Crystallization from solution examples

The propensity for iodine to catenate is well illustrated by the numerous polyiodides which crystallize from solutions containing iodide ions and iodine. The symmetrical and unsymmetrical 13 ions (Table 17.15) have already been mentioned as have the I5- and anions and the extended networks of stoichiometry (Fig. 17.12). The stoichiometry of the crystals and the detailed geometry of the polyhalide depend sensitively on the relative concentrations of the components and the nature of the cation. For example, the linear ion may have the following dimensions ... 

EXAMPLE 16.1. A solution at 0°C contains 119g of sodium acetate per lOOg of water. If more sodium acetate is added, it does not dissolve, and no sodium acetate crystallizes from solution either. Describe the following solutions as saturated, unsaturated, or supersaturated. (a) lOOg sodium acetate in lOOg water at 0°C. (b) 150g sodium acetate in lOOg water at 0°C. (c) 11.9g sodium acetate in lO.Og water at 0°C. 

Most solvents have unshared pairs of electrons, and they are polar. Therefore, they have the ability to attach to metal ions or interact with anions. As a result, when many solids crystallize from solutions, they have included a definite number of solvent molecules. When this occurs in water, we say that the crystal is a hydrate. An example of this is the well-known copper sulfate pentahydrate,... 

Such equilibria are driven by thermodynamics and therefore a selective synthetic route towards one of these species and isolation of such heteroleptic zincates in pure form is often very difficult or impossible. Only if one of the species has a sufficiently enhanced thermodynamic stability compared to the others in the equilibrium is its isolation as a pure compound possible. This is often the case when the various groups bound to zinc have a sufficiently different electronegativity, for example when one of the groups is bound to zinc via a heteroatom, or when the steric requirements of the groups bound to zinc are rather different. Sometimes it is possible to isolate one of the species present in the Schlenk equilibrium as a solid material, for example when one of the species preferentially crystallizes from solution. 

Crystallization from the melt often leads to a distinct (usually lamellar) structure, with a different periodicity from the melt. Crystallization from solution can lead to non-lamellar crystalline structures, although these may often be trapped non-equilibrium morphologies. In addition to the formation of extended or folded chains, crystallization may also lead to gross orientational changes of chains. For example, chain folding with stems parallel to the lamellar interface has been observed for block copolymers containing poly(ethylene), whilst tilted structures may be formed by other crystalline block copolymers. The kinetics of crystallization have been studied in some detail, and appear to be largely similar to the crystallization dynamics of homopolymers. 

It is noteworthy that solubility of the formed metal alkoxide is in many cases even more important for the reaction than the values of the metal standard electrode potentials or the mobility of protons of the alcohol. The following examples illustrate this statement. Despite higher acidity of MeOH in comparison with EtOH(pK= 15.5 and 16, respectively), the insoluble Ca(OMe)2 is formed very slowly in comparison with the soluble derivatives Ca(OEt)2 or Mg(OMe)2 (both latter compounds crystallize from solutions as solvates) [1646]. Aluminum readily reacts with PrOH with the formation of the highly soluble Al(OPrf)3 even in the absence of the catalyst (pK ROH = 18, E°AP7Al,ld = -1.66 V). On the other hand, polymeric Al(OMe)3 and Al(OEt)3 are formed only on prolonged refluxing of the metal with alcohols in xylene (140°C) in the presence of HgCl2 and I2 [1301]. 

Crystallization — The process of forming solid crystals from solution, melted or polycrystalline phase. Used to separate solid and liquid phase or preparing high purity materials. Crystallization from solution is the most common example of solid-liquid separation. In the process, the solid crystals are formed from supersaturated solution (the solution that contains more soluble molecules, ions etc. that it would under equilibrium conditions). Usually the supersaturated solution is obtained either by cooling the solution, evaporating the solvent, pH change, or adding another solvent. The crystallization process can be induced electrochemically (- electro deposition, electro crystallization). The most common ex-... 

Hexavalent. As with most reactions, the hydrolysis of U02 + is the best studied of the hexavalent actinides. The hydrolysis of U02 + begins at pH 3, while the onset for the hydrolysis of Np02 + and Pu02 + each occur at a higher pH. The monomeric hydrolysis products of the uranyl ion, U02(0H) n = 1, 2) can be studied in solutions with uranimn concentrations less than 10 M. For solutions with higher uranium concentrations, multinuclear cationic species dominate the speciation, for example, (U02)2(0H)22+, (U02)3(0H)42+, and (U02)3(OH)s+. These cations have been crystallized from solutions with the formulas (U02)2(at2-OH)2(OH2)6 + and (U02)3(M3-0)(/x2-0H)3(0H2)6+ (21). For Np and Pu, the dimer of the first hydrolysis product, (An02)2(OH)2 + (22), has also been identified but not fully stracturally characterized. 

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... 

Werner studied cobalt(III), chromium(III), platinum(II), and platinum(IV) compounds because they are inert and can be more readily characterized than labile compounds. This tendency has continued, and much of the discussion in this chapter is based on inert compounds because they can be more easily crystallized from solution and their structures determined. Labile compounds have also been studied extensively, but their study requires techniques capable of dealing with very short times (stopped flow or relaxation methods, for example, temperature or pressure jump, nuclear magnetic resonance). 

FIGURE 2.8. Some methods for growing crystals from solution, (a) Slow solvent evaporation, primarily used for small molecules, (b) Vapor diffusion, (c) The hanging drop method, primarily used for macromolecules, (d) A crystallization plate used for hanging drops there are slightly different crystallization conditions in each well, varied systematically with respect to, for example, pH and ionic strength. 

Liquid-liquid extraction also can be an attractive alternative to separation methods, other than distillation, e.g., as an alternative to crystallization from solution to remove dissolved salts from a crude organic feed, since extraction of the salt content into water ehminates the need to filter solids from the mother liquor, often a difficult or ejq)ensive operation. Extraction also may compete with process-scale chromatography, an example being the recovery of hydroxytyrosol (3,4-dihydrojq -phenylethanol), an antioxidant food additive, from olive-processing wastewaters [Guzman et al., U.S. Patent 6,849,770 (2005)]. 

A second approach to changing the composition of a mixture of two solvents is to use a less polar solvent which is also less volatile than the solvent in which the compound is initially dissolved. After the less polar solvent has been added to the filtered solution of the crude mixture, the mixture is concentrated under reduced pressure. The more volatile polar solvent is preferentially removed under these conditions, and the product will crystallize from solution. Typical solvent combinations which we have found to be valuable include benzene with 100-120 C ligroin, which can be conveniently used to crystallize neutral molecules which are soluble to aromatic solvents but not in hydrocarbons, and acetone with ethanol. Acetone will typically, for example, dissolve hexafluorophosphate salts of organometallic cations, which tend to be insoluble in the less volatile ethanol. 

FIGURE 14.7 Crystallization from solution. Approximate example of the nucleation rate, the average linear crystal growth rate, and the average crystal size resulting, as a function of supersaturation In [3. Arbitrary linear scales. 

The linear growth rates of crystal faces vary enormously. Some approximate examples of the average rate for crystallization from solution are... 

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