when HOT, and did NOT dissolve at room temperature, you re on the trail of a good recrystallization solvent. One last test. 

Place this last test tube in an ice-water bath, and cool it to about 5°C or so. If lots of crystals come out, this is good, and this is your recrystallization solvent. 

The Buchner funnel (Fig. 46) is used primarily for separating crystals of product from the liquid above them. If you have been boiling your recrystallization solvents dry, you should be horsewhipped and forced to reread these sections on recrystallization ... 

The faucet on the water aspirator should be turned on full blast This should suck down the filter paper, which you now wet with some of the cold recrystallization solvent. This will make the paper stick to the plate. You may have to push down on the Buchner funnel a bit to get a good seal between the rubber adapter and the funnel. 

Use a very small amount of the same cold recrystallization solvent and a spatula to remove any crystals left in the flask. Then you can use some of the fresh, cold recrystallization solvent and slowly pour it over the crystals to wash away any old recrystallization solvent and dissolved impurities. 

Oiling out is what it s called more work is what it means. Compounds usually oil out if the boiling point of the recrystallization solvent is higher than the melting point of the compound, though that s not the only time. In any case, if the oil solidifies, the impurities are trapped in the now solid oil, and you ll have to purify the solid again. 

Redissolve the oil by heating, then shake up the solution as it cools and begins to oil out. When these smaller droplets finally freeze out, they may form crystals that are relatively pure. They may not. You ll probably have to clean them up again. Just don t use the same recrystallization solvent. 

The recrystallization was carried out in a 40-ml. centrifuge tube. The recrystallization mixture, after cooling to room temperature, was cooled in a freezer and washed twice with 20-ml. portions of cold recrystallization solvent. 

These and succeeding preparations may occasionally be contaminated by small quantities of platinum metal, which may be removed by dissolving the product in the recrystallizing solvent, treating the hot solution with charcoal, filtering it, and allowing it to crystallize as usual. 

Since most of the bicyclic compounds and their derivatives considered here are solids, they are commonly purified by recrystallization. Methanol and ethanol are frequently chosen as recrystallization solvent. 

Carbon tetrachloride may also be used as a recrystallization solvent. The product should be stored in a tightly closed brown bottle to retard its slow decomposition. 

This procedure may be used for the preparation of a variety of a-amino ketones as is indicated in Table I, which summarizes most of the submitters experience with this reaction. Principal deviations from the procedure wifi be in the time required for a negative starch-iodide test and the nature and amount of extraction and recrystallization solvent. It is strongly recommended that any one using the reaction for the first time carry out the preparation on a-phenylethylamine before attempting to use it on other more valuable amines. 

The cyclohexane in the recrystallization solvent reduces the tendency for the product to come out of solution as an oil. 

Purification. Unsubsliluled di-p-xylylene (DPXN) is readily purified by recrystallization front xylene. It is a colorless, highly crystalline solid. The principal impurity is polymer, which is insoluble in the recrystallization solvent and easily removed by hot filtration. In purifying DPXC and DPXD, care is taken not to disturb the homologue composition, so that product uniformity is maintained. 

Since chlorocyanocyclobutanones are readily hydrolyzed, protic recrystallization solvents and silica gel chromatography should be avoided. Short path distillation is the method of choice for the purification of most of the cyclobutanones. 

Process groups expend considerable effort seeking out different crystal forms of drug candidates. Different polymorphs are often discovered during routine solubility studies as process groups search for an optimal recrystallization solvent. In one such study, a group uncovered 11 different polymorphs, some of which were likely hydrates or other solvates.8... 

When we are resolving a racemate by the diastereomeric method, the most important task is to find a suitable resolving agent and recrystallizing solvent with a great solubility difference between two of the diastereomers. 

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