Kinetics acid-base catalysis

The Brdnsted relations, as presented here, constitute a brief introduction to a vast and well-organized chapter of chemical kinetics acid-base catalysis. A recent survey of the field is The Proton in Chemistry, by R. P. Bell, Methuen Co. Ltd., London, 1959. 

Acid—Base Catalysis. Inexpensive mineral acids, eg, H2SO4, and bases, eg, KOH, in aqueous solution are widely appHed as catalysts in industrial organic synthesis. Catalytic reactions include esterifications, hydrations, dehydrations, and condensations. Much of the technology is old and well estabhshed, and the chemistry is well understood. Reactions that are cataly2ed by acids are also typically cataly2ed by bases. In some instances, the kinetics of the reaction has a form such as the following (9) ... 

Much of the study of kinetics constitutes a study of catalysis. The first goal is the determination of the rate equation, and examples have been given in Chapters 2 and 3, particularly Section 3.3, Model Building. The subsection following this one describes the dependence of rates on pH, and most of this dependence can be ascribed to acid—base catalysis. Here we treat a very simple but widely applicable method for the detection and measurement of general acid-base or nucleophilic catalysis. We consider aqueous solutions where the pH and p/f concepts are well understood, but similar methods can be applied in nonaqueous media. 

A reaction with a rate constant that conforms to Eq. (10-21)—particularly to the feature that the catalysts are H+ and OH-, and not weak acids and bases—is said to show specific acid-base catalysis. This phenomenon is illustrated by the kinetic data for the hydrolysis of methyl o-carboxyphenyl acetate16 (the methyl ester of aspirin— compare with Section 6.6) ... 

The rate of a reaction that shows specific acid (or base, or acid-base) catalysis does not depend on the buffer chosen to adjust the pH. Of course, an inert salt must be used to maintain constant ionic strength so that kinetic salt effects do not distort the pH profile. 

LaBarbera, D. V. Skibo, E. B. Solution kinetics of CC-1065 A-ring opening substituent effects and general acid/base catalysis, j. Am. Chem. Soc. 2006, 128, 3722-3727. 

Antibody 15C5 was able to catalyse the hydrolysis of the triester [105] with cat 2.65 x 10 3 min 1 whilst a second antibody from the same immunization programme was later found to hydrolyse the acetylcholinesterase inhibitor Paraoxon [106] with kcat = 1.95 x 10 3min-1 at 25°C (Appendix entry 6.2) (Lavey and Janda, 1996b). Antibody 3H5 showed Michaelis-Menten kinetics and was strongly inhibited by the hapten [104]. It exhibited a linear dependence of the rate of hydrolysis on hydroxide ion concentration, suggesting that 3H5 effects catalysis by transition state stabilization rather than by general acid/base catalysis. 

However, simple kinetic models, especially of the Langmuir—Hinshel-wood type, can serve with advantage for correlation of experimental data in spite of simplifying assumptions which are necessary for their derivation. Experience shows that heterogeneous acid—base catalysis is the very field where they fit best. Their most frequent general form... 

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... 

Effective concentration 65-72 entropy and 68-72 in general-acid-base catalysis 66 in nucleophilic catalysis 66 Elastase 26-30, 40 acylenzyme 27, 40 binding energies of subsites 356, 357 binding site 26-30 kinetic constants for peptide hydrolysis 357 specificity 27 Electrophiles 276 Electrophilic catalysis 61 metal ions 74-77 pyridoxal phosphate 79-82 Schiff bases 77-82 thiamine pyrophosphate 82-84 Electrostatic catalysis 61, 73, 74,498 Electrostatic effects on enzyme-substrate association rates 159-161... 

Figure 16.7 Mechanism of aspartyl proteases involving general acid-base catalysis and the formation of a protonated terahedral intermediate. Bottom Proposal by T. J. Rodriguez. T. A. Angeles, and T. D. Meek, Biochemistry 32, 12380 (1993), that the first step is peptide bond isomerization. This accounts for the observed inverse 15N/14N kinetic isotope effect, which implies that bonding with the N atom becomes stiffer in the transition state.

Although free radical reactions are found less often in solution than in the gas phase, they do occur, and are generally handled by steady state methods. There are also organic and inorganic reactions that involve non-radical intermediates in steady state concentrations. These intermediates are often produced by an initial reversible reaction, or a set of reversible reactions. This can be compared with the pre-equilibria discussed in Section 8.4, where the intermediates are in equilibrium concentrations. The steady state treatment is also used extensively in acid-base catalysis and in enzyme kinetics. 

Index Entries Reaction kinetics glucose decomposition dilute acid hydrolysis kinetic modeling acid-soluble lignin acid-base catalysis rules. 

Modeling of Chemical Kinetics and Reactor Design ACID-BASE CATALYSIS HOMOGENEOUS LIQUID PHASE... 

This enzyme, which is relatively stable under reaction conditions, will retain 70% of its activity after 10 days at pH 5 and 25°C. Although it is not yet commercially available, it has been overexpressed in E. coli, making large quantities easily accessible.68 The detailed mechanism of DERA has been determined based on the atomic structure (ca. 1.0 A) combined with site-directed mutagenesis, kinetic, and NMR studies136 (Scheme 5Alb). A proton relay system composed of Lys and Asp appears to activate a conserved active-site water that functions as the critical mediator for proton transfer in acid-base catalysis. 

To supplement the data on prolyl isomerization, I will draw on the literature describing rotation about the C-N bond in secondary amides. Early studies in this field were described by Stewart and Siddall in an excellent 1970 review. As we will see, these reactions are related to prolyl isomerization and support the mechanism to be proposed for prolyl isomerization. The mechanism is based on results from a variety of experimental approaches. In all cases, experiments employing kinetic-based probes will be used to obtain an accurate picture of the activated complex in the rate-limiting transition state. The experiments that will be described include thermodynamics, in which activation parameters (i.e., AG, AHt, and ASt) will be described solvent effects, in which the influence of organic solvents and deuterium oxide will be reviewed acid-base catalysis substituent effects and secondary deuterium isotope effects. 

One of the very first kinetic examples of this catalyzed prototropy was found in the halogenation of acetone. In polar solvents, it is found that the rate of halogenation of acetone is first-order in acetone, zero-order in halogen, X2, and subject to general acid-base catalysis ... 

Figure 3 Idealized pH dependence of a ribozyme reaction. Ideal pH-species plots and pH-/cobs profiles according to a kinetic model for general acid/base catalysis. The solid lines depict a mechanism in which the species with the lower pKa (pKa,1) acts as the general base (shown by blue lines), and the species with the higher pKa (pKa,2) acts as the general acid (shown by red lines). The black line indicates the observed pH dependence of the reaction rate. The dotted lines simulate a mechanism in which the catalytic roles of the species with pKa,1 and pKa,2 have been switched. Adapted from References 34 and 35.

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