Organic solvents donor number

Titanium Tetraiodide. Titanium tetraiodide [7720-83 ] forms reddish-brown crystals, cubic at room temperature, having reported lattice parameter of either 1200 (149) or 1221 (150) pm. Til melts at 150°C, boils at 377°C, and has a density of 440(0) kg/m. It forms adducts with a number of donor molecules and undergoes substitution reactions (151). It also hydrolyzes in water and is readily soluble in nonpolar organic solvents. 

The generalized procedure described here has been demonstrated with a large number of aromatic ketone substrates, including those described in Table 8.1. When the goal is the production of a particular (5)-cyanohydrin, specialized process improvements to parameters, such as the operating temperature and pH and the choice and concentration of organic solvent and cyanide donor, may further increase both the product ee and yield values. 

Tab. 1.5 Chemical properties of organic solvents of electrochemical interest. Donor numbers (DN), accepptor numbers (AN), and autoprotolysis constants (p/[Pg.18]

The acid-base properties of a mixed solvent is also an important factor influencing the behavior of solutes. Thus, the parameters of the acidity and basicity of mixed solvents have been studied to some extent [35], Figure 2.10 shows the donor numbers of mixtures of nitromethane and other organic solvents. Because ni-tromethane has very weak basicity (DN= 2.7), the addition of small amounts of basic solvents (HMPA, DMSO, pyridine) increase the donor number remarkably. 

Fig. 4.4 Relation between the half-wave potentials of K+ (V vs BCr+/BCr) in various organic solvents and the donor number of solvents [3]. For solvents, see Fig. 4.2.

The first silver(I) poly(pyrazolyl)borate complexes were reported in 1975.150 Since then, a number of silver complexes of the type Ag(L)(R B(pz)4 ) have been isolated and characterized.151,152 They were prepared by the reaction of the poly(pyrazolyl)borate anions with silver(I) salts, usually the nitrate, in the presence of donor ligands, L, and were isolated as white, thermally stable compounds. In most cases they were either insoluble, or at best only sparingly soluble, in common organic solvents. 

TABLE 7.2 Donor Number, DNSbCu of Some Organic Solvents... 

Separation of D and A centers by non-conducting media resulted in the strong dependence of the ET rate on distance between D and A and the marked effect of the chemical nature of saturated molecules and bonds between the pair. This dependence can be quantitatively characterized be the decay factor, (3, (Eq. 2.27). The following values of P (in A 1) were found 3-4 (vacuum), 1.6 - 1.75 (water), 1.2 (organic solvents) and 1.08 -1.2 (synthetic D-bridge-A molecules). The effects of distance and the number of intermediate saturated groups (n) on photoinduced electron transfer between a donor and acceptor are discussed in (Verhoeven, 1999). 

Table 2-3. Donor numbers (donicities) DN [199, 200, 212, 241, 339] and normalized DN values [200] of a selection of thirty-six organic EPD solvents , determined calorimetrically in dilute 1,2-dichloroethane solutions at room temperature and valid for isolated EPD solvent molecules . ...

These quantities have been termed acceptor number AN (or acceptivity) and they were obtained from the relative P NMR chemical shift values corr (n-hexane as reference solvent) with respect to that of the 1 1 adduct EtsPO—SbCls dissolved in 1,2-dichloroethane, which has been arbitrarily taken to have the value of 100. The acceptor numbers are dimensionless numbers expressing the acceptor property of a given solvent relative to those of SbCb, which is also the reference compound for assessing the donor numbers. A compilation of organic solvents in order of increasing acceptor number is given in Table 2-5. 

It should be mentioned that cation complexation by crown-type ligands can itself be solvent-dependent. For example, the dissociation rate of potassium [2.2.2]cryptate in EPD solvents increases with the donor number of the solvent [650]. Moreover, coronands themselves can interact with organic solvent molecules [651]. Such cation-solvent and ligand-solvent interactions can influence the formation of cation-ligand complexes. 

On the other hand, C0CI2 is insoluble in pure liquid SbClg since cation stabilization cannot take place. The majority of solvents that are extensively used in solution chemistry (particularly in the field of organic chemistry) are typical EPD solvents. Gutmann (25-28) has introduced the so-called donor number or donicity (DN) as a measure of the EPD properties of donor solvents. This is defined as the negative AH values for formation of the 1 1 adduct of the EPD with SbClg as reference standard EPA in a dilute solution of 1,2-dichloroethane. Donicities for various solvents are listed in Table I together with their dielectric constants e. 

The investigations carried out on the rate of ion-transfer electrode reactions in mixtures of water with solvents with a higher donor number than water, such as HMPA, DMF, or DMSO, revealed that the rate constant was decreased when the concentration of the organic solvent in the mixture increased [221-223, 225, 226, 228, 229, 231, 233, 275], but no minimum was found. The shape of this decrease depends on the mixed solvent and to some extent also on the nature of the reaction under study. 

Equation (67), which describes the change in the experimental rate constant k [228] in this model, is formally very similar to the equation which was used earlier to describe the inhibition [294, 295] of electrode reactions. Such an equation was used hy Kisova [296]. Later it was found that Eq. (68) better describes the rate constant-solvent composition dependence. However, it fails to describe this dependence at high concentrations of an organic solvent which has a donor number lower than that of water. In general, it fails when the rate constant-solvent composition dependence exhibits a minimum. 

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