Solvent Internal Pressure

For a given reaction studied in a series of solvents, (8r- 8 f) is essentially constant, and most of the change in In k will come from the term — AV (8j — 8s)". If AV is positive, an increase in 8s (increase in solvent internal pressure) results in a rate decrease. If AV is negative, the reverse effect is predicted. Thus reactivity is predicted by regular solution theory to respond to internal pressure just as it does to externally applied pressure (Section 6.2). This connection between reactivity and internal pressure was noted long ago," and it has been systematized by Dack. -" ... 

By and large, the solute exerts pressure on the solvent. Internal pressure can affect the liquid-phase ion-radicals reactions and requires special study. For instance, such pressure can determine the selectivity and even the stereochemistry of these reactions (Okamoto et al. 1998, Adam and Trofimov 2003). 

Comparison of the barrier heights in the gas phase with those found in the liquid phase can elucidate the role of solvent internal pressure on conformational interconversion when dielectrical interactions are minimal. The magnitude of these interactions can be estimated by considering an activation volume, AV, for the process, defined as,... 

A. F. M. Barton Handbook of Solubility Parameters and other Cohesion Parameters, CRC Press, Boca Raton/Florida, 1983. [232] M. R. J. Dack The Importance of Solvent Internal Pressure... 

The relative rate constants for reaction of Br with toluene (kj ) and cyclopropylbenzene (k(3) in SC-CO2 were assessed by direct competition (Scheme 9). The rate constant ratio k( /lqj in SC-CO2 (40 C, 1,300 psi) was 1.2 0.2 and is nearly identical to that found in CCI4 at the same temperature (1.3 0.1). These obseivations also support the hypothesis that Br properties (i.e., reactivity/selectivtiy) are unaffected by complexation to CO2 solvent. Earlier work has shown that kQ/kjj varies as a function of solvent internal pressure (57). We are presently examining the effect of CO2 pressure on kc/kjj. 

Dack MR (1975) The importance of solvent internal pressure and cohesion to solution phenomena. J Aust J Chem 28 211-229... 

A through-space F P coupling (formally across six bonds) has been observed by Kruck et al. in the spectra of one distereoisomer only, out of two possible iV,iV iimethylphosphoramidites, obtained from the reaction of 3 -trifluoromethylsulphonyl-2 2-dihydroxy-1,1 -binaphtalene with hexam-ethylphosphorous triamide. This coupling reveals a remarkable, up to 400%, dependence on temperature and solvent internal pressure. 

The solvent internal pressure can induce changes in the reaction kinetics when specific solvent-solute interactions are weak, and the activation volumes involved, which we will discuss later, are substantial. An example involves the kinetics of the Diels-Alder reaction between 4-bromonitrosobenzene and 2,3-dimethylbutadiene in solutions of various inorganic perchlorates in acetone [7]... 

Alternatively, authors have repeatedly invoked the internal pressure of water as an explanation of the rate enhancements of Diels-Alder reactions in this solvent ". They were probably inspired by the well known large effects of the external pressure " on rates of cycloadditions. However, the internal pressure of water is very low and offers no valid explanation for its effect on the Diels-Alder reaction. The internal pressure is defined as the energy required to bring about an infinitesimal change in the volume of the solvents at constant temperature pi = (r)E / Due to the open and... 

The pressure exerted by the solvent is called its internal pressure it, and it is defined by Eq. (8-34), where E is molar energy and V is molar volume. 

The internal pressure is a differential quantity that measures some of the forces of interaction between solvent molecules. A related quantity, the cohesive energy density (ced), defined by Eq. (8-35), is an integral quantity that measures the total molecular cohesion per unit volume. - p... 

Table 8-6. Internal Pressure and Cohesive Energy Density (ced) of Solvents...

TABLE 1.2 Internal Pressures and Cohesive Energy Densities for Some Common Solvents (25° C)9... 

An unopened bottle of chromous chloride solution exploded after prolonged storage [1]. This was most likely caused by internal pressure of hydrogen developed by slow reduction of the solvent water by the powerfully reducing Cr(II) ion [2],... 

In such two-phase systems, a higher pressure increases the solubility of the gas in the liquid phase. The changes in the solution itself are similar to those which occur when a low-density liquid is mixed with a high-density solvent the volume of the solution increases and its density and internal pressure decrease. Ill (see Figure la). These effects are opposite to those observed in condensed singlephase systems, in which a pressure increase causes a volume decrease and a density increase /2,3/ (see Figure 1 b). 

In cases where the solid reduction products of the solvent molecules form highly cohesive and adhesive surface films, the surface reactions are quickly blocked before further massive reduction of solution species (which also form the gas molecules) takes place. When passivation of the graphite is not reached quickly enough (as in the case of PC solutions), intensive surface reactions build up the internal pressure that cracks the particles and leads to their deactivation. 

However, at 200 °C or above, at atmospheric pressure, the choice of solvents is limited. High-boiling solvents are inconvenient to remove and to repurify. These disadvantages can be avoided by heating low-boiling solvents in closed systems that can withstand the increased internal pressure. After cooling and opening the vessel, concentration of products can be achieved readily by evaporation of the solvent, which usually can be redistilled for reuse. 

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