Acid-base catalysis distinction between

In acid-catalyzed reactions, the distinction between single-species and complex catalysis is not always clear-cut. The actual catalyst is the solvated proton, H30+ in aqueous solution, and H20 (or a molecule of the nonaqueous solvent) may thus appear as a co-product in the first step and as a co-reactant in the step reconstituting the original solvated proton, possibly also in other additional steps, e.g., if the overall reaction is hydrolysis or hydration. Moreover, the acid added as catalyst may not be completely dissociated, and its dissociation equilibrium then affects the concentration of the solvated proton. At high concentrations this is true even for fairly strong acids such as sulfuric, particularly in solvents less polar than water. Such cases are better described as acid-base catalysis (see Section 8.2.1). 

Some experimental criteria are mainly concerned with a distinction between unimolecular and bimolecular mechanisms in acid—base catalysis. One of the most straightforward criteria is based on the determination of the volume change on activation, AV, from the pressure dependence of the rate coefficient, according to the equation... 

The predominant application of metal oxide catalysts is due to their oxidation and acid-base behavior. In the following, these areas are discussed separately, although it is clear that in many materials, for example, heteropolyacids, which combine both strong acidity and oxidation efficacy (37,38), and the sulfated metal oxides, where controversy exists as to whether the observed low temperature isomerization pathways are catalyzed by superacid or redox mechanisms (39-41), the distinction between acid-base and oxidation properties is somewhat arbitrary. To illustrate their principles, a number of different reaction types are discussed. Dehydrogenation reactions, ammoxidation, and the WGS reaction have been included imder oxidation catalysts since they constitute major industrial applications of metal oxide-based catalysts. In the case of acid-base catalysis, some of the recent activity in the area of biodiesel is described as an illustration of the complementarity of both acid catalysis and base catalysis. There are a number of additional applications of oxides as catalysts, such as in photocatalysis (42), which have not been reviewed here because of limitations of space. Oxidation Activity. 

Although in catalytic reactions, in particular on the surface of solid catalysts, it is not formally correct to distinguish between acid-base and redox catalysis, because usually they are both involved, this distinction is often common. The two main classes of reactions are selective hydrogenation and selective oxidation. 

The term nucleophilicity refers to the relative rate of reaction of an electron donor with a given electrophile, as distinct from basicity, which refers to its relative affinity for a proton in an acid-base equilibrium. A quantitative relationship between rate and equilibrium constants was discovered by Brpnsted and Pedersen (1) in 1924. These authors found that the rate constants for the catalytic decomposition of nitramide by a family of bases, such as carboxylate ions (GCH2C02 ), could be linearly correlated with the acidities of their conjugate acids, pKHB. This observation led to the discovery of general base catalysis and the first linear free-energy relationship, which later became known as the Brpnsted equation ... 

If the first step of a reaction cycle is regarded as an acid base reaction between the catalyst and the organie substrate, then a distinction can be made between hard and soft catalysis, providing a simple basis for understanding transition metal catalyzed processes (Seheme 2-4). 

A distinction is often made between specific catalysis by hydrogen and hydroxyl ions specific hydrogen and hydroxide-ion catalysis) and catalysis by acids and bases in general general acid and base catalysis). Actually such a distinction is somewhat artificial, as we shall see. As an example of general base catalysis, consider the simple mechanism... 

XsH is the ionization constant of the second species in Eq. (8-49).] This type of ambiguity is quite general in acid-base reactions, and a distinction between general acid and general base catalysis cannot be made through the rate law... 

In the pH-range 6 to 8 the hydrolysis of salicylamide is faster than that of benzamide and the pH-profile is sigmoid. The plateau rate constant is 17.5 times that for 5-nitrosalicylamide [36] but this result does not allow a distinction to be drawn between possible mechanisms which involve intramolecular general-acid and intramolecular general-base catalysis [20]. 

Catalysis as usually defined has implied that the catalyst is recovered unchanged. There seems to be a tendency to broaden the definition. For example, Hammett points out that there is no important distinction between catalysis in the older sense and the kind of acceleration produced by a base which appears as its conjugate acid at the end of the reaction. A few of the examples presented here are of the conjugate type, but most of them conform to the older definition. 

A kinetic expression equivalent to general acid catalysis also occurs if a prior equilibrium between reactants and the acid is followed by a rate-controlling deprotonation. Each distinct conjugate base will appear in the overall rate expression ... 

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