Solvent pairs

If the substance is found to be far too soluble in one solvent and much too insoluble in another solvent to allow of satisfactory recrystallisation, mixed solvents or solvent pairs may frequently be used with excellent results. The two solvents must, of course, be completely miscible. Recrystallisation from mixed solvents is carried out near the boiling point of the solvent. The compound is dissolved in the solvent in which it is very soluble, and the hot solvent, in which the substance is only sparingly soluble, is added cautiously until a slight turbidity is produced. The turbidity is then just cleared by the addition of a small quantity of the first solvent and the mixture is allowed to cool to room temperature crystals will separate. Pairs of liquids which may be used include alcohol and water alcohol and benzene benzene and petroleum ether acetone and petroleum ether glacial acetic acid and water. 

The experimental details for recrystallisation from mixed solvents (or solvent pairs) will be evident from the account already given the best proportions of the two solvents are determined by preliminary small-scale experiments. 

The solubihty of random copolymers of monomers whose homopolymers are noncrystalline also varies quite regularly as the relative amounts of the comonomers are changed. The solubihty of random copolymers is often low in solvents for the respective homopolymers but high in solvent pairs (51). 

The solute 1 is dissolved in a solvent pair of 2 and 3. D are infinite dilution binary diffusivities estimated by the proper method discussed previously. The mixture viscosity can be predic ted by methods of the previous section. The average absolute error when tested on 40 systems is 25 percent. The method gives higher errors if the solute is gaseous. 

A more interesting example is given with PVC and the polycarbonate of bis-phenol A, both slightly crystalline polymers. It is noticed here that whilst methylene dichloride is a good solvent and tetrahydrofuran a poor solvent for the polycarbonate the reverse is true for PVC yet all four materials have similar solubility parameters. It would seem that the explanation is that a form of hydrogen bonding occurs between the polycarbonate and methylene dichloride and between PVC and tetrahydrofuran (Figure 5.7). In other words there is a specific interaction between each solvent pair. 

A solution of the acylated thiocyanatohydrin in a minimal amount of 5% potassium hydroxide in diglyme (other solvents such as methanol, ethanol or tetrahydrofuran have also been used) is stirred for 2 days at room temperature. Water is added to the reaction mixture to precipitate the product which is filtered or extracted with ether (or chloroform). The ether extract is washed several times with water, dried (Na2S04), and concentrated under vacuum. The thiirane usually can be crystallized from an appropriate solvent pair. Chromatography over alumina has been used for the purification of episulfides. 

When inserted into an oil bath at 200°, the compound undergoes an immediate change in crystal structure and melts at 232-234°. The mother liquor affords two further crops, mp 228-230° and 227-228.5°, amounting to 3.5 and 10.6 g, respectively. Further recrystallization of the first crop from the same solvent pair raises the mp to 244-245° (70% recovery). 

Carroll [82] discusses Henry s Law in detail and explains the limitations. This constant is a function of the solute-solvent pair and the temperature, but not the pres-... 

The methanol-ether filtrate has a slight yellow color. It is not known what impurity is removed by this solvent pair. However, the submitters found that this treatment improved the yield of several aryl fluorides prepared according to the present procedure. 

Theta temperature (Flory temperature or ideal temperature) is the temperature at which, for a given polymer-solvent pair, the polymer exists in its unperturbed dimensions. The theta temperature, , can be determined by colligative property measurements, by determining the second virial coefficient. At theta temperature the second virial coefficient becomes zero. More rapid methods use turbidity and cloud point temperature measurements. In this method, the linearity of the reciprocal cloud point temperature (l/Tcp) against the logarithm of the polymer volume fraction (( )) is observed. Extrapolation to log ( ) = 0 gives the reciprocal theta temperature (Guner and Kara 1998). 

The viscosity increment was determined as v = B v = 172.7 ( 2 5 for spheres) where B is the viscosity coefficient characteristic of a given solue-solvent pair, and amounts to (9.91 0.24)xl0" ff/ A g" for PGA in aqueous solution. is the partial specific volume of the macromolecular component equal to [Pg.612]

Reciprocal degrees of polymerization of polystyrenes prepared by thermal polymerization at 100°C in hydrocarbon solvents are plotted against [>8]/[itf] in Fig. 16. Conversions were sufficiently low to permit the assumption of constancy in this ratio, which is taken equal to its initial value. The linearity of plots such as these, including those for numerous other monomer-solvent pairs which have been investigated, affords the best confirmation for the widespread occurrence of chain transfer and for the bimolecular mechanisms assumed. It is... 

