Pressure, effect on reaction rate

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... 

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

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. 

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. 

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. 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

At this point we believe that changing local compositions is the dominant effect in the apparent rate increase for the reaction of benzophenone triplet and isopropanol in supercritical CO2 and this conclusion is supported by subsequent studies of the reaction of the triplet with other quenchers (Roberts et al., 1991). However, one must also consider the thermodynamic pressure effect on the rate constant in terms of transition state theory and possible cage effects. 

Figure 3. Pressure effect on the rate of the CH + N, reaction at room temperature...

External Pressure and Solvent Effects on Reaction Rates... 

Not only the internal pressure of a solvent can affect chemical reactions (see Section 5.4.2 [231, 232]), but also the application of external pressure can exert large effects on reaction rates and equilibrium constants [239, 429-433, 747-750]. According to Le Chatelier s principle of least restraint, the rate of a reaction should be increased by an increase in external pressure if the volume of the activated complex is less than the sum of the volumes of the reactant molecules, whereas the rate of reaction should be decreased by an increase in external pressure if the reverse is true. The fundamental equation for the effect of external pressure on a reaction rate constant k was deduced by Evans and Polanyi on the basis of transition-state theory [434] ... 

Volumes of activation and reaction are themselves also pressure-dependent as shown for the volume of activation in Figure l. There is no theory explaining this pressure dependence which would allow the volume of activation or reaction to be determined over a larger range of pressure. Therefore, several empirical relations are employed to fit the pressure dependencies of rate and equilibrium constants " from which the least-squares fit [hiA (p) = a + b- p, hi= 0) = a, A= -b-R-T orhi f(p) = a - -b p, hiA (p = 0) = a, AV = y RT] is the simplest and in many cases also the most reliable method of computing A and A V, It is only applicable in the low-pressure range (<2000 bar) where the dependencies of hi (p) or In if (p) on pressure p are usually linear. Thus, this method requires a very precise measurement of the rate constants at relatively low-pressures (1-2000 bar) where the pressure effect on the rate constants is relatively... 

It is clear that there is a wide scope for performing reactions in CO2 and expanded solvents. An improved knowledge of solvent effects on reaction rates and selectivity at high pressure will enable targeted research... 

Most reactions that have been investigated using PTC in supercritical fluids have been solid-SCF systems, not liquid-SCF. The first published example of PTC in an SCF is the displacement reaction of benzyl chloride 1 with potassium bromide in supercritical carbon dioxide (SCCO2) with 5 mol % acetone, in the presence of tetraheptylammonium bromide (THAB) [19-20] (Scheme 4.10-1) to yield benzyl bromide 2. The effects on reaction rate of traditional PTC parameters, such as agitation, catalyst type, temperature, pressure, and catalyst concentration were investigated. The experimental technique is described below. PTC appeared to occur between an SCF phase and a solid salt phase, and in the absence of a catalyst the reaction did not occur. With an excess of inorganic salt, the reaction was shown to follow pseudo-first order kinetics. 

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