Solvent selectivity correlation

NRTL-SAC has been demonstrated through the case study on Cimetidine as a valuable aid to solubility data assessment and targeted solvent selection for crystallization process design. The average model error is typically 0.5 Ln (x) [1] and is sufficient as a solvent screening tool. Methods that can deliver greater accuracy would increase the value and utility of these techniques. It is impressive in the case of Cimetidine that the NRTL-SAC correlation is capable of reasonable accuracy and predictive capability on the basis of just 2 fitted parameters. Further work to extend the solvent database and optimize the descriptive parameters will be beneficial, and are planned by the developers. 

There have been several attempts to correlate observed chemical, biological, or spectral properties of nitrenium ions with calculated proper-The correlations of azide-solvent selectivities with the... 

Table 4.5 suggests the individual areas of expertise necessary to make a strong analysis of solvent selection, using LCA principles. As previously emphasized, the areas impacted by the chemical process must be represented during the evaluation. There will be a direct correlation between the quality/composition of the cross-functional team and the quality of the conclusions reached. 

Since there are so many solvents to choose from, it is natural that the search for guidelines for solvent selection has been intense. Researchers have tried to correlate enzyme activity, stability, and selectivity with different solvent descriptors, such as logP, dielectric constant, dipole moment, Hildebrand solubility parameters, and many others. When this approach is successful, the search for the optimal solvent can be limited to those having suitable values of the selected solvent descriptor(s). A list of solvent descriptors of a range of commonly used solvents is given in Table 1.4. 

The main strength of the Snyder scheme is for the classification of solvent selectivity. We have seen from table 2.8 that solvents that are chemically similar yield similar selectivity parameters. This type of classification can be made on the basis of structural information alone. However, the Snyder scheme goes one step further, in that it classifies different chemical classes into a single selectivity group. From the definition equations (2.14 through 2.17) we see that the three selectivity parameters are correlated by the equation... 

The reactivity-selectivity relationship observed by Harris et al. (1973, 1974a) and illustrated in Fig. 10 provides additional support for this view. Thus 1-adamantyl, 2-adamantyl, and exo-2-norbonyl chlorides, which are hindered to back-side nucleophilic attack, show negative selectivity values. The reason for the negative selectivity value exhibited by 4,4 -dichlorodiphenylchloromethane and for its failure to conform to the reactivity-selectivity correlation is obscure. However, the result itself suggests that product formation is predominantly via the solvent-separated ion pair. 

Selecting the proper solvent by considering this criterion is still based on empirical approaches because of the large nonideality of the resulting mixtures. However, general selection patterns and rapid experimental techniques have been made available through the years. This paper presents a review of some of these methods to facilitate the solvent selection process in the chemical industry. Qualitative aspects are first considered, followed by empirical correlations and rapid experimental techniques. 

Correlations for various systems, developed by using experimental data on 265 systems, are available (II, 26). The relationships used, the numerical values of the constants, and the calculated and experimental values for y° are available (13) and should be used to study solvent selection. 

It has been stated that, when specific hydrogen-bonding effects are excluded, and differential polarizability effects are similar or minimized, the solvent polarity scales derived from UV/Vis absorption spectra Z,S,Ei 2Qi),n, Xk E- ), fluorescence speetra Py), infrared spectra (G), ESR spectra [a( " N)], NMR spectra (P), and NMR spectra AN) are linear with each other for a set of select solvents, i.e. non-HBD aliphatic solvents with a single dominant group dipole [263]. This result can be taken as confirmation that all these solvent scales do in fact describe intrinsic solvent properties and that they are to a great extent independent of the experimental methods and indicators used in their measurement [263], That these empirical solvent parameters correlate linearly with solvent dipole moments and functions of the relative permittivities (either alone or in combination with refractive index functions) indicates that they are a measure of the solvent dipolarity and polarizability, provided that specific solute/ solvent interactions are excluded. 

Fio, 15. Correlation of solvent selectivity effects with solvent-solute localization and Eq. (31a) (a) alumina, different solute pairs for each plot, 18 different polar solvents (b) silica, as in (a), except ternary- and quaternary-solvent mixtures used localizing agent , MTBE , ACN (O) chloroform or dichloromethane. The solute pairs are as follows (a) 1—1-naphthaldehyde and 1-cyanonaphthalene 2—1-nitronaphthalene and 1,2-dimeth-oxynaphthalene 3—1,5-dinitronaphthalene and 1-acetylnaphthalene (b) I—1-nitronaphthalene and 2-methoxynaphthalene 2—1,5-dinitronaphthalene and 1,2-dimethoxynaphtha-lene 3—methyl l-naphthoate and 2-naphthaldehyde. Reprinted from Snyder< f /. (/J. /it). 

Fig. 18. Correlation of solvent selectivity effects with solvent-solute localization and Eq. (31a. Data for selected diastereomeric solute pairs. Reprinted with permission from Snyder (39).

Fig. 19. Correlation of solvent-selectivity parameter m° for silica versus alumina. Values taken from Table I.

While there is no unique criterion for choosing 4 E, the selection must lead to an accurate theory of solvation dynamics without invoking two-time many-point correlation functions. We have found that this goal can be achieved with a new theory for the nonequilibrium distribution function in which the renormalized solute-solvent interactions enter linearly. In this theory and are chosen such that the renormalized linear response theory accurately describes the essential solute-solvent static correlations that rule the equilibrium solvation both at t = 0 (when solvent is in equilibrium with the initial charge distribution of the solute) and at 1 = oc (when the solvent has reached equilibrium with the new solute charge distribution). ... 

The PRISMA model developed by Nyiredy and co-workers (Nyiredy et al., 1985 Dallenbach-Tolke et al., 1986 Nyiredy and Fater, 1995 Nyiredy, 2002) for use in Over Pressured Layer Chromatography is a three-dimensional model that correlates solvent strength and the selectivity of different mobile phases. Silica gel is used as the stationary phase and solvent selection is performed according to Snyder s solvent classification (Tab. 4.7). 

Additional support for the proposed model was provided from the correlation obtained between the 2 computations of TK for the ethylene glycol, ethanol, NaA system when solvent selectivity data for the 2 systems, ethanol, water, NaA and ethylene glycol, water, NaA were employed in the model to evaluate ac and aeg. The a° values for ethylene glycol and ethanol, respectively, that were obtained (Table IV) were... 

The retention mechanism and solvent selectivity have been studied most carefully with alumina or silica as stationary phases. The knowledge of both for bonded phases used in normal-phase chromatography is much more limited. Nevertheless, it is safe to assume that similar selectivity rules for solvent strength and selectivity can be applied, especially since the results obtained for alumina and silica correlate well with each other. 

The ab initio hydration energies of a number of arylnitrenium ions, obtained by comparison with (79), have been correlated with their azide ion/solvent selectivities in aqueous solution, in an attempt to gain an understanding of their kinetic labilities. For the most part the linear correlation obtained was good, although not for the most reactive species. It was determined that the most important resonance contributor to... 

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