Ionizing power of a solvent

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)... 

Z-value (or polarity parameter Z) — is an index of the ionizing power of a solvent based on the frequency of the longest wavelength electronic absorption maximum of l-ethyl-4-methoxycarbonylpyridinium iodide in the solvent. The Z-value is defined by Z = 2.859 x 104/A where Z is in kcalmol-1 and A is in nm, respectively. 

Here, k is the rate constant for reaction of a compoxmd in a particular solvent and ko is the rate constant of the compound in the reference solvent system. Y is the ionizing power of a solvent, and m is the sensitivity of the substrate to solvent ionizing power. Defining m to be 1.00 for the reaction of f-butyl chloride in methanol, the value of Y for another solvent is then calculated from equation 8.18. 

Grunwald and Winstein tried to relate the rates of solvolysis reactions to the properties of the solvent when the process is of the SN type involving ionization of the reactant. The ionizing power of a solvent is measured by the quantity... 

Recognizing Y as the measure of the ionizing power of a solvent, it appeared probable that Y should also depend on the nature of the substrate undergoing solvolysis. It was therefore necessary to introduce an additional parameter m and thus correct eq. (1.11.8) into eq. (1.11.9) ... 

There are also other scales for characterization of solvents ionizing power. For instance, Kosower found that positions of certain absorption maxima ( charge transfer bands ) in the UV spectrum of l-methyl-4-carbomethoxy-pyridinium iodide depend very strongly on solvent polarity the better the ionizing power of a solvent, the better is the solvation of the ion pair ground state, which is, therefore, lower in energy relative to the ion pair in the gas phase. Thus, more energy is needed for formation of the excited state in solution... 

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 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. 

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. 

In either dimethyl-formamide or dimethyl sulfoxide, the reaction rates became too fast to measure even in the absence of a catalyst. It thus appears that while the ionizing power of the solvent as indicated by the dielectric constant is an important factor for the solvent effect, it is not the only one. The slow reaction in the case of acetonitrile may have been caused by the nitrile competing with the isocyanate for the electrons of the base catalyst and thereby neutralizing the catalyst by complexing. 

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