General acid catalyst, definition

When a Br nsted base functions catalytically by sharing an electron pair with a proton, it is acting as a general base catalyst, but when it shares the electron with an atom other than the proton it is (by definition) acting as a nucleophile. This other atom (electrophilic site) is usually carbon, but in organic chemistry it might also be, for example, phosphorus or silicon, whereas in inorganic chemistry it could be the central metal ion in a coordination complex. Here we consider nucleophilic reactions at unsaturated carbon, primarily at carbonyl carbon. Nucleophilic reactions of carboxylic acid derivatives have been well studied. These acyl transfer reactions can be represented by... 

The concept of acid site is based on the idea that protons are fixed at definite position. Thus, the measures of the acid strength, which are described so far, are basically based on the static properties of OH groups. However, the solid acid catalysed reactions are often carried out at higher temperatures than room temperature. In general, the catalysts undergo structural and chemical change under reaction conditions. Therefore, the characterization of properties of zeolites at high temperatures is more desirable. 

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. 

The first of these types of processes may be operated either in batches or continuously. Both methods require eflicient distilling columns, which may be of perforateiji plate or bell-cap design or even of the packed t3q>e. In every case, it is now customary to employ a catalyst, which is usually sulfuric acid, in admixture with the alcohol and acid that are to react. In making ethyl acetate industrially, ethyl alcohol of 95 per cent by volume and acetic acid of 80 per cent or less concentration are generally used. There being no definite lower limit of acid concentration, it is merely a matter of economic balance as to how far the exhaustion of the acetic acid may be carried. In a continuous process such as that of Backhaus, the concentration of the acetic acid may be reduced to 1 per cent. 

But there is another important type of acid the Lewis acid. These acids don t donate protons—indeed they usually have no protons to donate. Instead they accept electrons. It is indeed a more general definition of acids to say that they accept electrons and of bases that they donate electrons. Lewis acids are usually halides of the higher oxidation states of metals, such as BF3, AICI3, ZnCl2, SbFj, and TiCl4. By removing electrons from organic compounds, Lewis acids act as important catalysts in important reactions such as the Friedel-Crafts alkylation and acylation of benzene (Chapter 21), the S l substitution reaction (Chapter 15), and the Diels-Alder reaction (Chapter 34). 

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. 

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. 

Proton, by definition, is called specific acid, and if the overall energy barrier (activation energy) of a reaction is reduced in the presence of proton as a catalyst, then the reaction is said to involve specific acid catalysis. Generally, catalyst proton reacts with reactant (substrate) in a so-caUed acid-base reaction process, which, in turn, activates the reaction system (by either the preferential destabilization of reactant state or stabilization of transition state in the rate-determining step) for the product formation. The products do not contain any molecular site, which has enough basicity to trap the proton catalyst irreversibly or even reversibly. Thus, for a detectable S A catalysis, the basicity (measured by the magnitude of basicity constant, KJ of the basic site of electrophilic reactant, products, and solvent should vary in the order K, (for electrophilic reactant) > (for solvent) > Kb (for products). 

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