Acid-base catalysis concerted

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. ... 

As we have seen (Section 4, p. 191) the range of effective molarities associated with ring-closure reactions is very much greater than that characteristic of intramolecular general acid-base catalysis the main classification is therefore in terms of mechanism. By far the largest section (I, Tables A-D) gives EM s for intramolecular nucleophilic reactions. These can be concerted displacements (mostly at tetrahedral carbon), stepwise displacements (mostly addition-elimination reactions at trigonal carbon), or additions, and they have been classified in terms of the nucleophilic and electrophilic centres. 

The rich variety of active sites that can be present in zeolites (i) protonic acidic sites, which catalyze acid reactions (ii) Lewis-acid sites, which often act in association with basic sites (acid-base catalysis) (iii) basic sites (iv) redox sites, incorporated either in the zeolite framework (e.g., Ti of titanosHicates) or in the channels or cages (e.g., Pt clusters, metal complexes). Moreover, redox and acidic or basic sites can act in a concerted way for catalyzing bifunctional processes. 

Jencks (1972) has concluded that concerted bifunctional acid-base catalysis is rare or nonexistent because of the improbability of meeting simultaneously at two sites on reactant and catalyst the conditions of the rule which he has proposed for concerted reactions. The rule states that concerted general acid-base catalysis of complex reactions in aqueous solution can occur only (a) at sites that undergo a large change in pAT in the course of the reaction, and (b) when this change in pAf converts 2m unfavourable to a favourable proton transfer with respect to the catalyst, i.e., the pAT-value of the catalyst is intermediate between the initial and final pAf-vadues of the substrate site. 

Fig. 2. A pictorial representation of the ribonuclease reaction. The free enzyme (A) exists in two conformational states differing by small movements of the hinge region joining the two halves of the molecule. The substrate is bound (B) and a conformational change occurs closing the hinge (C). Concerted acid-base catalysis then occurs (D) products are formed ( ) the conformational change is reversed (F) and product(s) dissociate to give free enzyme.

Swain, C.G. (1950) Concerted displacement reactions. V. The mechanism of acid-base catalysis in water solution. J. Am. Chem. Soc., 72. 4578. 

The original experimental evidence for concerted acid-base catalysis of the mutarotation in benzene is now considered unsound133 134 and concerted acid-base catalysis has been difficult to prove for nonenzy-matic reactions in aqueous solution. However, measurements of kinetic isotope effects seem to support Swain and Brown s interpretation.135 Concerted acid-base catalysis by acetic acid and acetate ions may have been observed for the enolization of acetone136 and it may be employed by enzymes.1363... 

Gibbs energy of dissociation, table 293 pKa values of, table 293 strengths of 95-96 Acid -base catalysis 469,486 - 491 concerted 490 of mutarotation 487 Acid - base chemistry... 

Fumarase. See Fumarate hydratase Fumarase-aspartase family 685 Fumarate 481s, 516s, 683s Fumarate hydratase (fumarase) 526, 683,688 acid-base catalysis 471 concerted reaction 685 Fumarase A 688 Fumarase B 688 Fumarase C 683 mechanism 683 - 685 pH dependence 684 rates of substrate exchange 684 turnover number of 683 Fumarate reductase 785 Fumarylpyruvate 690s Function of state R 476 Fungal infections 20 Fungi 20... 

Serine Proteases Enzymes that Use a Serine Residue for Nucleophilic Catalysis Ribonuclease A An Example of Concerted Acid-Base Catalysis... 

Ribonuclease A An Example of Concerted Acid-Base Catalysis... 

In enzymes, the active site may possess acid and base groups intimately associated with the conjugate base and acid functions, respectively, of the complexed substrate the push-pull mechanism is possible but might not be a driving force. The halogenation of acetone in the presence of aqueous solutions of carboxylic acid buffers exhibits the rate law of Equation 11.2 where the third-order term, although small, has been shown to be significant and due to bifunctional concerted acid-base catalysis (Scheme 11.13) ... 

As emphasised recently by Hegarty and Jencks (1975), the interpretation of the third-order terms, kAp HAi[HAj][Af], in the rate law for acetone enolisation (4) has been the subject of much controversy and has played an important role in the development of ideas on the mechanism of acid-base catalysis in solution and at the active sites of enzymes . The problem has been not only to interpret this term, which can be accounted for by a concerted mechanism (17), but also to examine whether this mechanism, with H20 acting as an acid or a base, is valid for interpreting the other terms fcHA,[HAj] and fcA,lAj ]. 

Enzyme catalyzed mechanisms represent fundamentally familiar reactions from organic chemistry (Figure 2.17). Acid-base catalysis is associated with the donation or subtraction of protons. Acid catalysis is a process in which partial proton transfer from an acid lowers the free energy of the reaction transition state, while base catalysis is a process in which partial proton subtraction by a base lowers the free energy of the reaction transition state. Concerted acid-base catalysis, where both processes occur simultaneously, is a common enzymatic mechanism. 

Subst. carboxylic acid amides from inactive carboxylic acid esters Concerted acid-base catalysis... 

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