Acid-base models

The Lewis acid-base model is the most general of the three we have considered. 

Many years ago, geochemists recognized that whereas some metallic elements are found as sulfides in the Earth s crust, others are usually encountered as oxides, chlorides, or carbonates. Copper, lead, and mercury are most often found as sulfide ores Na and K are found as their chloride salts Mg and Ca exist as carbonates and Al, Ti, and Fe are all found as oxides. Today chemists understand the causes of this differentiation among metal compounds. The underlying principle is how tightly an atom binds its valence electrons. The strength with which an atom holds its valence electrons also determines the ability of that atom to act as a Lewis base, so we can use the Lewis acid-base model to describe many affinities that exist among elements. This notion not only explains the natural distribution of minerals, but also can be used to predict patterns of chemical reactivity. 

The reaction plane model with heterogeneous reactions was discussed at length for acid-base reactions in the previous section. The same modeling technique, of confining the reactions to planes, can be applied to micelle-facilitated dissolution. As with the acid-base model, one starts with a one-dimensional steady-state equation for mass transfer that includes diffusion, convection, and reaction. This equation is then applied to the individual species i, i.e., the solute, s, the micelle, m, and the drug-loaded micelle, sm, to yield... 

Drago and co-workers Introduced an empirical correlation to calculate the enthalpy of adduct formation of Lewis acids and bases ( 5). In 1971, he and his co-workers expanded the concept to a computer-fitted set of parameters that accurately correlated over 200 enthalpies of adduct formation ( ). These parameters were then used to predict over 1200 enthalpies of interaction. The parameters E and C are loosely Interpreted to relate to the degree of electrostatic and covalent nature of the Interaction between the acids and bases. This model was used to generalize the observations involved in the Pearson hard-soft acid-base model and render it more quantitatively accurate. 

Fundamental Difficulties with the Hard-Soft Acid Base Model... 

If the picture is correct then we see that the observed order of activation energies which is largest for molecular reactions, small for radical-molecule reactions and nearly zero for radical-radical reactions, falls into a 1 1 relation with the acid-base model. The radical-radical reactions have the open orbital and the electron donor, hence little promotion energy to form an attractive pair. The radical-molecule reactions have one open orbital but require polarization of the molecule in order to form the complimentary acid or base. For the molecule-molecule addition type reaction, complimentary polarization of both species must take place for an attractive transition state to form and the activation energy is the highest. 

The pHpzc (zero proton condition, point of zero charge) is not affected by the concentration of the inert electrolyte in the absence of a different specific supporting electrolyte ion boundary for cation and anion. The computations of Dzombak and Morel (1990) employ a difftise layer model coupled with acid-base surface reactions to describe Q versus pH. (This acid-base model incorporates variable capacitance.) As Figure 9.8 shows, there is a common intersection point of the titration curves obtained with different concentrations of inert electrolyte. 

Titrations involving weak acids and bases are far more complex to model, paticularly when the acids or bases are polyprotic and volume corrections are made. For detailed discussion of both simple and complex acid/base modeling with volume corrections, the reader is referred to Butler (1964), Pankow (1991), and Stumm and Morgan (1996). 

Note that the SO3 molecule, a Lewis acid, accepts an electron pair from the ion, a Lewis base. The Arrhenius, Bronsted-Lowry, and Lewis acid-base models are summarized in Table 18-2. 

The history of chanistry is marked with many acid-base models. A limitation of most early models is that they are applicable to only specific classes of compounds or a narrow set of conditions. One such limiting idea in the eighteenth century was that all acids contained oxygen oxides of nitrogen, phosphorus, sulfur, and the halogens aU form aqueous acids. However, by the early nineteenth century, this definition was regarded as too narrow many compounds had been discovered that did not contain oxygen but showed behavior associated... 

A plausible explanation for this is based on I. D. Brown s acid-base model (33), which is based on a valence model of Brown and Shannon (34), which in turn is an extension of Pauling s valence concepts (35). Briefly, one can use bond order-bond length relationships to give an order to any chemical bond. 

Auboiroux, M., F. Melon, F. Bergaya, and J. C. Touray. 1998. Hard and soft acid-base model applied to bivalent cation selectivity on a 2 1 clay mineral. Clays and Clay Minerals 46, no. 5 546-555. 

All Bronsted-Lowry acids (proton donors) are also Lewis acids, and all Bronsted-Lowry bases (proton acceptors) are also Lewis bases. But the Lewis acid-base model is far more general in that it applies to reactions beyond just proton transfers. 

Use the Lewis acid-base model to explain the following reaction. 

Compagnon I, Oomens J, Bakker J, Meijer G, vrai Helden G (2005) Vibrational spectroscopy of a non-aromatic amino acid-based model peptide identification of the gamma-turn motif of the peptide backbone. Phys Chem Chan Phys 7 13... 

---