Crystallization Purification of Solids

In most organic chemistry experiments, the desired product is first isolated in an impure form. If this product is a solid, the most common method of purification is crystallization. The general technique involves dissolving the material to be crystallized in a hot solvent (or solvent mixture) and cooling the solution slowly. The dissolved material has a decreased solubility at lower temperatures and will separate from the solution as it is cooled. This phenomenon is called either crystallization, if the crystal growth is relatively slow and selective, or precipitation, if the process is rapid and nonselective. Crystallization is an equilibrium process and produces very pure material. A small seed crystal is formed initially, and it then grows layer by layer in a reversible manner. In a sense, the crystal selects the correct molecules from the solution. In precipitation, the crystal lattice is formed so rapidly that impurities are trapped within the lattice. Therefore, any attempt at purification with too rapid a process should be avoided. Because the impurities are usually present in much smaller amounts than the compound being crystallized, most of the impurities will remain in the solvent even when it is cooled. The purified substance can then be separated from the solvent and from the impurities by filtration. 

The method described here for semimicroscale crystallizations is nearly identical to that used for crystallizing larger amounts of materials than those encountered in this textbook. Therefore, this technique can also be used to perform crystallizations at the macroscale level (more than several grams). 

The first problem in performing a crystallization is selecting a solvent in which the material to be crystallized shows the desired solubility behavior. In an ideal case, the material should be sparingly soluble at room temperature and yet quite soluble 

The solubility of organic compounds is a function of the polarities of both the solvent and the solute (dissolved material). A general rule is Like dissolves like. If the solute is very polar, a very polar solvent is needed to dissolve it if the solute is nonpolar, a nonpolar solvent is needed. Applications of this rule are discussed extensively in Technique 10, Section 10.2, and in Section 11.5. 

2 Theory of A successful crystallization depends on a large difference between the solubility 

---

Crystallization Purification of Solids Extractions, Separations, and Drying Agents... 

Technique 11 Crystallization Purification of Solids, Section 11.3 Technique 25 Infrared Spectroscopy, Part B... 

Concomitant crystallization is by no means limited to crystallization from solution, nor to preservation of constant molecular conformation. As noted in Section 2.2.5 the classic pressure vs temperature phase diagram for two solid phases (Fig. 2.6) of one material exhibits two lines corresponding to the solid/vapour equilibrium for each of two polymorphs. At any one temperature one would expect the two polymorphs to have different vapour pressures. This, in fact, is the basis for purification of solids by sublimation. Nevertheless there are examples where the two have nearly equal vapour pressures at a particular temperature and thus cosublime. This could be near the transition temperature or simply because the two curves are similar over a large range of temperatures or in close proximity at the temperature at which the sublimation is carried out. For instance, the compounds 3-VI and 3-Vn both yield two phases upon... 

Crystallization is the most important method for the purification of solid organic compounds. A crystalline organic substance is made up of a three-dimensional array of molecules held together primarily by van der Waals forces. These intramolecular attractions are fairly weak most organic solids melt in the range of room temperature to 250°C. 

Sublimation is the process whereby a solid evaporates from a warm surface and condenses on a cold surface, again as a solid (Figs. 13, 15). This technique is particularly useful for the small-scale purification of solids because there is so little loss of material in transfer. If the substance has the correct properties, sublimation is preferred over crystallization when the amount of material to be purified weighs less than 1(X) mg. 

Nevertheless, the use of NH4X for purification of solid salts is substantiated by the fact that it forms complex halides with salts of multivalent cations. As an example, we may consider the process for obtaining anhydrous MgCl2. Dissolution of MgO in HC1, followed by the evaporation of some water results in the crystallization of MgCl2-6H20 this salt is well known to lose HC1 and H20 at 400 °C, with MgO remaining ... 

Chemical vapor-phase transport is one of the earliest methods developed for the synthesis of new materials [19-26]. The method can also be used for the growth of high-quality single crystals or for the purification of solid-state compounds. The method has been used to synthesize a wide range of LMCs. 

RecrystaUization is an excellent method for the purification of solids and is the last of the major purification techniques. It is used by both inorganic and organic chemists. The preparation of many inorganic salts and complexes leads to the formation of solid crystals, which can be purified by recrystaUization and filtration. 

Solid organic compounds when isolated from organic reactions are seldom pure they are usually contaminated with small amounts of other compounds ( impurities ) which are produced along with the desired product. Tlie purification of impure crystalline compounds is usually effected by crystallisation from a suitable solvent or mixture of solvents. Attention must, however, be drawn to the fact that direct crystallisation of a crude reaction product is not always advisable as certain impurities may retard the rate of crystallisation and, in some cases, may even prevent the formation of crystals entirely furthermore, considerable loss of... 

Purification of the Methylamine HCI is in order now, so transfer all of the crude product to a 500mL flask and add either 250mL of absolute Ethanol (see end of FAQ for preparing this) or, ideally, n-Butyl Alcohol (see Footnote 4). Heat at reflux with a Calcium Chloride guard tube for 30 minutes. Allow the undissolved solids to settle (Ammonium Chloride) then decant the clear solution and cool quickly to precipitate out Methylamine HCI. Filter rapidly on the vacuum Buchner funnel and transfer crystals to a dessicator (see Footnote 3). Repeat the reflux-settle-cool-filter process four... 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

Modem refining technology uses tantalum and niobium fluoride compounds, and includes fluorination of raw material, separation and purification of tantalum and niobium by liquid-liquid extraction from such fluoride solutions. Preparation of additional products and by-products is also related to the treatment of fluoride solutions oxide production is based on the hydrolysis of tantalum and niobium fluorides into hydroxides production of potassium fluorotantalate (K - salt) requires the precipitation of fine crystals and finishing avoiding hydrolysis. Tantalum metal production is related to the chemistry of fluoride melts and is performed by sodium reduction of fluoride melts. Thus, the refining technology of tantalum and niobium involves work with tantalum and niobium fluoride compounds in solid, dissolved and molten states. 

Practical-grade 4-cthoxy-3-methoxybenzaldehyde (4-ethoxy-m-anisaldehyde) was obtained by the submitters from MC and B Manufacturing Chemists and by the checkers from Aldrich Chemical Company, Inc. This material was purified by distillation (b.p. 125-135°/0.1 mm.), followed by one recrystallization from cyclohexane (100 ml./lO g. crude solid). Colorless crystals, m.p. 60-62°, wrere obtained after filtration and vacuum drying. Purification of 20 g. of the commercial material gave about 15 g. of recrystallized product. 

The submitters purified the product by the following procedure. The residual pale yellow solid is dissolved in 50 ml of diethyl ether and the remaining solid is filtered off (Note 16). The filtrate is concentrated to a volume of ca. 25 mL, and the solution is allowed to crystallize at 0°C. Once crystallization begins, 50 mL of petroleum ether is added in two portions over 10 hr, and then crystallization is allowed to proceed overnight at 0°C. The white solid is collected by filtration and washed with a mixture of 3 1 petroleum ether-diethyl ether to afford 3.8 g of 4. Chromatographic purification of the mother liquor (5.5 x 18 cm of DSH silica gel 40-63 mm, elution with 1 L of petroleum ether/ethyl acetate 4 1 followed by 1.5 L of 3 1 petroleum ether-ethyl acetate) gives 2.5 g of 4 as a pale yellow solid. All the material is combined and recrystallized from diethyl ether/petrol as above to yield 5.2 g (47%) of 4 in two crops. 

---