Energetics of simple ions in solution

The remainder of this chapter is concerned with the stabilities of ions (mainly cations) in aqueous solution, with respect to oxidation, reduction and disproportionation. Ions in solution are surrounded by solvent molecules, oriented so as to maximise ion-dipole attraction (although there may be appreciable covalency as well). The hydration number of an ion in aqueous solution is not always easy to determine experimentally it is known to be six for most cations, but may be as low as four for small cations of low charge (e.g. Li+) or as high as eight or nine for larger cations (e.g. La3+). 

An important thermochemical quantity associated with the formation and stability of an ion in aqueous solution is its hydration enthalpy A//h yd, the enthalpy change under standard conditions for the process  

of course, is always negative, and plays the same role in aqueous thermochemistry as the lattice energy does in the energetics of ionic solids. The hydration enthalpy cannot be measured directly, and many thermodynamicists frown upon this or any other single-ion quantity. For example, the enthalpy of solution of sodium chloride can be measured and subjected to the following analysis  

For simple anions such as halide and S2, the experimental hydration enthalpies are in good agreement with the empirical formula  

Notice that the denominator does not include the addition of 80 pm, as is required for cations. This may be taken to reflect the different orientations of water molecules in the two cases  

---

Clearly, the outcome of such processes is determined by a diastereoisomerism existing somewhere along the reaction pathway. This could, for example, be in the approach of two materials to each other, in a transition state or reaction intermediate, or in the properties of the final product. The nature of the approach, interaction or bonding, is immaterial. For example, it has been shown recently that there are real and significant differences in the energetics of aggregation of chiral ions in solution to make diastereoisomeric ion pairs [31]. As shown in Table 1, even very simple chiral ion pairs can differ by 200-500 cal/mol in their heats of formation from the free ions. Such a difference can account for the difference between a reaction yield of 50 50 and 60 40. 

The simple triplet-triplet quenching mechanism requires that at low rates of light absorption the intensity of delayed fluorescence should decay exponentially with a lifetime equal to one-half of that of the triplet in the same solution. Exponential decay of delayed fluorescence was, in fact, found with anthracene, naphthalene, and pyrene, but with these compounds the intensity of triplet-singlet emission in fluid solution was too weak to permit measurement of its lifetime. Preliminary measurements with ethanolic phenanthrene solutions at various temperatures indicated that the lifetime of delayed flourescence was at least approximately equal to one-half of the lifetime of the triplet-singlet emission.38 More recent measurements suggest that this rule is not obeyed under all conditions. In some solutions more rapid rates of decay of delayed fluorescence have been observed.64 Sufficient data have not been accumulated to advance a specific mechanism but it is suspected that the effect may be due to the formation of ionic species as a result of the interaction of the energetic phenanthrene triplets, and the subsequent reaction of the ions with the solvent and/or each other to produce excited singlet mole-... 

In water, ions are not naked. They are more or less energetically solvated by water molecules. Here the adj ective solvated must be taken to have its usual meaning. The solvation of ions implicates phenomena that may be a simple ion-dipole interaction between the ion and water as well as a true chemical bond between both, as is the case in aqua complexes. Whatever the case, the reactivity of ions in aqueous solutions is strongly influenced by these solvation phenomena. As a result, it is also the case of the formation of complexes starting from the central metallic ion. 

The reaction with nitrite proceeds smoothly and with relatively high yields of the corresponding nitroarene (see Sec. 10.6). Obviously a major part of the driving force of this reaction is the formation of a stable, i. e., an energetically favorable, radical, nitrogen dioxide. With the hydroxide ion — a much stronger nucleophile than the nitrite ion — the reaction is expected to produce very unstable radicals, the hydroxy radical OH and the oxygen radical anion O, from the diazohydroxide (Ar - N2 — OH) and the diazoate (Ar-N20 ) respectively. Consequently, dediazoniation in alkaline aqueous solution does not follow the simple Scheme 8-41 with Yn = OH, but instead involves diazoanhydrides (Ar — N2 —O —N2 —Ar) as intermediates (see Sec. 8.8). 

---