Medium effects

A large amount of experimental evidence has been cited to show that the rates of several Diels-Alder additions are little dependent on the medium. Comparison of rates in the gas and liquid phases can only occasionally be made. The dimerisation of cyclopentadiene has the following coefficients (l.mole .sec ) at 20°C  

For the dimerisation of butadiene, some kinetic results for the reaction in the pure liquid phase are available. Rate coefficients can be calculated from experimental conversion data at low total conversion to a good approximation, and so compared with coefficients for the gas-phase reaction, calculated from kinetic results at higher temperatures , by using the appropriate Arrhenius equation the k values (I.mole. sec ) are gas phase 10 /t = 2.6 (60°C) 24 (82°C) pure liquid phase 10 = 3.4(60°C) 21 (82°C). 

Thus the rate of both cyclopentadiene and butadiene dimerisations are remarkably little affected by a change of phase. 

From the formula given in ref. 25 in a footnote on p. 202 density of butadiene taken from ref 119. 

EFFECT OF SOLVENT ON THE RATE COEFFICIENTS (L.MO L E s EC ) OF SOME DIELS-ALDER REACTIONS 

In his 1963 empirical classification of Lewis acids, Pearson put BF3 in the category of hard acids (see Section 1.11.1 in Chapter 1). In fact, semi-quantitative studies in the gas phase [3] show that the BF3 basicity of methyl derivatives of the elements of groups 15 and 16 decreases in the order 

This is confirmed by the quantitative BF3 affinity scale, which shows that trimethylamine and tetrahydrofuran have a higher BF3 affinity than trimethylphosphine and tetrahydroth-iophene, respectively. Moreover, the thiocarbonyl and thiophosphoryl sulfur bases show lower BF3 affinities than the corresponding carbonyl and phosphoryl oxygen bases. Indeed, the decrease in basicity (affinity) in descending groups 15 and 16 of the periodic table is characteristic of hard basicity scales. 

The hard character of the BF3 affinity scale has been confirmed theoretically. The theoretical absolute hardness of BF3, p = 9.7 eV, is fairly high compared with other molecular Lewis acids (see Table 1.17). The hardness of the interaction of BF3 with NH3, NMc3 and CO has been studied [70] through the local Hard-Soft Acid-Base principle. 

More generally, the nature of the dative bond in BF3 complexes has been characterized by various means valence-bond method [25], calculation of the amount of intermolecular electron transfer [71] and partitioning of the interaction energy [24, 72]. 

Comparison of the BF3 and SbCls affinities collected in Tables 3.3 and 2.2, respectively, shows that thermal effects are roughly equal for the same Lewis base. The relationship 3.11 confirms a linear trend with a slope of about 1 and a low value of the intercept  

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Chen P and Meyer T J 1998 Medium effects on charge transfer in metai compiexes Chem. Rev. 98 1439-78... 

Jorgensen W L 1983. Theoretical Studies of Medium Effects on Conformational Equilibria, jouniM Physical Chemistry 87 5304-5314. 

A micelle-bound substrate will experience a reaction environment different from bulk water, leading to a kinetic medium effect. Hence, micelles are able to catalyse or inhibit organic reactions. Research on micellar catalysis has focused on the kinetics of the organic reactions involved. An overview of the multitude of transformations that have been studied in micellar media is beyond the scope of this chapter. Instead, the reader is referred to an extensive set of review articles and monographs" ... 

The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentration of the reacting species in or at the micelle. Also for higher-order reactions the pseudophase model is usually adopted (Figure 5.2). However, in these systems the dependence of the rate on the concentration of surfactant does not allow direct estimation of all of the rate constants and partition coefficients involved. Generally independent assessment of at least one of the partition coefficients is required before the other relevant parameters can be accessed. 

Because of the uncertainties related to the parametrization of an sp C, this approach is unsuitable for the study of protomeric equilibria for structures 4 through 8. We must lay stress on the fact that this simple treatment does not include (a) medium effects which are known to be important and b) the existence of associated species (see Chapter VII, Section I. LB) whose consequences have been thoroughly studied in pyridone series (1688). 

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. 

AG = 0 that is, all substituent (or medium) effects on the free energy change vanish at the isokinetic temperature. At this temperature the AH and TAS terms exactly offset each other, giving rise to the term compensation effect for isokinetic behavior. 

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

We have defined the solvent effect (medium effect) on a reaction by 8mAC = 8G - hGl =... 

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