Base strength solvent effects

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

The structure of a molecule can affect its acidity or basicity in a number of ways. Unfortunately, in most molecules two or more of these effects (as well as solvent effects) are operating, and it is usually very difficult or impossible to say how much each effect contributes to the acid or base strength. Small differences in acidity or basicity between similar molecules are particularly difficult to interpret. It is well to be cautious when attributing them to any particular effect. 

W Williams, GT Young. Amino acids and peptides. XXXV. Effect of solvent on the rates of racemization and coupling of acylamino acid p-nitrophenyl esters. Base strengths of amines in organic solvents, and related investigations. J Chem Soc Perkin Trans 1 1194, 1972. 

Solvent effects and adduct formation of [VO(acac)2l and other [VO(j8-diketonato)2] complexes have been studied by several methods361,366,52 -532 and in coordinating solvents [VO(acac)2] is known to add a sixth ligand according to equation (36). Older reports include a spectrophotometric and calorimetric study of [VO(acac)2] and [VO(tfacac)2] adducts.521 With [VO(acac)2] in nitrobenzene, the enthalpy change for reaction (36) ranges from 44.3 kJ mol-1 for n-decylamine to 24.3 kJ mol-1 for methanol. Equilibrium constants K were between 1000 and —0.6. [VO(acac)2] is not a sensitive indicator of relative base strength.521 For [VO(acac)2], A0 decreases by —3 G (and go increases by —0.0004) as amine adducts are formed.522... 

Water has a limiting effect on the strength of acids and bases. All strong acids behave the same in water 1—M solutions of the strong acids all behave as 1 M solutions of the HsO+ ion and very weak acids cannot act as acids in water. Acid-base reactions don t have to occur in water, however. When other solvents are used, the full range of acid-base strength shown in the table below can be observed. 

Eq. (4-10) can be used only for solvents of equal acid and base strength, because only the effect of the solvent relative permittivity on the degree of ionization is considered. Under these conditions, Eq. (4-10) predicts that the logarithm of the ionization constant of HA should be inversely proportional to the relative permittivity of the solvent in which HA is dissolved. However, one has to take into account the fact that the relative permittivities near solute ions can differ considerably because of the effect of dielectric saturation, which hinders the precise calculation of electrostatic interactions. Because of these restrictions, Eq. (4-10) can be expected to yield only semiquantitative results. Nevertheless, it allows us to predict qualitatively how the charge type of an acid affects the ionization constant in solvents of different relative permittivities. 

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