Soft reactants

The concepts of electronegativity, hardness, and polarizability are all interrelated. For the kind of qualitative applications we will make in discussing reactivity, the concept that initial interactions between reacting molecules can be dominated by either partial electron transfer by bond formation (soft reactants) or by electrostatic interaction (hard reactants) is a useftxl generalization. 

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. 

Cn addition to the (a) and (b) species discussed above that provide the nucleus for a set of hard and soft acids and bases, it is possible to classify any given acid or base as hard or soft by its apparent preference for hard or soft reactants. For example, a given... 

The aim of specific poisoning is the determination of the chemical nature of catalytically active sites and of their number. The application of the HSAB concept together with eight criteria that a suitable poison should fulfill have been recommended in the present context. On this basis, the chemisorptive behavior of a series of hard poisoning compounds on oxide surfaces has been discussed. Molecules that are usually classified as soft have not been dealt with since hard species should be bound more strongly on oxide surfaces. This selection is due to the very nature of the HSAB concept that allows only qualitative conclusions to be drawn, and it is by no means implied that compounds that have not been considered here may not be used successfully as specific poisons in certain cases. Thus, CO (145, 380-384), NO (242, 381, 385-392, 398), and sulfur-containing molecules (393-398) have been used as probe molecules and as specific poisons in reactions involving only soft reactants and products (32, 364, 368). 

Calculations for the CO2 adsorption show that, on the naked surfaces, CO2 has to be considered as a basic species that binds to the exposed metallic cation[38, 39]. On Ti02 (110) CO2 is perpendicular to the surface building a OCO...Ti bond while on MgO(lOO) CO2 is flat and parallel to the surface bridging two adjacent Mg sites[24, 40]. The adsorption energies are nearly equal (7 kcal/mol. vs. 6.3 kcal/mol.) on MgO and Ti02- On MgO (100), it is an interaction between soft reactants and two bonds are built. 

Sharma and Millero (1988) determined the corresponding second-order rate constants k 0 = 2.1 104 and iCi=8.7 102 A/-1s-1 in sea water. The di and ii iehlorocomplexes were not sufficiently reactive to produce detectable rate constants. Thus the chloride ion, which stabilizes the soft reactant Cu(I) inhibits the oxygenation, whereas OH, which stabilizes the product Fe(III), accelerates i lie rale of Fe(II) oxidation. The reaction of Cu(I) with 02 represents an interesting test case because the reverse reaction has been measured by pulse tadiolysis. We may therefore apply the principle of microscopic reversibility to the electron-transfer step ... 

Pearson " designated the class (a) metal ions of Ahrland, Chatt, and Davies as hard acids and the class (b) ions as soft acids. Bases are also classified as hard or soft on the basis of polarizability the halide ions range from F , a very hard base, through less hard Cl and Br to I , a soft base. Reactions are more favorable for hard-hard and soft-soft interactions than for a mix of hard and soft reactants. Hard adds and bases are relatively small, compact, and nonpolarizable soft acids and bases are larger and more polarizable. The hard acids include cations with a large positive charge (3-t or larger) or those whose... 

In the brief guidelines given above for what makes a good nucleophile and electrophile, we touched on the energy and accessibility of the electrophilic and nucleophilic orbitals. This brings us to another related concept, that of "hard" and "soft" acids and bases. In this definition, the acids and bases are best viewed as being of the Lewis type. Here we examine the "hardness" and "softness" of the acid and base to predict reactivity. In this analysis, the character of a nucleophile or electrophile is most often correlated with the polarizability of the species hard reactants are non-polarizable, whereas soft reactants are polarizable. The... 

Hard or Soft or soft by its apparent preference for hard or soft reactants. For example, a given... 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

The standard polymers used for rubber linings consist of materials that are cross-linkable macromolecules which, on mixing with suitable reactants that form strong chemical bonds, change from a soft deformable substance into an elastic material. These polymers include natural rubber and its corresponding synthetic, c/s-polyisoprene, styrene-butadiene rubber, polychloroprene, butyl rubber, halogenated butyl rubbers, acrylonitrile-... 

Second, sensors are often intended for a single use, or for usage over periods of one week or less, and enzymes are capable of excellent performance over these time scales, provided that they are maintained in a nfild environment at moderate temperature and with minimal physical stress. Stabilization of enzymes on conducting surfaces over longer periods of time presents a considerable challenge, since enzymes may be subject to denaturation or inactivation. In addition, the need to feed reactants to the biofuel cell means that convection and therefore viscous shear are often present in working fuel cells. Application of shear to a soft material such as a protein-based film can lead to accelerated degradation due to shear stress [Binyamin and Heller, 1999]. However, enzymes on surfaces have been demonstrated to be stable for several months (see below). 

Perhaps the most successful application of Fukui function and local softness is in the elucidation of the region-selective behavior of different types of pericyclic reactions including the 1,3-dipolar cycloadditions (13DC), Diels-Alder reactions, etc. These reactions can be represented as shown in Scheme 12.4. Considering the concerted approach of the two reactants A and B, there are two possible modes of addition as shown in Pathway-I and Pathway-II. 

Addition of radicals to a different unsaturated substrate is an important class of organic reactions. To understand its regiochemistry, one needs to examine the condensed Fukui function (f°) or atomic softness (.v°) for radical attack of the different potential sites within the reactant substrate. We consider now a simple problem summarized in Example 3. 

The value of A, < 1 for this Cr(II) reduction, as with the reactions of Eu(II), indicates that the substitution of bridging F by I" is unfavorable in the bridged trrmsition complex in both cases. The two sets of reactivity patterns noted above thus disappear. It has been noted that K < 1.0 when both metal centers are hard acids, whereas AT, > 1 when one reactant is soft e.g., Cu+. These relationships have been rationalized. The much better bridging properties of chloride than water are shown by the data in Table 5.7 and Table 5.9. 

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