Primary medium effect

The kinetic solvent-isotope effects on these reactions are made up of primary and secondary kinetic isotope effects as well as a medium effect, and for either scheme it is difficult to estimate the size of these individual contributions. This means that the value of the isotope effect does not provide evidence for a choice between the two schemes (Kresge, 1973). The effect of gradual changes in solvent from an aqueous medium to 80% (v/v) Me2SO—H20 on the rate coefficient for hydroxide ion catalysed proton removal from the monoanions of several dicarboxylic acids was interpreted in terms of Scheme 6 (Jensen et al., 1966) but an equally reasonable explanation is provided by Scheme 5. 

Some of the most important evidence for the two-step mechanism comes from studies of base catalysis, in this regard, reactions involving primary and secondary amines have played a central role1-5. The initially formed cx-adduct, 1, is zwitterionic and contains an acidic proton, which can be removed by a base which may be the nucleophile itself. Conversion of 1 to products can then occur via the uncatalysed k2 pathway or via the base-catalysed hl pathway. The influence of Brpnsted base catalysis, the experimental observation of 1,1- and 1,3-cr-adducts, the sensitivity of the system to medium effects, are some experimental evidence of the mechanism depicted in equation 1. 

Extensions of EET theories to account for coherence in the rate expression [28-30] and complicated donor-acceptor aggregates [23-27] have been described in the literature. To understand the effect of the medium on the EET rate we need to focus on the electronic coupling component of the theory [11]. A primary assumption of Forster theory, relevant to the medium effect, is that the electronic coupling can be approximated as a dipole-dipole interaction between transition dipole moments of the donor and acceptor molecules,... 

Solvent isotope effects are complicated by the fact that primary, secondary and medium effects are multiplied together and... 

The primary medium effect of an electrolyte can also be calculated from the standard potentials of a galvanic cell. The difference of the standard electromotive forces E and E " of the galvanic cells... 

Once the standard potential of Cell I has been determined precisely, calculations of the mean activity coefficient, y , of HC1 and the primary and secondary medium effects using well-known relations are relatively simple tasks. Using empirical equations of the type E = a - - bT + cT2 and E° = a0 + b0T + c0T2, it is possible to calculate the molal enthalpies and heat capacities. These types of calculations are demonstrated in many... 

From a thermodynamic point of view, the ionization constants of a weak acid in two solvents are related to each other by primary medium effect. Thus,... 

Step 2 is defined as the primary medium effect and is obviously by definition the same as the free energy of transfer, AG °, of the ion from the infinitely dilute solution in one solvent to the infinitely dilute solution in another solvent. 

The primary medium effect may be expressed now by subtracting Equation 23 from Equation 24. Thus,... 

Acidity scales are used commonly to assess the chemical and biological state of seawater. The international operational scale of pH fulfills the primary, requirement of repro ducibility and leads to useful equilbrium data. Nevertheless, these pH numbers do not have a well defined meaning in respect to all marine processes. Seawater of 35%o salinity behaves as a constant ionic medium, effectively stabilizing both the activity coefficients and the liquid junction potential. It may be possible, therefore, to determine hydrogen ion concentrations in seawater experimentally. One method is based on cells without a liquid junction and is used to establish standard values of hydrogen ion concentration (expressed as moles of H /kg of seawater) for reference buffer solutions. 

The primary medium effect or transfer activity coefficient m7 is the value of y in the new standard state (pm e seawater), referred to the aqueous standard state it is therefore identical with y . Values of log in seawater I and E° for cell A are summarized in Table IV. 

If we consider the studied solution to be infinitely diluted we can neglect the term connected with the activity coefficients of the charged particles in the solution and obtain finally that the Hammett function is dependent only on the proton activity and the primary medium effects for the conjugate indicator pair ... 

Now let us return to the approaches connected with the estimation of the primary medium effect for protons, log y0 n+, that are used for obtaining quantitative information on the acidity of pure protolytic or aprotic solvents relative to the standard solution of a strong acid in water. From the thermodynamics, these are known to be a measure of the Gibbs free energy of proton transfer from the standard solution in water to the one in a non-aqueous solvent (M). This parameter is connected with the energy of proton resolvation in the following way ... 

This expression means that the values of pAM and the H0 values obtained for the standard solutions should coincide in the case when the primary medium effects for acid and base are the same, i.e. log y0 BH+ - log 7o,b = 0. Practical investigations of proton acidity in different solvents gives the evidence that this is not usually the case [50]. 

Izmailov et al. developed methods for the estimation of the primary medium effect of protons [51], which permit determination of the relative acidic properties of different solvents and solutions. The essence of their proposition consisted in the division of the primary medium effect of a separate ion into two terms... 

Tremillon introduced the common oxoacidity function, ft, as a measure of the relative acidity (basicity) of solutions of various substances in molten media. This parameter is similar to the well-known protic acidity functions, H0 (Hammett function) and pA (Izmailov s acidity function) considered in Part 1. In terms of the primary medium effects, the function ft may be represented as the sum of the primary medium effect of O2- for the given medium (—logy0O2 ) and the instrumentally measured pO (pOl) in this medium ... 

It is obvious that the expression enclosed in the brackets by the author of the present book is nothing but the primary medium effect of O2- expressed via the difference in the values of the equilibrium constants of equation (1.3.6) for the media compared the molten equimolar KCl-NaCl mixture, which was chosen as a reference melt, and for which pKHa/H20 was found to be 14 at 700 °C, and the melt studied. As to the physical sense of the common acidity function Cl, this is equal to the pO of the solution in the molten equimolar KCl-NaCl mixture, whose acidic properties (oxide ion activity) are similar to those of the solution studied. Moreover, from equation (1.3.7) it follows that solutions in different melts possess the same acidic properties (f ) if they are in equilibrium with the atmosphere containing HC1 and H20 and Phc/Ph2o — constant. This explanation confirms that the f function is similar to the Hammett function. Therefore, Cl values measured for standard solutions of strong bases in molten salts allow the prediction of the equilibrium constants on the background of other ionic solvents from the known shift of the acidity scales or the f value for the standard solution of a strong Lux base in the solvent in question. According to the assumption made in Refs. [169, 170] this value may be obtained if we know the equilibrium constant of the acid-base reaction (1.3.6) in the solvent studied. 

If the f) values obtained in Refs. [169, 170] may be attributed to the primary medium effect of O2-, they allow us to consider the above-mentioned melts as extremely aggressive acidic media, analogous to anhydrous sulfuric acid in comparison with water (H0 -log y0ff+ = -11.1) [174], In particular, such melts should dissolve appreciable quantities of solid oxides, e.g. MgO or NiO, up to concentrations of the order of 1 mol kg-1. For example, MgO is practically insoluble in the equimolar KCl-NaCl mixture (pP = 9.33 0.06) [175], but should be characterized by values of the order of pP 1 (since 9.33 — 8 1) in the KCl-LiCl eutectic that corresponds to the value of the solubility product of the order of 10 1. For the KCl-NaCl-CaCl2 melt, the solubility-product value should be close to 1. On the contrary, most metal oxides which are slightly soluble in the KCl-NaCl melt were shown by... 

The considerations stated above show that the oxobasicity index of ionic solvents is the concentration analogue of the primary medium effect for H+ in molecular solvents [174], which allows us to compare the acidic properties of different protolytic and aprotic solvents with respect to H+ as an acidic particle. 

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