Solvents, acceptor number aprotic

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]. 

Figure 2.11, on the other hand, shows the acceptor numbers of mixtures of water and aprotic solvents. Because water is protic and selectively interacts with Et3P = 0 (strong Lewis base), many of the relations curve upward. However, with HMPA, the relation curves downward, because HMPA is a strong base and easily interacts with water to weaken the interaction between water and Et3P = O. The acidity and basicity of mixed solvents are influenced not only by the acidity and basicity of the constituent solvents but also by the mutual interactions between the molecules of constituent solvents. At present, however, this cannot be treated theoretically. 

Fig. 2.11 Acceptor numbers of mixtures of water and aprotic solvents [13]. Aprotic solvents are shown on the curves.

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. 

Figure 2. Correlation of the peak height h of the Fourier-ttansform spectrum for Br ion with the acceptor number/4j j of the solvent ( ) protic and (O) aprotic solvents. Taken from ref. 5.

Extensive studies have been made of solvent effects on atom transfer reactions involving ions [12]. In the case of reaction (7.3.23), the rate constant decreases from 250M s in A-methylpyrrolidinone to 3 x 10 M s in methanol. This effect can be attributed to solvation of the anionic reactant Cl and the anionic transition state [12]. Since the reactant is monoatomic, its solvation is much more important. It increases significantly with solvent acidity leading to considerable stabilization of the reactants. As a result the potential energy barrier increases and the rate decreases with increase in solvent acidity. As shown in fig. 7.7, this leads to an approximate linear relationship between the logarithm of the rate constant and the solvent s acceptor number AN, an empirical measure of solvent acidity (see section 4.9). Most of the results were obtained in aprotic solvents which have lower values of AN. The three data points at higher values of AN are for protic solvents. 

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). ... 

In contrast to the above prochiral donor reactions, only a limited number of prochiral acceptor reactions have been reported (Scheme 3) [13]. Nitromethane did not add to chalcone (13) in the presence of 1 in aprotic solvents. Although... 

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