Electron-pair donicity

For example, according to Reichardt and references quoted by him (Reichardt 1988), the enol form of ethyl acetoacetate, CH3C(OH)=CHCOOC2H5, constitutes 65% of it in cyclohexane, 28% in toluene, 11.5% in acetone, and 5% in dimethylsulfoxide, due to competition between intra- and inter-molecular hydrogen bonding of the enol form. When such a competition is precluded, as in 5,5-dimethyl-l,3-cyclohexanedione, the opposite trend is observed there is 7% enol in toluene, 81% in acetone and 99% in dimethylsulfoxide (Reichardt 1988). In this case, the solvent with a higher electron pair donicity favours the hydrogenbonding enol form. 

The solvatochromic effects on UV/visible spectra of certain solutes are so large, that they can conveniently be employed as probes for certain solvating properties of the solvents. Those that have enjoyed widespread application in this capacity are discussed in Chapter 4. They include 2,6-diphenyl -4-(2,4,6-triphenyl-l-pyridino)-phenoxide, 4-methoxynitrobenzene, 4-(dimethylamino)-nitrobenzene, for the estimation of the polarity of solvents, acetylacetonato-N,N,N, N -tetramethylethylenediamino-copper(II) perchlorate, 4-nitrophenol, and 4-nitroaniline, for the estimation of the electron pair donicity of solvents, 4-carboxymethyl-l-ethylpyridinium iodide, 4-cyano-l-ethylpyridinium iodide, and bis-c/.v-1, lO-phenanthrolinodicyano-iron(II) for the estimation of the hydrogen bond donation abilities of solvents (Marcus 1993). 

The DN and P electron pair donicity scales are closely related to each other, as is to be expected. The correlation expression, established for 25 solvents for which both measures were known at the time, was DN = -0.9 + 39.18P (Kamlet et al. 1985). A correlation involving many more solvents has since been established ... 

TABLE 3.9 Indexes of Solvent Solvation Ability Polarity and Ji, Electron Pair Donicity, DN and /, Hydrogen Bond Donicity, AA and a, and the Softness Parameter, /t... 

Electron Pair Donicity and AbiUty to Accept a Hydrogen Bond The ability of solvent molecules to donate a free electron pair from their donor atoms (O, N, or S) to coordinate with acceptor atoms of solutes is a measure of the solvent donicity. It can also be construed as its basicity in the Lewis and the Brbnsted senses, because it also describes the ability of the solvent molecules to accept a proton from a Bronsted acid to be protonated or to form a hydrogen bond. 

The electron pair donicity and acceptance ability of solvent molecules are reflected in their bulk properties dealt with in Sections 3.3.2.2 and 3.3.2.3. They are, of course, related to their reactions as acids and bases pertaining specifically to the deproton-ation or protonation in the gas phase, on the one hand, and these reactions in dilute solutions in a reference solvent (water) on the other, as well as their autoprotolysis as pure liquids. These topics are dealt with in the present section. 

The actual value of the double-layer capacitance depends on many variables including electrode type, electrochemical potential, oxide layers, electrode surface heterogeneity, impurity adsorption, media type, temperature, etc. [1, pp. 45-48]. Capacitance of the double layer also largely depends on the intermolecular structure of the analyzed media, such as the dielectric constant (or high-frequency permittivity), concentration and types of conducting species, electron-pair donicity, dipole moment, molecular size, and shape of solvent molecules. Systematic correlation with dielectric constant is lacking and complex, due to ionic interactions in the solution. In ionic aqueous solutions with supporting electrolyte ("supported system") the values of -10-60 pF/ cm are typically experimentally observed for thin double layers and solution permittivity e - 80. The double-layer capacitance values for nonpolar dielec-... 

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