Effect of ion-pair formation

Ion-pair formation needs its due recognition because it very often gives rise to unexpected extractions. In true sense, ion-pair may be regarded as a close association of an anion and cation, and therefore, it usually takes place either in a polar or a non-polar solvent. In reality, the ion-pairs are invariably formed by virtue of the union between comparatively large organic anions and (much smaller) cations. Interestingly, the resulting ion-pairs have been found to show their appreciable solubility in polar solvents and hence, these species may be extracted conveniently under such experimental parameters where neither individual component ion could. 

A few vital criteria towards the formation of an improved aqueous extractable ionic species are, namely  

A few typical examples shall be discussed here to explain the chelate-formation  

Example 1 Cu2+ with acetylacetonate forms a fairly stable ring compound  

Copper (II) -acetylacetonate complex (Resonating ring compound) 

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Hi) Effect of ion-pair formation, and (/v) Effect of synergistic extraction... 

Clearly, the effects of ion pair formation in matrix isolation spectra are apparent, particularly in the removal of degeneracy of vibrational modes. However, these can be fit quite well with normal coordinate calculations which show that ion pairing effects cause significant force constant changes. Moreover, it is apparent that the energetics of ion pair formation do provide a limit as to the species which can be studied in this fashion. However, for the species which can be stabilized in this manner, the technique is still effective and provides a model for both ion-molecule reactions and CsF catalysis. 

The effect of ion pair formation to the hard-sphere contributions can be evaluated with the help of thermodynamic perturbation theory [25] and is included into the MAL terms,... 

Analysis of the effect of ion pair formation on ESR parameters made by using McLachlan MO s combined with McClelland approximation. 

The same group published another contribution to this topic which describes the complexation of oppositely charged PEs, focusing on the effect of ion pair formation [98]. The free energy was derived from the law of mass action, where the Flory-Huggins parameter was influenced by the polymer fraction [98]. The PE cluster structure, as well as the precipitate properties were described to be salt dependent, which is also in agreement with other theories [16, 98]. 

Kudlay, A., Ermoshkin, A.V., Olvera de la Cruz, M. Complexation of oppositely charged polyelectrolytes effect of ion pair formation. Macromolecules 37, 9231-9241 (2004). doi 10.1021/ma048519t... 

Pseudo-first-order rate constants (k bs) for the reaction of anionic fV-hydrox-yphthaUmide (NHP) with HO increased by > 3-fold and 15-fold in the presence of inert monocations (LP, Na+, K+, and Cs+) and dication (Ba ), respectively, in aqueous solvent containing 2% v/v acetonitrile." Catalytic effects of these cations increased with the increase in the contents of acetonitrile in mixed aqueous solvents." The presence of anions such as CE and C03 did not show a kinetically detectable effect on for the alkaline hydrolysis of NHP-. The catalytic effects have been explained quantitatively in terms of an ion-pair mechanism in which cations produced a predominantly stabilizing effect on TS rather than on GS. The overall catalytic effect of inert cations is apparently the combined effect of ion-pair formation between cation and anionic reactants, which causes the increase in electrophilicity of carbonyl carbon of NHP for nucleophilic attack and decrease in the nucleophilicity of nucleophile (HO ). 

The equation does not take into account such pertubation factors as steric effects, solvent effects, and ion-pair formation. These factors, however, may be neglected when experiments are carried out in the same solvent at the same temperature and concentration for an homogeneous set of substrates. So, for a given ambident nucleophile the rate ratio kj/kj will depend on A and B, which vary with (a) the attacked electrophilic center, (b) the solvent, and (c) the counterpart cationic species of the anion. The important point in this kind of study is to change only one parameter at a time. This simple rule has not always been followed, and little systematic work has been done in this field (12) stiH widely open after the discovery of the role played by single electron transfer mechanism in ambident reactivity (1689). 

A brief discussion of the systematics of solvent effects on the p/, pr, and values of Tables II and III is presented in the discussion section. However, it is worthy of note here that sets 7, 37, 38, 39, 40, and 41, which involve nonhydroxylic solvents, are fitted with comparable precision to that for reaction series in aqueous or mixed aqueous organic solvents. The present analysis does not support the previous assignment (7b) of ion-pair formation of benzoic acids... 

