Reaction rates, medium effects, catalysis

The energy required to proceed from reactants to products is AG, the free energy of activation, which is the energy at the transition state relative to the reactants. We develop the theoretical foundation for these ideas about reaction rates in Section 3.2. We first focus attention on the methods for evaluating the inherent thermodynamic stability of representative molecules. In Section 3.3, we consider general concepts that interrelate the thermodynamic and kinetic aspects of reactivity. In Section 3.4, we consider how substituents affect the stability of important intermediates, such as carbocations, carbanions, radicals, and carbonyl addition (tetrahedral) intermediates. In Section 3.5, we examine quantitative treatments of substituent effects. In the final sections of the chapter we consider catalysis and the effect of the solvent medium on reaction rates and mechanisms. 

A catalyst is a substance that increases the rate of a reaction, other than by a medium effect, regardless of the ultimate fate of this substance. For example, in hydroxide-catalyzed ester hydrolysis the catalyst OH is consumed by reaction with the product acid some writers, therefore, call this a hydroxide-promoted reaction, because the catalyst is not regenerated, although the essential chemical event is a catalysis. 

With 77 % aqueous acetic acid, the rates were found to be more affected by added perchloric acid than by sodium perchlorate (but only at higher concentrations than those used by Stanley and Shorter207, which accounts for the failure of these workers to observe acid catalysis, but their observation of kinetic orders in hypochlorous acid of less than one remains unaccounted for). The difference in the effect of the added electrolyte increased with concentration, and the rates of the acid-catalysed reaction reached a maximum in ca. 50 % aqueous acetic acid, passed through a minimum at ca. 90 % aqueous acetic acid and rose very rapidly thereafter. The faster chlorination in 50% acid than in water was, therefore, considered consistent with chlorination by AcOHCl+, which is subject to an increasing solvent effect in the direction of less aqueous media (hence the minimum in 90 % acid), and a third factor operates, viz. that in pure acetic acid the bulk source of chlorine ischlorineacetate rather than HOC1 and causes the rapid rise in rate towards the anhydrous medium. The relative rates of the acid-catalysed (acidity > 0.49 M) chlorination of some aromatics in 76 % aqueous acetic acid at 25 °C were found to be toluene, 69 benzene, 1 chlorobenzene, 0.097 benzoic acid, 0.004. Some of these kinetic observations were confirmed in a study of the chlorination of diphenylmethane in the presence of 0.030 M perchloric acid, second-order rate coefficients were obtained at 25 °C as follows209 0.161 (98 vol. % aqueous acetic acid) ca. 0.078 (75 vol. % acid), and, in the latter solvent in the presence of 0.50 M perchloric acid, diphenylmethane was approximately 30 times more reactive than benzene. 

The nature of antibody catalysis remains to be elucidated, and antibodies will not reach the efficiency of enzymes until they can emulate the conformational changes, acid/base, redox, and/or nucleophilic/electro-philic reactivities of catalytic residues along the entire reaction coordinate. It is worthy of note that Hollfelder et al recently demonstrated that serum albumins catalyze the eliminative ring-opening of a benzoisoxazole at rates that are similar to those observed with catalytic antibodies. They suggest that formal general base catalysis contributes only modestly to the efficiency of both systems, and they favor the view that the antibody catalysis may be enhanced in some cases by nonspecific medium effects. 

An important problem in catalysis is to predict diffusion and reaction rates in porous catalysts when the reaction rate can depend on concentration in a non-linear way.6 The heterogeneous system is modeled as a solid material with pores through which the reactants and products diffuse. We assume for diffusion that all the microscopic details of the porous medium are lumped together into the effective diffusion coefficient De for reactant. 

There have been a number of studies of the reaction of diazoacetic ester in aprotic solvents, mainly with carboxylic acids (Bronsted and Bell, 1931 Hartman et al., 1946 and references cited). However, the information available hardly justifies conclusions about the mechanism. Addition of relatively basic phenols causes an acceleration in rate which can be interpreted in terms of nucleophilic catalysis of a rate-determining displacement of nitrogen, but the kinetic order in acid varies between one and two. Formally, a mixed order would result if proton loss from the diazonium ion was effected by carboxylate ions alone, while the less discriminating displacement of nitrogen involved competition between anions and unionized molecules. However, there are examples of high or mixed orders in other acid-catalysed reactions (Bronsted and Bell, 1931 Bell, 1941 1959) and in all probability large medium effects play a role. 

With substrate 8a, the use of SCCO2 leads to a significant improvement of the performance of the catalyst compared to the reaction in CH2CI2. Hence, significantly lower amounts of iridium complex are required for efficient catalysis in the supercritical medium and turnover frequencies (TOF, mole product per mole metal center per hour) above 2,000 h were achieved at catalysts loadings as low as 0.014 mol%. It is important to note that this effect is not related to a simple increase in reaction rate, but rather to a change in the overall kinetic behavior of the catalytic system. 

Akinetic study was also performed in a variety of vesicular solutions (DDAB, DODAB, DODAC [NaOH] = 2.25mM, 25 °C). Interestingly, the vesicles possess stronger catalytic reaction environments than the micelles. The rate-determining proton transfer from carbon to the hydroxide ion was accelerated up to 850 fold in di- -dodecyldimethylam-monium bromide (DDAB) vesicles. This is evidence that the reaction sides are less aqueous than those in micelles, as anticipated. Application of the pseudophase model afforded the bimolecular rate constants in the vesicles (kves). For the different vesicles, ves is significantly higher (ca. 12 times for DODAB) than the second-order rate constant in water. This shows that the catalysis is due to both a medium effect and a concentration effect. It was assumed that there was a fast equilibrium for substrate binding to the inner and outer leaflets of the bilayer, in accord with the fact that no two-phase kinetics were found. 

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