Specific acid catalysis definition

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. 

Specific acid catalysis as illustrated in the mechanism of Eqs. 4-7 is the most common form of catalysis encountered in environmental systems. However, we may ask whether the reaction of hydrogen peroxide with thiourea is truly acid-catalyzed. According to some of the early definitions of chemical catalysis, this reaction would not be categorized as a catalytic, since the proton is a stoichiometric reactant, but in accord with the IUPAC definition the pathway involving H +... 

For the definition of general and specific acid catalysis see H. MaskUl, The physical basis of organic chemistry, Oxford University Press, New York, 1993. 

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. 

The specific acid is defined as the protonated form of the solvent in which the reaction is being performed. For example, in water the specific acid is hydronium. In acetonitrile, the specific acid is CHaCNHh and in DMSO the specific acid is CHaSOlH )CH3. The specific base is defined as the conjugate base of the solvent. As examples, in water, acetonitrile, and DMSO, the specific bases would be hydroxide, CH2CN, and CH3SOCH2, respectively. These definitions lead to strict definitions for specific catalysis. Specific-acid catalysis refers to a process in which the reaction rate depends upon the sjrecific acid, not upon other acids in the solution. Specific-base catalysis refers to a process in which the reaction rate depends upon the specific base, not upon other bases in the solution. To understand the kinds of reaction mechanisms that would depend only upon the specific add or base, we need to examine some possible mechanisms and the associated kinetic analyses. 

We start the kinetic analysis with Eq. 9.12, and substitute for RH using Eq. 9.13. Eq. 9.14 gives the kinetic expression for the mechanism of Eq. 9.11, assuming that the equilibrium between R and RH " is completely established. The kinetic expression contains [HsO ], as the definition of specific-acid catalysis implies. Hence, the reaction rate depends upon the pH. The expression also contains the acid dissociation constant (KaRHO of RH , which is an important factor that we will return to below. Note that k, [HsO""], and KaRH are constants during the reaction. Hence, we create a new rate constant, showing that the reaction appears... 

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). 

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... 

Figure 10.15 A shows the steps and relative rates involved in the addition with the strongly basic amines. The addition step has three possible pathways (step 1) direct addition, general-acid catalyzed addition, or specific-acid catalyzed addition. Because the amines are good nucleophiles, they add directly at all pHs, but below pHs around 4 this direct addition becomes rate-determining. This is because there is a low concentration of unproton-ated amine present at low pHs. In some circumstances, enforced general-acid catalysis of the first step is found (see Section 9.3.6 for the definition of enforced catalysis). At the high pHs...

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... 

A definite connection exists between the concepts of acids and bases and the idea of nucleophiles and their complementary substances, electrophiles. A Lewis acid is an electrophile, and a Lewis base is a nucleophile. Catalysis by enzymes, including their remarkable specificity, is based on these well-known chemical principles operating in a complex environment. 

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. 

If we consider the more generic Bronstedt s definition of a base and acid, protons and hydroxide ions do not have specific properties in comparison to other bases. For example, with an acid.4H and its conjngated base ", with generic acid-base catalysis, the apparent rate coefficient will have the form ... 

By definition, hydroxide ion is called a specific base and, hence, SB catalysis involves hydroxide ion as a catalyst. SB-catalyzed reactions must be only inter-molecular, because in the intramolecular catalytic reaction, the catalyst (hydroxide ion) is required to be covalently or electrovalently attached with the substrate, and under such circumstances, substrate-bound O or OH group becomes the GB catalyst. There are two possible modes of SB catalysis (1) hydroxide ion acts as a base in an acid-base reaction of overall catalytic process as shown in Scheme 2.28, and (2) hydroxide ion acts as a nucleophile in the overall catalytic process as shown in Scheme 2.29. It is evident from Scheme 2.28 that Y must be a stronger base than HO" so that hydroxide ion could not be consumed during the... 

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