Acid-base catalysis definition

As implied above, there is nothing dramatically special about photocatalysis. It is simply another type of catalysis alongside, as it were, redox catalysis, acid-base catalysis, enzyme catalysis, thermal catalysis and others. Consequently, it is worth reemphasising that any description of photocatalysis must correspond to the general definition of catalysis. This said, it could be argued that the broad label photocatalysis simply describes catalysis of a photochemical reaction. 

The classical theory of catalysis supposed that the hydrogen and hydroxyl ions were the only effective catalysts in solutions of acids and bases. In a few instances early attempts were made to remedy some of the discrepancies encountered by attributing some catalytic power to undissociated acid molecules, but these attempts were mostly based on incorrect values for degrees of dissociation, and they did not take into account the possibility of primary or secondary salt effects. However, later work has shown definitely that species other than hydrogen and hydroxyl ions often can exert a catalytic effect, and the development of these ideas was closely linked with a closer understanding of the nature of the hydrogen ion in solution, and with the clarification of acid-base definitions (cf. Bell, 11). 

The position is similar in basic catalysis. The hydroxyl ion has no strong claims to uniqueness, being merely the anion of a weak acid. According to the Bronsted-Lowry acid-base definition, a base is any species which has a tendency to accept a proton. This obviously includes anions like OH-, CH3COO, HPCL , as well as uncharged basic molecules like ammonia and the amines. Catalysis by all these species was first found in the decomposition of nitramide (Bronsted and Pedersen, 15), and subsequently in many other reactions. 

Another definition of acids and bases is due to G. N. Lewis (1938). From the experimental point of view Lewis regarded all substances which exhibit typical acid-base properties (neutralisation, replacement, effect on indicators, catalysis), irrespective of their chemical nature and mode of action, as acids or bases. He related the properties of acids to the acceptance of electron pairs, and bases as donors of electron pairs, to form covalent bonds regardless of whether protons are involved. On the experimental side Lewis definition brings together a wide range of qualitative phenomena, e.g. solutions of BF3, BC13,... 

General acid catalysis occurs when the rate law includes a concentration term due to added acid (rate = AhaIHA]). Specific acid catalysis involves a rate law with only the oxonium ion (rate = itHlHjOq). Similar definitions apply to general base and specific base catalysis involving base and hydroxide ion respectively. 

The basics of general acid and general base catalysis are described clearly and in detail in Chapter 8 of Maskill [1]. Acid-base catalysis is termed specific if the rate of the reaction concerned depends only on the acidity (pH, etc.) of the medium. This is the case if the reaction involves the conjugate acid or base of the reactant preformed in a rapid equilibrium process - normal behavior if the reactant is weakly basic or acidic. The conjugate acid or base is then, by definition, a strong... 

Many chemical reactions involve a catalyst. A very general definition of a catalyst is a substance that makes a reaction path available with a lower energy of activation. Strictly speaking, a catalyst is not consumed by the reaction, but organic chemists frequently speak of acid-catalyzed or base-catalyzed mechanisms that do lead to overall consumption of the acid or base. Better phrases under these circumstances would be acid promoted or base promoted. Catalysts can also be described as electrophilic or nucleophilic, depending on the catalyst s electronic nature. Catalysis by Lewis acids and Lewis bases can be classified as electrophilic and nucleophilic, respectively. In free-radical reactions, the initiator often plays a key role. An initiator is a substance that can easily generate radical intermediates. Radical reactions often occur by chain mechanisms, and the role of the initiator is to provide the free radicals that start the chain reaction. In this section we discuss some fundamental examples of catalysis with emphasis on proton transfer (Brpnsted acid/base) and Lewis acid catalysis. 

To discuss acid-base catalysis, it is helpful to recall the definitions of acids and bases. In the Brpnsted-Lowry definition, an acid is a proton donor and a base is a proton acceptor. The concept of general acid-base catalysis depends on donation and acceptance of protons by groups such as the imidazole, hydroxyl, carboxyl, sulfhydryl, amino, and phenolic side chains of amino acids all these functional groups can act as acids or bases. The donation and acceptance of protons gives rise to the bond breaking and re-formation that constitute the enzymatic reaction. 

