Solvent classification chemical constitution

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. 

Some other classification schemes are provided in a work by Kolthoff (Kolthoff, 1974). It is according to the polarity and is described by the relative permittivity (dielectric constant) e, the dipole moment p (in 10 ° C.m), and the hydrogen-bond donation ability Another suggested classification (Parker, 1969) stresses the acidity and basicity (relative to water) of the solvents. A third one (Chastrette, 1979), stresses the hydrogen-bonding and electron-pair donation abilities, the polarity, and the extent of self-association. A fourth is a chemical constitution scheme (Riddick et al., 1986). The differences among these schemes are mainly semantic ones and are of no real consequence. Marcus presents these clearly (Marcus, 1998). 

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. 

Solvents can be classified as EPD or EPA according to their chemical constitution and reaction partners [65]. However, not all solvents come under this classification since e.g. aliphatic hydrocarbons possess neither EPD nor EPA properties. An EPD solvent preferably solvates electron-pair acceptor molecules or ions. The reverse is true for EPA solvents. In this respect, most solute/solvent interactions can be classified as generalized Lewis acid/base reactions. A dipolar solvent molecule will always have an electron-rich or basic site, and an electron-poor or acidic site. Gutmann introduced so-called donor numbers, DN, and acceptor numbers, AN, as quantitative measures of the donor and acceptor strengths [65] cf. Section 2.2.6 and Tables 2-3 and 2-4. Due to their coordinating ability, electron-pair donor and acceptor solvents are, in general, good ionizers cf. Section 2.6. 

The diversity of solvents makes classification very complex and many different ways of classifying solvents have been used. Broadly, solvents may be classified as aqueous, nonaqueous molecular, nonaqueous ionic, and atomic. The ways in which solvents are classified according to their chemical constitution and then according to their physical properties are briefly discussed below. 

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