Solvent pressures, activation volume from

Supercritical solvents can be used to adjust reaction rate constants (k) by as much as two orders of magnitude by small changes in the system pressure. Activation volumes (slopes of In k vs P) as low as —6000 cm3/mol were observed for a homogeneous reaction (97). Pressure effects can also be pronounced on reversible reactions (17). In one example the equilibrium constant was increased from two- to sixfold by increasing the solvent pressure. The choice of supercritical solvent can also dramatically affect an equilibrium constant. An obvious advantage of using supercritical fluid solvents as a media for chemical reactions is the adjustability of the reaction kinetics and equilibria owing to solvent effects. 

The postulated C-N heterocoupling requires diffusion of the two radicals either in the solvent-solute surface layer or in the bulk solution. In both cases one expects that the reaction rate should decrease with increasing solvent viscosity. To achieve the latter, the CdS-Si02-catalyzed photoaddition of 2,5-DHF to azobenzene was conducted at pressures ranging from 0.1 to 120 MPa [164]. Both the formation rates of the addition and reduction products 13c and 16 (R = R = Ph) decrease with increasing pressure and from a plot of In(rate) vs. pressure activation volume AF are obtained as 17.4 + 3.4 and 15.8 + 2.3 cm mol , for 13c and 16, respectively (Figure 25). 

As expected from the mechanism, the reaction was strongly accelerated in aprotic as well as in protic solvents. The activation volumes were —21 and —22 cm mol in ethanol and methyl acetate, respectively. Similar pressure-induced accelerations were also observed in MPD and GTA at lower pressures. Flowever, the direction of the pressure effect was reversed at higher pressures. Figures 3.12 and 3.13 illustrate pressure effects observed for DNAB in the two solvents. The results for DMNAB were qualitatively identical. 

Figure S. Activation volume from empirical differential solvent pressures (16)...

Extreme cases were reactions of the least stabilized, most reactive carbene (Y = CF3, X = Br) with the more reactive alkene (CH3)2C=C(CH3)2, and the most stabilized, least reactive carbene (Y = CH3O, X = F) with the less reactive alkene (1-hexene). The rate constants, as measured by LFP, were 1.7 x 10 and 5.0 X lO M s, respectively, spanning an interval of 34,000. In agreement with Houk s ideas,the reactions were entropy dominated (A5 —22 to —29e.u.). The AG barriers were 5.0 kcal/mol for the faster reaction and 11 kcal/ mol for the slower reaction, mainly because of entropic contributions the AH components were only —1.6 and +2.5 kcal/mol, respectively. Despite the dominance of entropy in these reactive carbene addition reactions, a kind of de facto enthalpic control operates. The entropies of activation are all very similar, so that in any comparison of the reactivities of alkene pairs (i.e., ferei)> the rate constant ratios reflect differences in AA//t, which ultimately appear in AAG. Thus, car-benic philicity, which is the pattern created by carbenic reactivity, behaves in accord with our qualitative ideas about structure-reactivity relations, as modulated by substiment effects in both the carbene and alkene partners of the addition reactions. " Finally, volumes of activation were measured for the additions of CgHsCCl to (CH3)2C=C(CH3)2 and frani-pentene in both methylcyclohexane and acetonitrile. The measured absolute rate constants increased with increasing pressure Ayf ranged from —10 to —18 cm /mol and were independent of solvent. These results were consistent with an early, and not very polar transition state for the addition reaction. 

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