Kinetics general catalysis

E. Crezee, B.W. Hoffer, R.J. Berger, M. Makkee, F. Kapteijn and J.A. Motrlijrt, Three-phase hydrogerration of D-glucose over a carbon supported mtherrirrm catalyst - mass transfer and kinetics, Applied Catalysis A General 251 (2003) 1. 

Based on the results obtained in the investigation of the effects of modulation of the electron density by the nuclear vibrations, a lability principle in chemical kinetics and catalysis (electrocatalysis) has been formulated in Ref. 26. This principle is formulated as follows the greater the lability of the electron, transferable atoms or atomic groups with respect to the action of external fields, local vibrations, or fluctuations of the medium polarization, the higher, as a rule, is the transition probability, all other conditions being unchanged. Note that the concept lability is more general than... 

Nevertheless the results quoted above for the breakdown of dimethyl hemiorthobenzoate indicate that (kinetic) general acid catalysis should be detectable in the methanolysis of methyl benzoate (Bransted a = 0.49) and probably in the analogous hydrolysis of methyl benzoate. Therefore any mechanism proposed for these reactions must be able to account for this. 

Kinetic data are available for the hydrolysis of benzophenone diethyl ketal in various dioxane—water mixtures [164]. General catalysis cannot be detected with certainty [165, 188,189]. The activation parameters for the H30+ catalyzed reaction are AH = 13.9 kcal. mole-1 and AS = +1.4 eu as expected for an A1 reaction. Another interesting example is the acid catalyzed hydrolysis of the tetramethyl ketal of p-benzoquinone which takes place in two stages [190]... 

G. Sivahngam, G. Madras, Photocatal5dic degradation of poly(bisphenol-A-carbonate) in solution over combustion-S3mthesized Ti02 mechanism and kinetics , Apphed Catalysis A General, 269, 81-90, (2004). 

Klabunovskii, E.l. (1992) On some general regularities of action of enantiomeric catalysts of hydrogenation, Kinet. Katal., (Kinetics and Catalysis, mss) 33,292-303, Chem. Abstr. 1992,117, 170494r. 

Laosiripojana, N., Assabumrungrat, S. (2005). Methane steam reforming over Ni/ Ce-Zr02 catalyst influences of Ce-Zr02 support on reactivity, resistance toward carbon formation, and intrinsic reaction kinetics. Applied Catalysis A General, 290, 200-211. 

The addition of water to carbon dioxide, CO2 + H2O OC(OH)2 H +HCOJ, is formally very similar to its addition to aldehydes and ketones, although here only 0.2% of the carbon dioxide is hydrated at equilibrium, and observations make use of the further equilibrium with and HCO3. A summary of work up to 1958 has been given by Edsall and Wyman,and several later kinetic studies have been made " the hydration process shows general catalysis by basic anions. It is particularly interesting that the enzyme carbonic anhydrase, which is active in maintaining the carbon dioxide-bicarbonate equilibrium in the body, is also an effective catalyst for the hydration of acetaldehyde and other carbonyl compounds. ... 

Beretta A, Groppi G, Majocchi L, Forzatti P. Potentialities and drawbacks of the experimental approach to the study of high T and high GHSV kinetics. Applied Catalysis A General 1999 187 49-60. 

Eq. 9.26 shows a scenario in which the first step involves an acid that facilitates a reaction of water witli the substrate. This is the rate-determining step. An intermediate is formed that results from the addition of water to the reactant [HRCHaO) ] the structural details are unimportant here. The intermediate then loses a proton in a second fast step to regenerate the acid HA and give the product. General catalysis does not require that water be one of the reactants. This just simplifies our current analysis because we assume that its concentration is constant and is incorporated into the rate constant k. The kinetic expression for this reaction is given in Eq. 9.27, which reflects only the first step of Eq. 9.26 because it is rate-determining. Alternatively, one can substitute the relationship [HA] = [HjO llA I/K, to achieve Eq. 

One of the important mechanistic considerations involved in addition and addition-elimination reactions of carbonyl compounds is the precise sequence of events. In particular, a major focus is on whether specific or general catalysis is involved in these reactions. In Chapter 10 we will consistently state whether the reactions are subject to general or specific catalysis. Let s examine the factors under which these various mechanisms operate. Figure 9.13 A shows a two-step process involving nucleophilic addition followed by protonation. The first step is rate-determining. The acid is not part of the kinetic equation, and therefore there is no acid catalysis of any kind, specific or general. This mechanism occurs for strong nucleophiles. As we will see in Chapter 10, the addition of cyanide to an aldehyde is one example. 

Related Reading General Texts on Kinetics and Catalysis.271... 

RELATED READING GENERAL TEXTS ON KINETICS AND CATALYSIS... 

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

The role that acid and base catalysts play can be quantitatively studied by kinetic techniques. It is possible to recognize several distinct types of catalysis by acids and bases. The term specie acid catalysis is used when the reaction rate is dependent on the equilibrium for protonation of the reactant. This type of catalysis is independent of the concentration and specific structure of the various proton donors present in solution. Specific acid catalysis is governed by the hydrogen-ion concentration (pH) of the solution. For example, for a series of reactions in an aqueous buffer system, flie rate of flie reaction would be a fimetion of the pH, but not of the concentration or identity of the acidic and basic components of the buffer. The kinetic expression for any such reaction will include a term for hydrogen-ion concentration, [H+]. The term general acid catalysis is used when the nature and concentration of proton donors present in solution affect the reaction rate. The kinetic expression for such a reaction will include a term for each of the potential proton donors that acts as a catalyst. The terms specific base catalysis and general base catalysis apply in the same way to base-catalyzed reactions. 

A kinetic expression which is equivalent to that for general acid catalysis also occurs if a prior equilibrium between reactant and the acids is followed by rate-controlling proton transfer. Each individual conjugate base will appear in the overall rate expression ... 

Alkenes lacking phenyl substituents appear to react by a similar mechanism. Both the observation of general acid catalysis and the kinetic evidence of a solvent isotope effect are consistent with rate-limiting protonation with simple alkenes such as 2-metlQ lpropene and 2,3-dimethyl-2-butene. 

A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

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