Acid-Base Catalysis and Proton-Transfer Reactions

Acid-Base Catalysis and Proton-Transfer Reactions... 

Since acid-base catalysis involves proton-transfer between the catalyst and the substrate, Bronsted-type relations should also apply to the variation of reaction velocity with the acid-base strength of the substrate, and the same statistical corrections should apply, as may be seen from the following general considerations. 

Now let s look at what we can do with the water. Because it has more negative charge (a higher electron density), OH is more reactive than HOH. By providing an appropriately placed base to at least partially remove one of the protons from the attacking water molecule, we can increase the reactivity of this water and make the reaction go faster. This is known as acid-base catalysis and is widely used by enzymes to help facilitate the transfer of protons during chemical reactions. 

There has been a resurgence of interest in proton-coupled redox reactions because of their importance in catalysis, molecular electronics and biological systems. For example, thin films of materials that undergo coupled electron and proton transfer reactions are attractive model systems for developing catalysts that function by hydrogen atom and hydride transfer mechanisms [4]. In the field of molecular electronics, protonation provides the possibility that electrons may be trapped in a particular redox site, thus giving rise to molecular switches [5]. In biological systems, the kinetics and thermodynamics of redox reactions are often controlled by enzyme-mediated acid-base reactions. 

This chapter will be concerned mainly with the relation between the equilibrium constants of acid-base reactions and their forward and reverse rates. Relations between equilibrium constants and structure have already been considered in Chapter 6, so that the present discussion also implies relations between rates and structure. Moreover, there are many cases in which rates are easier to measure (though more difficult to interpret) than equilibria and can be compared directly with structures. We shall first consider the general basis and experimental evidence for this type of relation, followed by its molecular interpretation, with special reference to exceptional cases. We have seen in the two preceding chapters that the rates of proton-transfer reactions can be measured either directly, or indirectly through the study of acid-base catalysis, and in the following discussion information from both sources will be used indifferently. 

Now that we have examined various forms of acid-base catalysis, and we have looked at how the thermodynamics of proton transfer are related to the kinetics via the Bronsted catalysis law, let s examine the mechanisms and rates of proton transfer in more detail. The transfer of a proton from an acid to a base is one of the simplest of all chemical reactions, and yet, even this reaction has been found to have several subtle mechanistic twists. The rate of the reaction generally depends upon the driving force (the thermodynamics) of the reaction, but there are cases where intrinsic barriers exist, making even very exothermic reactions slower than one might expect. 

In this section we would like to approach what it concerns the general acid-base catalysis and its associated proton transfer. For example, what determines whether a reaction proceeds by stepwise acid-base catalyzed or concerted reaction mechanisms , or does catalysis take place in such a way as to avoid the most unstable intermediate , etc. Answers to these queries may be found in a rule which states that "Concerted general acid-base... 

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

Rates of addition to carbonyls (or expulsion to regenerate a carbonyl) can be estimated by appropriate forms of Marcus Theory. " These reactions are often subject to general acid/base catalysis, so that it is commonly necessary to use Multidimensional Marcus Theory (MMT) - to allow for the variable importance of different proton transfer modes. This approach treats a concerted reaction as the result of several orthogonal processes, each of which has its own reaction coordinate and its own intrinsic barrier independent of the other coordinates. If an intrinsic barrier for the simple addition process is available then this is a satisfactory procedure. Intrinsic barriers are generally insensitive to the reactivity of the species, although for very reactive carbonyl compounds one finds that the intrinsic barrier becomes variable. ... 

In acid-base catalysis there is at least one step in the reaction mechanism that consists of a generalized acid-base reaction (a proton transfer between the catalyst and the substrate). The protonated or deprotonated reactant species or intermediate then reacts further, either with... 

Acid and base catalysis of a chemical reaction involves the assistance by acid or base of a particular proton-transfer step in the reaction. Many enzyme catalysed reactions involve proton transfer from an oxygen or nitrogen centre at some stage in the mechanism, and often the role of the enzyme is to facilitate a proton transfer by acid or base catalysis. Proton transfer at one site in the substrate assists formation and/or rupture of chemical bonds at another site in the substrate. To understand these complex processes, it is necessary to understand the individual proton-transfer steps. The fundamental theory of simple proton transfers between oxygen and nitrogen acids and... 

Proton transfer reactions on the aqua oxo complex are described by Eq. (8) (acid catalysis or protolysis), Eq. (9) (base catalysis or hydrolysis), and Eq. (10) (direct proton exchange). 

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