Simple crystallization

Simple crystallization of an impure solid is a routine operation, which nevertheless requires care and good judgement if good results are to be obtained. The basic procedure can be broken down into six steps, which are listed below, together with some tips on how to overcome common problems. 

Find a suitable solvent by carrying out small scale tests. Remember that like dissolves like. The most commonly used solvents in order of increasing polarity are petroleum ether, toluene, chloroform, acetone, ethyl acetate, ethanol, and water. Chloroform and dichloromethane are rarely useful on their own because they are good solvents for the great 

Dissolve the compound in the minimum volume of hot solvent Remember that most organic solvents are extremely flammable and that many produce very toxic vapour. 

This step is often problematic and should NOT be carried out unless an unacceptable (use your judgement) amount of insoluble material is suspended in the solution. The difficulty here is that the compound tends to crystallize during the filtration so an excess of solvent (ca. 5%) should be added, and the apparatus used for the filtration should be preheated to about the boiling point of the solvent. Use a clean sintered funnel of porosity 2 or 3, or a Hirsch or Buchner funnel, and use the minimum suction needed to draw the solution rapidly through the funnel. If the solution is very dark and/or contains small amounts of tarry impurities, allow it to cool for a few moments, add ca. 2% by weight of decolorising charcoal, reflux for a few minutes, and filter off the charcoal. Charcoal is very finely divided so it is essential to put a 

Icm layer of a filter aid such as Celite on the funnel before filtering the suspension. Observe the usual precautions for preventing crystallization in the funnel. Very dark or tarry products should be chromatographed through a short (2-3cm) plug of silica before attempted crystallization. 

Simple crystallization works weU with large quantities of material (100 mg and up), and it is essentially identical to that of the macroscale technique. 

Step 1. Place the solid in a small Erlenmeyer flask or test tube. A beaker is not recommended because the rapid and dangerous loss of flammable vapors of hot solvent occurs much more easily from the wide mouth of a beaker than from an Erlenmeyer flask. Furthermore, solid precipitate can rapidly collect on the walls of the beaker as the solution becomes saturated because the atmosphere above the solution is less likely to be saturated with solvent vapor in a beaker than in an Erlenmeyer flask. Step 2. Add a minimal amount of solvent and heat the mixture to the solvent s boiling point in a sand bath. Stir the mixture by twirling a spatula between the thumb and index finger. A magnetic stir bar may be used if a magnetic stirring hot plate is used. 

Step 3. Continue stirring and heating while adding solvent dropwise until all of the material has dissolved. 

Step 4. Add a decolorizing agent (powdered charcoal, 2% by weight or better, activated-carbon Norit pellets, -0.1% by weight), to remove colored minor impurities and other resinous byproducts. 

Step 5. Filter (by gravity) the hot solution into a second Erlenmeyer flask (pre-heat the funnel with hot solvent). This removes the decolorizing agent and any insoluble material initially present in the sample. 

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This structure is called close packed because the number of atoms per unit volume is quite large compared with other simple crystal structures. 

For example, consider a simple crystal witii one atom per lattice point the total ionic potential can be written as... 

For Iran sition metals th c splittin g of th c d orbitals in a ligand field is most readily done using HHT. In all other sem i-ctn pirical meth -ods, the orbital energies depend on the electron occupation. HyperCh em s m oiccii lar orbital calcii latiori s give orbital cri ergy spacings that differ from simple crystal field theory prediction s. The total molecular wavcfunction is an antisymmetrized product of the occupied molecular orbitals. The virtual set of orbitals arc the residue of SCT calculations, in that they are deemed least suitable to describe the molecular wavefunction, ... 

Peroxohydrates are usually made by simple crystallization from solutions of salts or other compounds in aqueous hydrogen peroxide. They are fairly stable under ambient conditions, but traces of transition metals catalyze the Hberation of oxygen from the hydrogen peroxide. Early work on peroxohydrates has been reviewed (92). 

We begin by looking at the smallest scale of controllable structural feature - the way in which the atoms in the metals are packed together to give either a crystalline or a glassy (amorphous) structure. Table 2.2 lists the crystal structures of the pure metals at room temperature. In nearly every case the metal atoms pack into the simple crystal structures of face-centred cubic (f.c.c.), body-centred cubic (b.c.c.) or close-packed hexagonal (c.p.h.). 

The crucial experiments that determined the structures of a number of very simple crystals, beginning with sodium chloride, were done, not by von Laue and his helpers, but by the Braggs, William (1862-1942) and Lawrence (1890-1971), father... 

The alkali metal halides are all high-melting, colourless crystalline solids which can be conveniently prepared by reaction of the appropriate hydroxide (MOH) or carbonate (M2CO3) with aqueous hydrohalic acid (HX), followed by recryslallization. Vast quantities of NaCl and KCl are available in nature and can be purihed if necessary by simple crystallization. The hydrides have already been discussed (p. 65). 

Tenacity. Whilst some plastics are rigid and others flexible, all commercial materials show a degree of strength and toughness superior to simple crystals and common glass when rapidly stressed. 

At temperatures only slightly below the liquefaction temperatures, the liquids freeze. The solids are all simple crystals in which the atoms are close-packed in a regular lattice arrangement. The narrow temperature range over which any one of these liquids can exist suggests that the forces holding the crystal together are very much like the forces in the liquid. 