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... 

Most of the acid-base concepts, developed for solution chemistry, are applied to solid acids and bases. The present lectures, however, concern the acid-base catalytic properties of zeolites and their modification. Consequently, for clarity, attention is focussed on catalytic sites which results in somewhat restrictive definitions of acid-base interactions. In a comprehensive picture of zeolite catalysis several features other than the nature of particular sites can strongly influence overall activity and selectivity. These other features (2f) which include the wider implications of sorbate-sorbent interactions, both locally and globally, and the role of diffusion, are not considered here. 

Acid/base catalysis is probably the oldest type of homogenous catalytic reactions. Following the definitions by Bronstedt [6] and Lowry [7], the acids are proton donators and the bases are proton acceptors. Let us consider a bimolecular catalytic reaction with the equation presented as following ... 

The solid/gas interface was traditionally studied with respect to adsorption and catalysis. Here the assertion that the Bronsted definition of acidity is a particular case of the Lewis definition is neither obvious nor even helpful. It suffices to say that many reactions in heterogeneous catalysis require specifically the presence of either Bronsted or Lewis acidic (or basic) sites, and the reaction mechanisms depend on the nature of the surface site. A long-term goal of surface studies for the characterization of solid catalysts was to distinguish and quantify the number of Bronsted or Lewis sites with potential catalytic activity for gas-phase reactants. For that reason, when discussing the acid-base behavior of solid surfaces, it is no longer possible, nor desirable, to adopt the viewpoint that subsumes Bronsted acid-base properties in the more general Lewis definition. 

The success of these quantitative developments helped to obscure some logical weaknesses in the qualitative definitions. For example, it was not clear whether a pure non-conducting substance like anhydrous hydrogen chloride should be called an acid or whether it became one only on contact with water. The definition did not apply directly to non-aqueous solvents, where the ions formed differed from those in water, and this difficulty was particularly acute when it was realized that typical acid-base properties such as neutralization, indicator effects, and catalysis often appeared in solvents such as benzene and chloroform where free ions could barely be detected by conductivity methods. A particular ambiguity appears in the definition of bases, some of which (e.g., metallic hydroxides) contain a hydroxyl group, whereas others (e.g., amines) produce hydroxide ions in solution by abstracting a proton from... 

The compounds considered as bases according to the Lewis classification are practically the same as those by the Brpnsted definition Species, which are able to add a proton, possess a pair of electrons and, naturally, they react with electron-pair acceptors. In contrast, there are substantial differences when acids are considered. First of all, Brpnsted acids are able to split off proton, whereas Lewis acids do not necessarily contain a hydrogen atom. Acid-base reactions, correspondingly, are much simpler in the Br0nsted sense, since they involve proton transfer, that is, the transfer of a nucleus without electrons. Furthermore, the reaction of any Brpnsted acid with a base B produces the same species, the conjugate acid BH+ irrespective of the acid. That is why the behavior of Brpnsted acids is similar toward all bases and indicators, and their catalytic effect is the same in specific acid catalysis. The products of the reaction of Lewis acids with a base, in turn, are different with each acid-base complex having specific properties. The behavior of Lewis acids toward bases and indicators, and their catalytic effect, therefore, may be substantially different. It is quite interesting to point out that an acid is a proton donor in the Brpnsted sense, but the proton itself is the acid in the Lewis picture due to its vacant orbital. 

Many examples of proximity effects are known. In general, whenever an intramolecular acid or base is invoked in acid-base catalysis, proximity effects can be a factor. Further, when any catalyst holds a substrate near a catalytic group at its active site, or holds two separate substrates next to each other, proximity effects can be relevant. Proximity effects are definitely prevalent in organometallic catalysis, as we will see in Chapter 12. Hence, proximity effects are key to many forms of catalysis. 

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