Classification of solvents

Solvents can be classified as polar, non-polar and apolar, with the definitions being as above. However, a more general classification uses the relative permittivity. There are high relative permittivity solvents such as (at 25°C)  

Solvents may be classified according to their physical and chemical properties at several levels. The most striking differences among Uquids that could be used as solvents are observed between molecular liquids, ionic liquids (molten salts or salt mixtures, room-temperature ionic liquids), and metals. They can be considered as extreme types, and represented as the three vertices of a triangle (Tremillon, 1974) (see Fig. 1.2). Intermediate types or mixtures can then be located along edges or within the triangle. The room-temperature ionic liquids (see later. Section 8.3), which 

FIGURE 1.2 Ternary diagram for classification of liquids (schematic location of points is conjectural) [bmimjPF represents a room-temperature ionic liquid (see Section 8.3). After Tremillon (1974). 

Inert-polarizable Benzene (x-EPD), tetrachloromethane, carbon disulfide, tetracyanoethene (jt-EPA) 

Amphiprotic Water, alcohols ammonia is more protophilic than protogenic, while hydrogen fluoride is the reverse 

Dipolar-aprotic Dimethylformamide, acetonitrile (EPD, weak HBA), dimethyl sulfoxide, hexamethylphosphortriamide 

The values of AN are also included in Table 1.5.6) The solvent acidity increases with the increase in the AN value. Here, it should be noted that neither DN nor AN can be correlated with the relative permittivity of the corresponding solvents. 

Lewis acids are electron pair acceptors and Lewis bases are electron pair donors. However, according to the Hard and Soft Acids and Bases (HSAB) concept [17], Lewis acids are classified into hard and soft acids, while Lewis bases are classified into hard and soft bases. Hard acids interact strongly with hard bases, soft acids with soft bases. 

6) Riddle and Fowkes [19] considered that the values of AN, determined by the NMR method, are partly due to the van del Waals forces between Et3P = O and solvent molecules and attributed somewhat large AN values of strongly basic solvents like pyridine to it. 

They proposed a new acceptor number, which was corrected for the influence of the van der Waals forces. 

Studies of solvent structure are usually carried out by analyzing radial distribution functions that are obtained by X-ray or neutron diffraction methods. Monte Carlo (MC) or molecular dynamics (MD) calculations are also used. Studies of the structure of lion-aqueous and mixed solvents are not extensive yet but some of the results have been reviewed. Pure and mixed solvents included in the reviews 

Due to the physieal and ehemical differences between the numerous organic and inorganic solvents it is diffieult to organise them in a useful seheme. Here, five attempts at a elassifieation of solvents are presented, which should prove useful to the chemist. Due to broad definitions, some overlapping of these is unavoidable. As has been customary in previous reviews non-aqueous organic solvents will reeeive partieular attention [1-15, 103-108, 172, 174-177]. Extensive compilations of chemical and physical properties of non-aqueous solvents can be found in references [11-14, 104, 106, 175, 177]. 

---

We now consider a type of analysis in which the data (which may consist of solvent properties or of solvent effects on rates, equilibria, and spectra) again are expressed as a linear combination of products as in Eq. (8-81), but now the statistical treatment yields estimates of both a, and jc,. This method is called principal component analysis or factor analysis. A key difference between multiple linear regression analysis and principal component analysis (in the chemical setting) is that regression analysis adopts chemical models a priori, whereas in factor analysis the chemical significance of the factors emerges (if desired) as a result of the analysis. We will not explore the statistical procedure, but will cite some results. We have already encountered examples in Section 8.2 on the classification of solvents and in the present section in the form of the Swain et al. treatment leading to Eq. (8-74). 

Based on previous classifications and according to criteria discussed in Ref. [15], we have extended the classification of solvents into eight classes ... 

The dielectric constant and refractive index parameters and different functions of them that describe the reactive field of solvent [45] are insufficient to characterize the solute-solvent interactions. For this reason, some empirical scales of solvent polarity based on either kinetic or spectroscopic measurements have been introduced [46,47]. The solvatochromic classification of solvents is based on spectroscopic measurements. The solvatochromic parameters refer to the properties of a molecule when its nearest neighbors are identical with itself, and they are average values for a number of select solutes and somewhat independent of solute identity. 