Figure S.2. Basal planes formed by cutting the unit cells of the simple crystal structures.

Why do we get differences in crystal shape or habit This may be a matter of directional rates of growth. Factors affecting directional rates will then affect the habit. Directional rates of growth can be illustrated with a relatively simple crystal structure, that of sodium chloride. 

In a simple crystal-field description, a spin-quartet ground state (S = 3/2) of iron(III) is energetically stabilized over the spin-sextet (S = 5/2) when the energy separation... 

The term crystal structure in essence covers all of the descriptive information, such as the crystal system, the space lattice, the symmetry class, the space group and the lattice parameters pertaining to the crystal under reference. Most metals are found to have relatively simple crystal structures body centered cubic (bcc), face centered cubic (fee) and hexagonal close packed (eph) structures. The majority of the metals exhibit one of these three crystal structures at room temperature. However, some metals do exhibit more complex crystal structures. 

Initially, 50 was converted into the benzoxazinone 51 by reaction with phosgene in the presence of triethylamine and 51 was isolated in 95% yield upon crystallization from methanol. Deprotection of the pMB group from 51 was accomplished with ceric ammonium nitrate (CAN) in aqueous acetonitrile. Efavirenz was isolated in 76% yield after crystallization from EtOAc-heptane (5 95), as shown in Scheme 1.19. There were two issues identified in this route. First, lequiv of ani-saldehyde was generated in this reaction, which could not be cleanly rejected from product 1 by simple crystallization to an acceptable level under the ICH guideline. Anisaldehyde was removed from the organic extract as a bisulfite adduct by washing with aqueous Na2S205 twice, prior to the crystallization of 1. Secondly,... 

The fact that PTAD readily reacts with 5,7-steroidal dienes to form the Diels-Alder adducts, whereas the isomeric 4,6-diene remains unchanged can be used for their separation. Owing to the significant differences in the polarity of these compounds, they can be separated sometimes by simple crystallization <2004JOC8529>. Similarly, the reaction can be used for purification of the 5,7-dienes from other impurities <2005H(65)2107>. 

Perhaps the most simple crystals in this class are the alkaline earth oxides. They are II-VI compounds and have rocksalt crystal structures. Data for their hardnesses versus their bond moduli (optical band gaps per molecular volumes) are displayed in Figure 11.4. 

Fig. 2.3. Some simple crystal structures showing space-filling. The metal ion is shaded.

By means of the radius ratio, we have already described the type of local environment around the ions in several types of simple crystals. For example, in the sodium chloride structure (not restricted to NaCl itself), there are six anions surrounding each cation. The sodium chloride crystal structure is shown in Figure 7.4. 

In simple crystal field theory, the electronic transitions are considered to be occurring between the two groups of d orbitals of different energy. We have already alluded to the fact that when more than one electron is present in the d orbitals, it is necessary to take into account the spin-orbit coupling of the electrons. In ligand field theory, these effects are taken into account, as are the parameters that represent interelectronic repulsion. In fact, the next chapter will deal extensively with these factors. 

Four simple crystal structural types encompass the majority of elemental or binary semiconductors. The high symmetry of the structures has important consequences for the NMR spectra in several respects ... 

We recognize that there are applications in two- and three-dimensional waveguides (12,13) which do not have the same criteria of phase-matching as in simple crystals or that one may just as well be interested in screening these materials for the related electrooptic performance by the simple SHG powder method. (It has been shown for several organic materials that although the electro-optic and SHG x tensors are in principle unequal, due to dispersion and due to the possible contribution of atomic and molecular distortions... 

The heat capacity models described so far were all based on a harmonic oscillator approximation. This implies that the volume of the simple crystals considered does not vary with temperature and Cy m is derived as a function of temperature for a crystal having a fixed volume. Anharmonic lattice vibrations give rise to a finite isobaric thermal expansivity. These vibrations contribute both directly and indirectly to the total heat capacity directly since the anharmonic vibrations themselves contribute, and indirectly since the volume of a real crystal increases with increasing temperature, changing all frequencies. The constant volume heat capacity derived from experimental heat capacity data is different from that for a fixed volume. The difference in heat capacity at constant volume for a crystal that is allowed to relax at each temperature and the heat capacity at constant volume for a crystal where the volume is fixed to correspond to that at the Debye temperature represents a considerable part of Cp m - Cv m. This is shown for Mo and W [6] in Figure 8.15. 

The importance of dislocations becomes evident when we consider the strain on the microstructure of a simple crystal. The atoms or ions in a crystal are in symmetric energy wells and so vibrate around their lattice site. When we track across a crystal plane, the potential energy increases and decreases in a regular fashion with the minima at the lattice points... 

Their comparatively simple crystal structures may be described as a primitive packing of quasi-molecular units, and, in a way, they represent the border line between molecular and infinitely extended units in a solid (Cheetham and Day 1992). The Mo6 type core is completely surrounded by X atoms and inter-cluster bonding essentially occurs through the Mo-X interactions. The Mo—Mo bonding between clusters is very weak. 

Some of the phenomena of differentiation of silicate rocks are probably to be treated as results of simple crystallization backed up by the effects of gravity in causing sinking or flotation of crystals. This is the only method of differentiation that has been experimentally proven, but the separation of two or more liquid phases, and perhaps other phenomena also, may take part in this little-known process. 

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