Ionizing power of solvents, 57

In order to gain a better insight into the basic relationships between ionizing power of solvents and their coordination chemical properties, it is necessary to study both the EPD effect at the cation A and the EPA effect at the anion B separately. The role of the EPD effect has been successfully studied by the use of conductometric techniques for investigating the ionization equilibrium,... 

Grunwald- Winstein Y Ionization of f-BuCl in 80 20 EtOH/HjO Ionizing power of solvent III > 1, more sensitive III < 1, less sensitive... 

Table 4.11 lists the Y values for some alcohol-water mixtures and for some other solvents. The Y value reflects primarily the ionization power of the solvent. It is largest for polar... 

The ionization eonstant should be a function of the intrinsic heterolytic ability (e.g., intrinsic acidity if the solute is an acid HX) and the ionizing power of the solvents, whereas the dissoeiation constant should be primarily determined by the dissociating power of the solvent. Therefore, Ad is expeeted to be under the eontrol of e, the dieleetrie eonstant. As a consequenee, ion pairs are not deteetable in high-e solvents like water, which is why the terms ionization constant and dissociation constant are often used interchangeably. In low-e solvents, however, dissociation constants are very small and ion pairs (and higher aggregates) become important species. For example, in ethylene chloride (e = 10.23), the dissociation constants of substituted phenyltrimethylammonium perchlorate salts are of the order 10 . Overall dissociation constants, expressed as pArx = — log Arx, for some substanees in aeetie acid (e = 6.19) are perchloric acid, 4.87 sulfuric acid, 7.24 sodium acetate, 6.68 sodium perchlorate, 5.48. Aeid-base equilibria in aeetie acid have been earefully studied beeause of the analytical importance of this solvent in titrimetry. 

Ideally, Y should measure only the ionizing power of the solvent, and should not reflect any backside attack by a solvent molecule in helping the nucleofuge to leave (nucleophilic assistance k, p. 411). Actually, there is evidence that many solvents... 

It is clear that the ionizing power of the solvent used is important in many of these reductions. When 2,4,6-trimethylbenzyl chloride is heated with diphenylsi-lane in nitrobenzene at temperatures as high as 130°, no isodurene is formed.193 Not unexpectedly, the same lack of reactivity is reported for a series of benzyl fluorides, chlorides, and bromides substituted in the para position with nitro or methyl groups or hydrogen when they are heated in nitrobenzene solutions with triethylsilane, triethoxy silane, or diphenylsilane.193... 

In 1948, Grunwald and Winstein2 4 attempted to define the ionizing power of a solvent by the Y parameter, based on the comparison of the rate for the solvolysis of t-butyl chloride. In 1956, Kosower5 made an attempt to define the polarity of a medium (solvent) by introducing the Z parameter based on the spectroscopic properties (in various solvents)... 

In solution of pure HMPA,DMSO, or DMF, (CH3)3SnI is found to be completely ionized as 1 1 electrolyte. This observation leads to the conclusion that the equivalent conductivities are a measure of the relative ionizing power of the donor. Thus we can say that the relative ionizing power of a donor solvent increases with an encrease in the donicity of the solvent molecules. 

Gropp measured the conductivity of liquid and frozen soln. of lithium, sodium, and potassium chlorides. F. Ratig studied the electro-chemical action—vide alkali chlorates. The electrical conductivity of soln. of lithium chloride in several non-aqueous solvents has been investigated. Formic acid as a solvent exerts an ionizing power of the same order of magnitude as water in acetic acid, the lithium chloride seems to be partially associated to double molecules, > and in some solvent,... 

In equation (10), Y is the ionizing power of the solvent and N its nucleophilic power with m and / measuring the sensitivity of the substrate to these factors. By selecting model compounds, e.g. t-butyl chloride, where only Y should be important and methyl chloride, where both N and Y would be important, then it is possible to obtain values of Y and N for various solvent mixtures. An examination of the variation of the rate of reaction of a given substrate in these solvents according to equation (10) would then reveal the relative importance of the associative and dissociative character to the reactions. 

Actually, two factors are relevant in regard to the ionizing ability of solvents. First, a high dielectric constant increases ionizing power by making it easier to separate ions. This is because the force between charged particles varies inversely with the dielectric constant of the medium.7 Thus water, with a dielectric constant of 80, is 40 times moreveffective than a hydrocarbon with a dielectric constant of 2. Second, and usually more important, is the ability of the solvent to solvate the separated ions. Cations are solvated most effectively... 

The quantity Y = log k - log k0 characterizes the ionizing power of the solvent (Grunwald and Winstein 1948). Here k0 is the rate constant at 25°C in the reference solvent, 80% v/v ethanol + 20% v/v water, and k is the rate constant in any other solvent studied. Representative values of Y are shown in Table 2.5 (Reichardt 1998 Grunwald and Winstein 1948 Abraham 1972, 1985 and Parker 1978). 

The difference in mechanisms of addition of trifluoroacetic acid (see above) and HC1 (in acetio acid) to 3-hexyne has been tentatively explained as due to the different nucleophilic and ionizing power of the two solvents. 

The relative yields of cyclized products increase greatly with the decreasing nucleophilicity (2a-c, 3 and 4 of Table 3) and with the increasing ionizing power of the solvent. Cyclobutanone derivatives (32) are usually obtained in much greater amounts than cyclopropyl ketones (33) but, when mercuric ions are added, formation of 33 is overwhelming. The effect of R in 31 on the product distribution is illustrated in cases 3b, 4a, 5a, 6a, and 7. 

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