Solvents hydrogen-bond donicity

The solvents used for electroanalytical determinations vary widely in their physical properties liquid ranges (e.g., acetamide, N-methyl-acetamide and sulfolane are liquid only above ambient temperatures), vapour pressures (Table 3.1), relative permittivities (Table 3.5), viscosities (Table 3.9), and chemical properties, such as electron pair and hydrogen bond donicities (Table 4.3), dissolving ability of the required supporting electrolyte to provide adequate conductivity, and electrochemical potential windows (Table 4.8). A suitable solvent can therefore generally be found among them that fits the electroanalytical problem to be solved. 

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... 

The solvent index , (30) depends on both the polarity and the hydrogen bond donicity of the solvent or binary aqueous mixture with a cosolvent. In all the mixtures, except aqueous acetonitrile, this index is smaller than expected for ideal mixtures, as the negative < values indicate for the equimolar mixtures. The polarity/polarizability index % may be smaller (for most mixtures) or larger (for aqueous pyridine, form-amide, DMF, and DMSO) than expected for ideal mixtures, as the sign of shows for the equimolar mixtures. The latter option (p >0) occurs where the cosolvent is aromatic (pyridine) or has the =0 group (except acetone) that contributes to the polarizability. 

The half-wave potentials of K+, Tl+ and Ca2+ in water are slightly more negative and thosefor Zn2+, Cd2+, Mn2+, Ni2+ and Co2+ considerably more negative than is expected according to the donicity rule. It has been shown in the previous sections that water is a rather unique solvent. The effect in question may be interpreted by the so-called Katzin-effect according to which water forms a royal core of coordinated water molecules which are hooked together by hydrogen bonds 70,71>122,1231. 

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). 

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. 

Hence the one donicity measure can be estimated for a solvent when the other is known, but outliers do occur. It was recommended [38] to abandon the averaged P values (listed in Table 3.9) in favor of those obtained by the 4-nitrophenol probe alone. It is also to be noted that for solvents associated by hydrogen bond networks (water and alkanols), their molecules in dilute solution in inert solvents have considerably lower values than those for the bulk solvents, in which the cooperative effect of the hydrogen bonding enhances the donicity. 

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