The solvatochromic classification of solvents takes into consideration only the polar interactions of the solvents and not their cohesion. The transfer of a solute from one solvent to another occurs with the cancellation of dispersion interactions [38]. 

Snyder s classification of solvent properties is important in the selection of the chromatographic conditions and the optimization of the chromatographic processes. 

Fig. 30.10. Hierarchical agglomerative classification of solvents according to solvent-solute and solvent-solvent interactions [13].

To begin with, molecular solvents with high permittivities will be considered. Classification of solvents on the basis of their permittivities agrees roughly with classification as polar and non-polar, and the borderline between these two categories is usually considered to be a relative dielectric constant of 30-40. Below this value ion pairs are markedly formed. From... 

Liquid chromatography (LC) activated alumina applications, 2 400 adsorption, 1 610-611 of ascorbic acid, 25 760 basic principles, 4 603-606 classification of solvents for, 23 87... 

Table 4.8 Classification of solvent based on hydrogen bonding...

Classification of Solvents using Physical Constants The following physical constants can be used to characterize the properties of a solvent melting and boiling point, vapor pressure, heat of vaporization, index of refraction, density, viscosity, surface tension, depose moment, dielectric constant, polarizability, specific conductivity, and so on. 

Classification of Solvents in Terms of Specific Solute-Solvent Interactions Parker divided solvents into two groups according to their specific interactions with anions and cations, namely dipolar aprotic solvents and protic solvents (Parker, 1969). The distinction lies principally in the dipolarity of the solvent molecules and their ability to form hydrogen bonds. It appears appropriate to add to these two groups a third one, namely, the apolar aprotic solvents. 

Classification by Chemical Constitution Classification of solvents according to chemical constitntion allows certain qualitative predictions. In general, a compound dissolves far more easily in a solvent possessing related functional groups than in one of a completely different nature (see table 3.11). A proper choice of solvent, based on the knowledge of its chemical reactivity, helps to avoid undesired reactions between solute and solvent. 

The classification of solvents has been dealt with in various books on non-aque-ous solvents [25, 26]. In the classification of solvents, it is usual to use some solvent properties as criteria. In order to discuss solvent effects on chemical reactions, it is convenient to use relative permittivities and acid-base properties as the criteria. 

For practical reasons the different solubility can be used as a basis for a classification of solvent dyes, although there is no strict differentiation. Chemical constitution is defined here as a structure which meets the corresponding solvent requirements. 

Classification of solvent-free techniques in the context of sample preparation for analysis. 

Protic character. The protic character of the solvent is an important consideration because electrochemical intermediates (particularly radical anions) frequently react rapidly with protons. The classification of solvents into protic or aptotic solvents is somewhat arbitrary. A simple classification1 is that protic solvents (such as hydrogen fluoride, water, methanol, formamide, and ammonia) are strong hydrogen-bond donors, exchange protons rapidly, and in-... 

A classification of solvents can be developed on the basis of the stability of the radial anion produced by reduction of aromatic hydrocarbons, such as naphthalene and anthracene. The solvent reactions of such anions have been widely studied2 and have generally been found to go by a sequence of reactions in either a protic solvent or in the presence of a proton donor in an aprotic solvent 3... 

Classification of Solvents. Solvent classification helps to identify properties useful in solvent selection for individual applications for example, the study of acid-base reactions, oxidation-reduction reactions, inorganic coordination chemistry, organic nucleophilic displacement reactions, and electrochemistry. 

Kg is the experimental distribution coefficient and K g the corrected value. This correction is required, because any measure for the interactions that occur in certain solvents should be more related to the ratio of mole fractions than to the ratio of concentrations of the solute in the liquid phase and in the gas phase. We may assume the molar volume of the gas phase to be constant and hence irrelevant if our purpose is a classification of solvents. However, the molar volumes of solvents vary a great deal. The Kg values for n-octane in various hydrocarbon solvents vary up to a factor of 3.9 between cyclohexane and squalane [216]. The Kg values vary by a more realistic factor of 1.5 [214]. 

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... 

Table 19. Classification of solvents according to their ability of promoting the formation of stereocomplex between iso-PMMA and synd-PMMA299)...

---