Acid-base catalysis reaction types

In aqueous solution, the rates of many reactions depend on the hydrogen-ion (H+ or h3o+) concentration and/or on the hydroxyl-ion (OH-) concentration. Such reactions are examples of acid-base catalysis. An important example of this type of reaction is esterification and its reverse, the hydrolysis of an ester. 

This type of alkoxylation chemistry cannot be performed with conventional alkali metal hydroxide catalysts because the hydroxide will saponify the triglyceride ester groups under typical alkoxylation reaction conditions. Similar competitive hydrolysis occurs with alternative catalysts such as triflic acid or other Brpnsted acid/base catalysis. Efficient alkoxylation in the absence of significant side reactions requires a coordination catalyst such as the DMC catalyst zinc hexacyano-cobaltate. DMC catalysts have been under development for years [147-150], but have recently begun to gain more commercial implementation. The use of the DMC catalyst in combination with castor oil as an initiator has led to at least two lines of commercial products for the flexible foam market. Lupranol Balance 50 (BASF) and Multranol R-3524 and R-3525 (Bayer) are used for flexible slabstock foams and are produced by the direct alkoxylation of castor oil. 

Yeast enolase (Mr 93,316) is a dimer with 436 amino acid residues per subunit. The enolase reaction illustrates one type of metal ion catalysis and provides an additional example of general acid-base catalysis and transition-state stabilization. The reaction occurs in two steps (Fig. 6-23a). First, Lys345 acts as a general base catalyst,... 

The acid-base catalysis is carried out in BRC, with H+ or 2H+ ion transfer. Poltorak notes [99] that when H+ and e or H+ and H are transferred, the reaction mechanisms relate to H+-dependent redox type. BRC with electron transfer describes heterolytic oxidative processes. 

There is a substantial body of literature on the hydrolysis of alkoxy-silanes (16) including a study of ETES (15). Note that all three reactions, (1, 2a, and 2b), are probably subject to acid-base catalysis, but only for reactions of type 2a has catalysis been demonstrated (15,16). 

The most common type of biocatalytic reactions is proton transfer (115). Nearly, every enzymatic reaction involves one or more proton-coupled steps. Transition-state proton bridging and intramolecular proton transfer (general acid-base catalysis) are important strategies to accelerate substrate conversion processes. Moreover, proton transfer also plays a fundamental role in bioenergetics (116). 

The most common and most thoroughly studied type of homogeneous catalysis is acid-base catalysis. It includes hydrolysis, alcoholysis, esterification, and condensation reactions among many others. It is characterized by the fact that the equilibrium between base and conjugate acid, or between acid and conjugate base, is coupled with the actual catalytic cycle. 

In numerous reactions the organic compounds possess either acidic or basic properties — even though these may be weak — and thus they can either add a proton to form a cationic intermediate or expel a proton to form an anionic intermediate. The cationic intermediates may easily undergo nucleophilic attack, and the anionic intermediates are more susceptible to electrophilic attack. Consequently, reactions of organic compounds may be acid or base catalyzed. Actually, acid-base catalysis is the most frequent type of catalysis, and it has been studied in much detail [1, 2]. For these reasons, this article will be mainly concerned with such catalysis. 

This article will describe the different chemical strategies used by enzymes to achieve rate acceleration in the reactions that they catalyze. The concept of transition state stabilization applies to all types of catalysts. Because enzyme-catalyzed reactions are contained within an active site of a protein, proximity effects caused by the high effective concentrations of reactive groups are important for enzyme-catalyzed reactions, and, depending on how solvent-exposed the active site is, substrate desolvation may be important also. Examples of acid-base catalysis and covalent (nucleophilic) catalysis will be illustrated as well as examples of "strain" or substrate destabilization, which is a type of catalysis observed rarely in chemical catalysis. Some more advanced topics then will be mentioned briefly the stabilization of reactive intermediates in enzyme active sites and the possible involvement of protein dynamics and hydrogen tunneling in enzyme catalysis. 

Two types of kinetics have been observed in the oxidation of 1,2-diols by periodate. The first of these, exemplified by ethane-1,2-diol, is of a mixed-order type, and is consistent with the reversible formation of the cyclic diester as an intermediate. The second type, exemplified by pinacol, is second-order kinetics, the reaction showing general acid-base catalysis and a complex dependence of rate on pH. 

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. 

More O Ferrall-Jencks diagrams were devised to deal with the varying effects of the same substituent on differing types of elimination reaction, only later being applied to acid-base catalysis. Figure 6.66 represents such a diagram. 

This view of acid-base catalysis is now generally accepted, and specific mechanisms have been proposed for a large number of types of reaction. For the purpose of illustration a few of these are given in the next subsection, but no attempt has been made at completeness. In many instances the mechanism involves two successive proton transfers, and it may be a matter of some difficulty to decide the relative rates of the two successive steps. This question is considered in Sec. III.3. 

Apart from protein sequence and structure, temperature, pH, and the type of buffers can all influence the deamidation and isoaspartate formation. The effect of temperature on the deamidation reaction rate generally follows the Arrhenius law. Deamidation activation energies around 21-22kcalmoH have been reported for two model peptides under alkaline conditions [19, 20]. The deamidation is also subject to general acid/base catalysis, as evidenced by an increase in deamidation rate with an increase in buffer concentration [2]. 

There are several types of pH-dependent kinetic behavior that can be interpreted in terms of one or more of the various forms of the specific acid-base catalysis relation [equation (7.3.2)]. Skrabal (33) classified the various possibilities that may arise in reactions of this type, and Figure 7.3 is based on this classification. The various forms of the plots of log k versus pH reflect the relative importance of each of the various terms in equation (73.2) as the pH shifts. Curve a represents the most general type of behavior. This curve consists of a region where add catalysis is superimposed on the noncatalytic reaction, a region where neither acid nor base catalysis is significant. 

We have indicated how to determine the various kinetic constants appearing in the expression for specific acid and base catalysis. Let us now consider how to evaluate the various contributions to the rate constant in the case of general acid-base catalysis. For reactions of this type in a solution of a weak acid or base and its corresponding salt, the possible catalysts indicated by equation (7.3.3) are the hydro-nium ion, the hydroxide ion, the undissociated weak acid (or base), and the conjugate base (or acid), In the case of acetic acid the general acid would be the neutral CHjCOOH species and the conjugate base would be the acetate ion (CH3COO"). In this case the apparent rate constant can be written as... 

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