Values obtained for and a for a number of polymer-solvent pairs are given in Table XXX. It will be observed that the exponent a varies with both the polymer and the solvent. It does not fall below 0.50 in any case and seldom exceeds about 0.80. Once K and a have been established for a given polymer series in a given solvent at a specified temperature, molecular weights may be computed from intrinsic viscosities of subsequent samples without recourse to a more laborious absolute method. 

The configuration of the polymer molecule must depend also on its environment. In a good solvent, where the energy of interaction between a polymer element and a solvent molecule adjacent to it exceeds the mean of the energies of interaction between the polymer-polymer and solvent-solvent pairs, the molecule will tend to expand further so as to reduce the frequency of contacts between pairs of polymer elements. In a poor solvent, on the other hand, where the energy of interaction is unfavorable (endothermic), smaller configurations in which polymer-polymer contacts occur more frequently will be favored. 

The presenee of intramoleeular interaetions ean be eheeked by eomparing calculated and experimental log P values. Sueh identifieation ean yet only be obtained for the water-OCT system, sinee eomputed lipophilieity values have only been derived for this solvent pair. Due to its simplicity and its easy adaptation to eomplex structures, Rekker s calculation method is often appreciated, but more sophistieated approaches like that evaluating log P using the molecular lipophilicity potential (MLP) has also been employed. 

To eliminate the need to recover the product by distillation, researchers are now looking at thermomorphic solvent mixtures. A thermomorphic system is characterized by solvent pairs that reversibly change from being biphasic to monophasic as a function of temperature. Many solvent pairs exhibit varying miscibility as a function of temperature. For example, methanol/cyclohexane and n-butanol/water are immiscible at ambient temperature, but have consolute temperatures (temperatures at which they become miscible) of 125°C and 49°C, respectively (3). 

The partition coefficient of a substance between several Immiscible solvent pairs can be combined with retention time data to confirm the identity of a substance when a pure standard is available [706]. Devised by Bowman and Beroza, the substance specific partition coefficient ("p-value") was defined as the fractional amount of substance partitioning into the less polar phase of an equal-volume, two-phase system. Only nanogram quantities of sample are required for the measurement and p-values are often sufficiently characteristic to distinguish between closely related substances. 

The method given is similar to those described elsewhere -7 but has the advantages that the solvent pair used for recrystallization enables large well-formed single crystals suitable for spectroscopic studies to be grown very easily, and that this method is generally applicable to the isolation of bis- or tris(czs-l,2-dicyano-l,2-ethylenedithiolato) complexes which are not subject to reduction by the solvent. 

The complexes [(n-C4H9)4N]2[MS4C4(CN)4], where M = Co, Ni, Cu, and Rh are isomorphous, and mixed single crystals can be grown very easily using the acetone-isobutyl alcohol solvent pair and only slight modifications of the recrystallization procedure. 

Any solvent pair that behaves the same way can be used. The addition of hot solvents to one another can be tricky. It can be extremely dangerous if the boiling points of the solvents are very different. For the water-methanol mixed solvent, if 95 °C water hits hot methanol (B.P. 65.0 °C), watch out ... 

There are other miscible, mixed-solvent pairs, pet. ether and diehyl ether, methanol and water, and ligroin and diethyl ether among them. 

The structural constraints used in the first case study namely, Eqn s 27,28 and 29 are used again. The melting point, boiling point and flash point, are used as constraints for both solvent and anti-solvent. Since the solvent needs to have high solubility for solute and the anti-solvent needs to have low solubility for the solute limits of 17 <8 < 19 and 5 > 30 (Eqn s. 33 and 37) are placed on the solubility parameters of solvent and anti-solvents respectively. Eqn.38 gives the necessary condition for phase stability (Bernard et al., 1967), which needs to be satisfied for the solvent-anti solvent pairs to be miscible with each other. Eqn. 39 gives the solid-liquid equilibrium constraint. 

The miscibility of solvent anti-solvent pairs is considered in sub-problem 4M through constraint represented by Eqn.38. Only 6 pairs were found to be mutually miscible with each other. 

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