Solvents, acceptor number dipolar

For an organic compound (Q) in dipolar aprotic solvents, the half-wave potential ( 1/2) of the first reduction step tends to shift to the positive direction with an increase in solvent Lewis acidity (i.e. acceptor number). This is because, for the redox couple Q/Q, the reduced fonn (Q ) is energetically more stabilized than the oxidized fonn (Q) with increasing solvent acidity. The positive shift in E1/2 with solvent acceptor number has been observed with quinones [57 b], benzophenone [57 a, c] and anthracene [57 c], With fullerene (C60), the positive shift in E1/2 with solvent acidity parameter, ET, has been observed for the reductions of C60 to Qo, Qo to Clo, and Cf)0 to Cli, [54c], However, the positive shift in E1/2 is not apparent if the charge in Q is highly delocalized, as in the cases of perylene and fluoren-9-one [57 c]. 

Acceptor numbers of various solvents are also listed in Table 3. The values range from zero, for the reference solvent -hexane, to about 130, for trifluoro-methane sulfonic acid. For instance, the acceptor number of aliphatic alcohols varies between 27 and 41 (methanol). Within the group of dipolar aprotic solvents there are considerable differences in acceptor properties. Solvents such as propylenecarbonate, tetramethylene-sulfone, acetonitrile, dimethylsulfoxide, or nitromethane are stronger acceptors than solvents such as acetone, A-methylpyrroli-done, or dimethylacetamide. The acceptor strengths of amine solvents vary considerably with the degree of substitution. For instance, triethylamine has no acceptor properties. 

Acceptor numbers are less than 10 for nonpolar non-HBD solvents, they vary between about 10... 20 for dipolar non-HBD solvents, and they cover a wide range of about 25... 105 for protic solvents cf. Table 2-5). Surprisingly, benzene and tetra-chloromethane have stronger electrophilic properties than diethyl ether and tetrahy-drofuran. Acceptor numbers are also known for binary solvent mixtures [70, 213]. 

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. 

Table 17 lists a number of reactions of hydroxide ion and, for comparison, superoxide ion with electron donors. These reactions are classified conventionally and according to the categories shown in Scheme 14. When water is replaced by a dipolar aprotic solvent (e.g. MeCN or DMSO), many of these electron donor-electron acceptor reactions proceed at dramatically faster rates and produce much larger yields of free-radical products. (DMF is generally avoided because of the possible hydrolysis of the amide bond by hydroxide ion). ... 

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