Transfer activity coefficient solvents

Quantitative Separation of Protic and Dipolar Aprotic Solvent Effects for Reaction Rates by Means of Solvent-Transfer Activity Coefficients... 

When solvent-transfer activity coefficients are applied to reaction rates in terms... 

Table 5-19. Some representative values of solvent-transfer activity coefficients for anions and cations at 25 °C with water (W) as reference solvent molar concentration scale f For the corresponding Gibbs energies of transfer, see Table 2-9 in Section 2.3. 

The specific rate eonstant for this bimolecular reaction in a solvent S is related to the rate eonstant in the reference solvent O through the appropriate solvent-transfer activity coefficients. Eq. (5-115) shows to what extent solvent effects on the reaction rate are due to changes in the solvation of reactant anions, Y , of reactant nonelectrolytes, RX, and of anionic activated complexes, [YRX ]. Anionic and uncharged activated complexes will behave in exactly the same way upon solvent transfer as real anions and non-electrolytes of comparable structure. Anionic activated complexes such as [YRX ] should behave like large polarizable anions, and, therefore, should be better solvated in polarizable, dipolar solvents than in protic solvents. 

Table 5-20. Relative reaction rates and solvent-transfer activity coefficients for reactants and activated complex of Sn2 reaction (5-116) upon solvent transfer from methanol (abbreviated to M) to Af,Af-dimethylformamide (DMF) at 25 °C [6, 291].

If the transition state does involve this sort of interaction then the simple considerations based on solvent transfer activity coefficients are invalidated. The mechanism of substitution in square planar platinum(II) complexes, and in particular the role of the fifth and sixth positions in relation to reactant and transition state stability, is one of the most interesting and challenging mechanistic problems in transition-metal substitution kinetics, and there is no doubt that a systematic application of solvent activity coefficients to a range of neutral and charged reactant complexes will lead to a better insight into these problems. 

Solvents. Solvent transfer parameters have been used from time to time in diagnosis of mechanisms of organic reactions. Now their first use in an inorganic system has been discussed. Comparisons between kinetic results and solvent transfer activity coefficients for solvolysis of [Cr(NCS)e] in DMSO, DMF, and dimethylacetamide have been considered in terms of a dissociative mechanism. The DMSO results can be accommodated by a dissociative model for the transition state, but those in DMF and dimethylacetamide fit less well. It is interesting to compare these conclusions with, for instance, the dissociative mechanism proposed by the same authors for anation of the [Cr(DMSO)e] + and [Cr(DMF)6] + cations, and the associative mechanism suggested on the basis of the determined activation volume for exchange of DMSO with [Cr(DMSO)e +. ... 

By combining these ions with other counterions, single ion transfer activity coefficients are calculated. By these techniques transfer free energies or activity coefficients have been determined for many ions and nonelectrolytes in a wide variety of solvents.Parker has discussed the extrathermodynamic assumptions that lead to single ion quantities. 

Table 8-8 gives some nonelectrolyte transfer free energies, and Table 8-9 lists single ion transfer activity coefficients. Note especially the remarkable values for anions in dipolar aprotic solvents, indicating extensive desolvation in these solvents relative to methanol. This is consistent with the enhanced nucleophilic reactivity of anions in dipolar aprotic solvents. Parker and Blandamer have considered transfer activity coefficients for binary aqueous mixtures. 

This coefficient has various names (medium effect, solvation activity coefficient, etc.) the name recommended by the responsible IUPAC commission is the transfer activity coefficient. In this book the effect of solvation in various solvents will be expressed exclusively in terms of standard Gibbs transfer energies. 

The various factors that contribute to ion solvation were discussed in Section 2.2.1. In this section, we deal with the solvent effects on chemical reactions more quantitatively [5, 22]. To do this, we introduce two quantities, the Gibbs energy of transfer and the transfer activity coefficient. 

When we consider the solvent effect on the reactivity of a chemical species, it is convenient to use the transfer activity coefficients9 instead of the Gibbs energies of transfer. The transfer activity coefficient, yt(i,R->S), is defined by... 

The influence of ionic strength on yCii is great in solvents of lower permittivities. When we compare ionic activities in different solvents, we have to consider this activity coefficient, in addition to the transfer activity coefficient yt. However, in reality, the influence of is usually negligibly small compared to that of yt. 

We can use the transfer activity coefficients to predict solvent effects on chemical reactions and equilibria [22]. Some examples are shown below. 

Tab. 2.7 Transfer activity coefficients of ions from water to non-aqueous solvents [log yt (i,W— S)] ... 

Table 3.4 shows the pKa values of various acids in some popular organic solvents and in water [6]2. Solvent effects on pKa values are significant. For example, the pKa of acetic acid is 4.76 in water, 12.6 in DMSO, and 22.3 in AN. The solvent effects can be predicted by using the transfer activity coefficients for the species participating in the dissociation equilibrium (Section 2.3). The relation between the pKa values of acid HA in solvents R (water in this case) and S is given by Eq. (2.7) ... 

The hydrogen ion in protophobic aprotic solvents is very reactive. For example, judging from the values of transfer activity coefficient, H+ in AN is 10s times more reactive than in water. Thus, if basic substances are added to the solution in AN, they easily combine with H+. Table 3.6 shows the complex formation con-... 

Equation (96) now expresses the medium-dependence of the fractionation factor cj>LX (or K95). However, we note that the quotient of activity coefficients contains ratios of the form J/ha/2(da which really represent isotope effects on transfer activity coefficients. For this reason, the activity coefficient quotient in equation (96) is expected to vary less rapidly with the isotopic composition of the solvent than the factor Y. Furthermore as a practical step, the inclusion of the variation of fractionation factors due to the transfer effect is an unrealistic refinement at the present time. 

At equilibrium the ratio of concentrations of P in a mixed solvent and in water will be equal to the inverse of the transfer activity coefficient,... 

Besides the standard free energy of transfer AG , also the medium activity coefficient or transfer activity coefficient yf is used quite often to relate the ion activities aj and aj of an ion referred to the standard states in the two solvents ( and ") ... 

Also called the medium effect, solvent activity coefficient, or transfer activity coefficient, and also written as y (MX, O —> S). It is a constant characteristic of the solute MX (or the solute ions M and X ) and the two solvents O and S. 

SO that Ytr(S<—0) measures how strongly the solute is solvated in the reference solvent 0 compared with solvent S. The transfer activity coefficient for the transition state is accessible via Eq. (8-26) or by combining Eqs. (8-57) and (8-59). 

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