HSAB theory, relative

Further examination of the results indicated that by invocation of Pearson s Hard-Soft Acid-Base (HSAB) theory (57), the results are consistent with experimental observation. According to Pearson s theory, which has been generalized to include nucleophiles (bases) and electrophiles (acids), interactions between hard reactants are proposed to be dependent on coulombic attraction. The combination of soft reactants, however, is thought to be due to overlap of the lowest unoccupied molecular orbital (LUMO) of the electrophile and the highest occupied molecular orbital (HOMO) of the nucleophile, the so-called frontier molecular orbitals. It was found that, compared to all other positions in the quinone methide, the alpha carbon had the greatest LUMO electron density. It appears, therefore, that the frontier molecular orbital interactions are overriding the unfavorable coulombic conditions. This interpretation also supports the preferential reaction of the sulfhydryl ion over the hydroxide ion in kraft pulping. In comparison to the hydroxide ion, the sulfhydryl is relatively soft, and in Pearson s theory, soft reactants will bond preferentially to soft reactants, while hard acids will favorably combine with hard bases. Since the alpha position is the softest in the entire molecule, as evidenced by the LUMO density, the softer sulfhydryl ion would be more likely to attack this position than the hydroxide. 

However, a A H calculation usually predicts the C-reacted compound to be thermodynamically more stable than the Z-reacted compound (mainly because of the greater C-Z bond strength in the C-reacted product compared to the C=C in the Z-reacted). However, this does depend on the relative C-E vs. 0-E bond strength. It is important to determine which is the dominant effect, product formation based upon product thermodynamic stability or upon kinetic direction from HSAB theory. To do this we need to determine whether the reaction is under kinetic or thermodynamic control. Figure 9.1 gives a flowchart for the decision for a common ambident nucleophile, an enolate anion (Z equals oxygen). 

Solution. Typically, the relative magnitude of the overall stability constants can be determined from HSAB theory. 

Figures 11(a) and 11(b) [112] show the variation of Ni-Ge-P deposition rate and Ge content as a function of aspartic acid and Ge(IV) concentration, respectively. A relatively low P content, ca. 1-2 at%, was observed in the case of films exhibiting a high concentration of Ge (> 18 at%). Like other members of its class, which includes molybdate and tungstate, Ge(IY) behaves a soft base according to the hard and soft acids and bases theory (HSAB) originated by Pearson [113, 114], capable of strong adsorption, or displaying inhibitor-like behavior, on soft acid metal surfaces. In weakly acidic solution, uncomplexed Ge(IV) most probably exists as the hydrated oxide, or Ge(OH)4, which, due to acid-base reactions, may be more accurately represented as [Gc(OH)4 nO ] ".

To the extent that the N+ correlation is successful it means that the pattern of nucleophilic reactivity is not influenced by the nature of the electrophilic center at which substitution takes place. On the other hand, according to the concepts of the theory of hard and soft acids and bases (HSAB) as applied to nucleophilic substitution reactions (Pearson and Songstad, 1967) one would expect that a significant change in the HSAB character of the electrophilic center as an acid should lead to changes in the pattern of nucleophilic reactivity observed. Specifically, in substitutions occurring at soft electrophilic centers, soft-base nucleophiles should be more reactive relative to other nucleophiles than they are in substitutions at harder electrophilic centers, and in substitutions at hard electrophilic centers hard-base nucleophiles should appear relatively more reactive compared to other nucleophiles than they do in substitutions at softer electrophilic centers. 

Modern advances in solvation theory are reviewed.A second critical review of the thermodynamic functions and crystallographic data of some solid solvates (e.g. NaI,3MeOH LiCl,py and CoCl2,6NHg) shows that the relative acid-base properties of the constituent cation, anion, and solvate molecule can well be described by the HSAB (hard and soft acids and bases) concept. ... 

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