Lewis acid-base catalysis

A second form of acid—base catalysis reflects another, more general definition of acids and bases. In the Lewis formulation, an acid is an electron-pair acceptor, and a base is an electron-pair donor. Metal ions, including such biologically important ones as Mn +, Mg +, and Zn, are Lewis acids. Thus, they can play a role in metal-ion catalysis (also called Lewis acid-base catalysis). The involvement of Zn + in the enzymatic activity of carboxypeptidase A is an example of this type of behavior. This enzyme catalyzes the hydrolysis of... 

As an example involving Lewis acid-base catalysis, we can cite the case of Friedel-Crafts alkylation of benzene hydrocarbons catalyzed by aluminum chloride, which is a Lewis acid. A four-step mechanism reports the results ... 

Lewis acid-base mechanism, 233, 234 Lewis acid catalysts, 13, 546 Lewis-acid-catalyzed ammonolysis, of nylon, 571 Lewis acids, 224 catalysis by, 68-69 Lewis bases, 338... 

The second important solvent effect on Lewis acid-Lewis base equilibria concerns the interactions with the Lewis base. Since water is also a good electron-pair acceptor129, Lewis-type interactions are competitive. This often seriously hampers the efficiency of Lewis acid catalysis in water. Thirdly, the intermolecular association of a solvent affects the Lewis acid-base equilibrium242. Upon complexation, one or more solvent molecules that were initially coordinated to the Lewis acid or the Lewis base are liberated into the bulk liquid phase, which is an entropically favourable process. This effect is more pronounced in aprotic than in protic solvents which usually have higher cohesive energy densities. The unfavourable entropy changes in protic solvents are somewhat counterbalanced by the formation of new hydrogen bonds in the bulk liquid. 

The rich variety of active sites that can be present in zeolites (i) protonic acidic sites, which catalyze acid reactions (ii) Lewis-acid sites, which often act in association with basic sites (acid-base catalysis) (iii) basic sites (iv) redox sites, incorporated either in the zeolite framework (e.g., Ti of titanosHicates) or in the channels or cages (e.g., Pt clusters, metal complexes). Moreover, redox and acidic or basic sites can act in a concerted way for catalyzing bifunctional processes. 

CHIRAL BRONSTED BASE-LEWIS ACID BIFUNCTIONAL CATALYSIS... 

General acid-base catalysis is often the controlling factor in many mechanisms and acts via highly efficient and sometimes intricate proton transfers. Whereas log K versus pH profiles for conventional acid-base catalyzed chemical processes pass through a minimum around pH 7.0, this pH value for enzyme reactions is often the maximum. In enzymes, the transition metal ion Zn2+ usually displays the classic role of a Lewis acid, however, metal-free examples such as lysozyme are known too. Good examples of acid-base catalysis are the mechanisms of carbonic anhydrase II and both heme- and vanadium-containing haloperoxidase. 

In acid-base catalysis,proton addition to or abstraction from reactant molecules (with Bronsted acids or bases), or formation of coordination bonds (with Lewis acids), and subsequent bond breaking and rearrangement are the key reaction processes. Most cases involve ionic reaction intermediates bound to the surface by electrostatic interactions. 

Lagtime, 75 Laplace transform, 82 Larmor precessional frequency, 155, 165 Laser pulse absorption, 144 Lattice energy, 403 Law of mass action, 60, 125 Least-squares analysis linear, 41 nonlinear, 49 univariate, 44 unweighted, 44, 51 weighted, 46, 51, 247 Leaving group, 9, 340, 349, 357 Lennard-Jones potential, 393 Lewis acid-base adduct, 425 Lewis acid catalysis, 265 Lewis acidity, 426... 

Although boron trifluoride, sulphur trioxide and the like may be titrated against bases in inert solvents, there is no unequivocal instance of a Lewis acid causing catalysis in the absence of protons. Thus the concept is... 

---