Br0nsted catalysis

PROTON TRANSFERS BETWEEN ACIDS AND BASES IN GENERAL, AND THE BR0NSTED CATALYSIS LAW... 

Fig. 8. (a) Potential energy curves giving a molecular interpretation of the Br0nsted catalysis law. (b) The limiting case for AH a much weaker acid than BH. (c) The limiting case for AH a much stronger acid than BH. 

When a wider range of chemical structures is chosen for the catalytic bases, significant deviations from the Br0nsted catalysis law are found . 

Br0nsted Catalysis Law. As might be expected, there is a relationship between the effectiveness of general acid catalysts and the acidic strength of a proton donor, as measured by its acid dissociation constant K. The stronger acids are more effective catalysts. This relationship is expressed by the Br0nsted catalysis law ... 

There have been numerous studies of the correlation between equilibrium acidity or basicity and the efficiency of various catalysts (see the discussion of the Br0nsted catalysis law in Chapter 4, section 4.6, p. 156). The correlations are generally good within related classes of acids and bases. Several water molecules may be involved in the transition state, in which case a concerted mechanism can be written for general acid-catalyzed hydration ... 

A number of studies of the acid-catalyzed mechanisms of enolization have been done. We can consider cyclohexanone as a typical case. The reaction is catalyzed by various carboxylic acids and substituted ammonium salts. The effectiveness of the various acids as catalysts correlates with their pKa values. When plotted according to the Br0nsted catalysis law, the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization /ch/ d 5. This isotope effect indicates that the C-H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

By definition, Lewis acid catalysts involve a metal center as an electron pair acceptor that accepts the electron pair from a nucleophile. This property makes them effective in many organic reactions and indispensable for the production of a large category of chemicals from simple alkylated compounds to complicated polymers or pharmaceuticals [6]. Also, the differences between the energy levels of the highest occupied molecular orbital (HOMO) of the reactant (nucleophile) and the lowest unoccupied molecular orbital (LUMO) of the Lewis acid (electrophile) make Lewis acid catalysis more complicated than the corresponding Br0nsted catalysis [7]. 

Bligaard T, Nprskov JK, Dahl S, Matthiesen J, Chistensen CH, Sehested J. 2004. The Br0nsted-Evans-Polanyi relation and the volcano curve in heterogeneous catalysis. J Catal 224 206-217. 

Although the concepts of specific acid and specific base catalysis were useful in the analysis of some early kinetic data, it soon became apparent that any species that could effect a proton transfer with the substrate could exert a catalytic influence on the reaction rate. Consequently, it became desirable to employ the more general Br0nsted-Lowry definition of acids and bases and to write the reaction rate constant as... 

Obviously, in such cases the CD is acting as a true catalyst in esterolysis. The basic cleavage of trifluoroethyl p-nitrobenzoate by a-CD occurs by both pathways approximately 20% by nucleophilic attack and approximately 80% by general base catalysis (GBC) (Komiyama and Inoue, 1980c). The two processes are discernible because only the former leads to the observable p-nitrobenzoyl-CD. For the ester, Ks = 3.4 mM and kjka = 4.4 for the GBC route (1.25 for the nucleophilic route), and so KTS = 0.77 mM. For reaction within the ester CD complex [28], it was estimated that the effective molarity of the CD hydroxyl anion was 21-210 m (for Br0nsted /3 = 0.4 to 0.6 for GBC). Such values are quite reasonable for intramolecular general base catalysis (Kirby, 1980). 

Figure 11.4. Hydrogen-bonding and Br0nsted acid complexation modes for the LUMO-lowering activation of substrates inherent to the field of Brpnsted acid catalysis.

Br0nsted acid catalysis, the substrate electrophile is reversibly protonated in a pre-equilibrium step, prior to the nucleophilic attack (Scheme 2). In general acid catalysis, however, the proton is (partially or fuUy) transferred in the transition state of the rate-determining step (Scheme 2). Clearly, the formation of a hydrogen bond precedes proton transfer. 

X 0, NR Nu Scheme 2 Specific and general Br0nsted-acid catalysis... 

The key feature of Br0nsted acid catalysis is often the choice of a catalyst with the appropriate acidity for particular substrate classes. Whereas less reactive substrates require stronger Brpnsted acids than the widely used phosphoric acids for activation, acid-sensitive substrates tend to decompose under strongly acidic conditions. Thus, weaker Brpnsted acid catalysts may prove beneficial. 

Any chemical reaction that is accelerated through the catalyic participation of base. See Acidity Br0nsted Relation General Acid Catalysis General Base Catalysis... 

Br0nsted plot for base catalysis of the mutarotation of glucose. 

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