The extent to which steric effects adversely affect the attainment of such intimate ion-pair structures would be reflected in an increase in the work term and concomitant diminution of the inner-sphere rate. This qualitative conclusion accords with the reactivity trend in Figure 16. However, Marcus theory does not provide a quantitative basis for evaluating the variation in the work term of such ion pairs. To obtain the latter we now turn to the Mulliken theory of charge transfer in which the energetics of ion-pair formation evolve directly, and provide quantitative informa-... 

If ion pairs but not free ions are formed, the extent of ion pair formation may be estimated from the dielectric properties of the solution. This method has been used in studying the effect of Lewis acids on alkyl halides.164... 

Anion-based organic dye nanoparticles can be also synthesized on the basis of the ion-association method. In this case, hydrophobic phosphazenium cations such as tetrakis[tris(dimethylamino) phosphoranylideneamino]phosphonium cation are effective for ion-pair formation with anionic dyes. A neutral polymer stabilizer polyvinylpyrrolidone (PVP) that is soluble in water is sometimes added for preventing agglomeration... 

The constants of ion pair formation of 33 amines with 2,4-dinitrophenol in benzene (Ab) have been compared with the pAa in water56. The effects of structural variations on basicity are larger in water than in benzene for primary and secondary cyclic amines, but similar for tertiary amines. The Taft and Hancock equation [where <7 has the usual meaning and E° (R ) is the steric effect of a component substituent]... 

Simple methods can, and need to, still be employed, however. The effects of added ions on the rates of complex ion reactions, discussed by Wahl at this meeting, need to be further explored. In the substitution area, the observations of ion pair formation concommitant with the substitution reactions of... 

Energetic considerations based on the separation of solvated ions at the encounter distance a show that solvated ion-pair formation from 1M is sufficiently exothermic in polar solvents to effectively prevent the production of excited singlet states 1M by the reverse process. Table XVIII lists values for free energies AGIM of ion-pair formation in acetonitrile estimated24 from the oxidation and reduction potentials, D/D+ and EA-tA, of donor and acceptor using the relationship... 

Note that the mechanism of ion-pair formation by y particles is not the same as that by a or 0 particles. A y ray itself will not leave behind it a trail of ion pairs but rather collides with free or bound electrons imparting to them all or part of its electromagnetic energy (photoelectric and Compton effects). These secondary electrons are then responsible for formation of ion pairs in matter through which y radiation passes. In cases where the energy of radiation exceeds 1.02 Mev, another process is possible, the creation of electron->positron pairs. 

An understanding of the concentration dependence of activity coefficients required the postulation of the concepts of ion-pair formation and complex formation. Certain structural questions, however, could not be answered unequivocally by these considerations alone. For instance, it was not possible to decide whether pure Cou-lombic or chemical forces were involved in the process of ion association, i.e., whether the associated entities were ion pairs or complexes. The approach has been to postulate one of these types of association, then to work out the effect of such an association on the value of the activity coefficient, and finally to compare the observed and calculated values. Proceeding on this basis, it is inevitable that the postulate will always stand in need of confirmation because the path from postulate to fact is indirect. 

The treatment of partition equilibrium was further generalized to the cases in the presence of ion-pair formation [19] and ion-ionophore complex formation [21]. An important corollary of this theory of partition equilibrium based on standard ion transfer potentials of single ions is to give a new interpretation to liquid extraction processes. Kakutani et al. analyzed the extraction of anions with tris(l,10-phenan-throline) iron(II) cation from the aqueous phase to nitrobenzene [22], which demonstrated the effectiveness of the theory and gave a theoretical backbone for ion-pair extraction from an electrochemical point of view. 

Effect (1) is the main topic of this review effect (2) will not be discussed. Effect (3) can be extremely important in certain cobalt(III) complexes, examples being the effect of the counterion on bond lengths and ir spectra in the solid. state and of ion-pair formation on the kinetics of ligand substitution. These will be discussed, where relevant, together with the internal effects. 

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