Pressure effects on reaction

It is customary to justify the study of pressure effects on reaction rates on the grounds that it can elucidate reaction mechanisms. A somewhat different and, I think, more... 

In this spirit, an attempt will be made to account for the magnitude of pressure effects on ligand substitution reaction rates. Attention will necessarily be confined to a few simple model systems two recent reviews (1, 2) of pressure effects on reactions of transition metal complexes in solution may be consulted for more comprehensive surveys of the field. 

Jenner [275] has presented a thorough description of several possible contributions to both the intrinsic and the environmental parts of the activation volumes, based on accurate experimental observation of pressure effect on reactions in solutions. The intrinsic contribution to the activation volume essentially derives from the differences in structure between the transition state and the reacting species, so it is directly related to the partial cleavage and formation of chemical bonds in the transition state. In cases where the environmental contribution is negligible, the activation volume variation gives a direct insight in the molecular mechanism [275, 280]. In this case in fact, considering... 

Temperature and Pressure Effect on Reaction Rate Coefficients and Diffusivities... 

It should be pointed out however that Kramer and Leder (15) used CO2 as the SCF solvent because it also acts as a catalyst activator. Thus, factors other than temperature and pressure effects on reaction equilibrium can also dictate the choice of the SCF solvent. 

The fundamental principles developed for gas-phase or liqnid-phase reactions may be applied to supercritical phase reactions as well. When the reaction medium density is gas-like, the concepts developed for gas-phase reactions (such as kinetic theory of gases) may be applied. For liquid-like reaction mixtures (ie, dense supercritical reaction media), principles of liqnid-phase kinetics have been applied. Parameters such as the solvent s solubility parameter, dielectric constant or solvatochromic shift, routinely used to interpret liquid-phase reactions, have been employed to understand the effect of a given supercritical solvent on chemical reaction (42,43). In the vicinity of the critical point, supercritical reaction media admit some unique phenomena such as local enhancement of density (the so-called clustering phenomenon) and sensitive pressure effects on reaction rate and equilibrium constants. 

Pressure and residence time have relatively Htde effect on reaction selectivity, at least within the ranges normally encountered. Poor mixing and excessive residence time result in increased carbonization of the reactor. 

Flynn and Dickens [142] have translated the relaxation methods of fluid kinetics into terms applicable to solid phase thermogravimetry. The rate-determining variables such as temperature, pressure, gas flow rate, gas composition, radiant energy, electrical and magnetic fields are incremented in discrete steps or oscillated between extreme values and the effect on reaction rate determined. 

Jenner investigated the kinetic pressure effect on some specific Michael and Henry reactions and found that the observed activation volumes of the Michael reaction between nitromethane and methyl vinyl ketone are largely dependent on the magnitude of the electrostriction effect, which is highest in the lanthanide-catalyzed reaction and lowest in the base-catalyzed version. In the latter case, the reverse reaction is insensitive to pressure.52 Recently, Kobayashi and co-workers reported a highly efficient Lewis-acid-catalyzed asymmetric Michael addition in water.53 A variety of unsaturated carbonyl derivatives gave selective Michael additions with a-nitrocycloalkanones in water, at room temperature without any added catalyst or in a very dilute aqueous solution of potassium carbonate (Eq. 10.24).54... 

Processes with gaseous reactants are excluded here. Due to the large compressibility of gases an increase of pressure (up to 1 kbar) leads essentially only to an increase of gas concentration, and hence to an acceleration of bimolecular processes in which gases are involved as reactants. The effect of pressure on a chemical reaction in compressed solution is largely determined by the volume of reaction (AV) and the volume of activation (AV ). It is not the purpose of this chapter to provide a complete survey of reactions of dienes and polyenes which have been investigated at elevated pressures. There are many excellent monographs (e.g. References 1-4) and reviews (e.g. References 5-16) on this topic which cover the literature up to early 1990. After a short introduction into the basic concepts necessary to understand pressure effects on chemical processes in compressed solutions, our major objective is to review the literature of the past ten years. 

Laverman and coworkers have reported activation parameters for the aqueous solution reactions of NO with the iron(II) and iron(III) complexes of the water soluble porphyrins TPPS andTMPS (21). These studies involved systematic measurements to determine on and kQ as functions of temperature (298—318 K) and hydrostatic pressure (0.1—250 MPa) to determine values of AH, AS and AV for the on and off reactions of the ferri-heme models and for the on reactions of the ferro-heme models (Table II). Figure 2 illustrates hydrostatic pressure effects on kOTL and kQff for Fem(TPPS). 

The usual starting point in enzyme kinetics is the Michaelis-Menten equation for the reaction rate v. This also seems a convenient starting point for interpretation of pressure effects on enzyme mechanisms. It will be shown that this formalism may be deceptive if the definitions and interpretations have not been made clear from the beginning. For the mechanism... 

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