The Concept of Ion Pairs

The ionization and direct displacement mechanisms can be viewed as the extremes of a mechanistic continuum. At the 8 1 extreme, there is no covalent interaction between the reactant and the nucleophile in the transition state for cleavage of the bond to the leaving group. At the 8 2 extreme, the bond formation to the nucleophile is concerted with the bondbreaking step. In between these two limiting cases lies the borderline area, in which the degree of covalent interaction between the nucleophile and the reactant is intermediate between the two limiting cases. The concept of ion pairs is important in the consideration of... 

The concept of ion pairs in nucleophilic substitution is now generally accepted. Presumably, the barriers separating the intimate, solvent-separated, and dissociated ion pairs are quite small. The potential energy diagram in Fig. 5.4 depicts the three ion-pair species as being roughly equivalent in energy and separated by small barriers. 

Both of these expressions are catered for in the thermodynamic approach to the concept of ion pairs outlined by Denison and Ramsey (44). Their treatment assumes at the outset that the two oppositely charged ions of an electrolyte (1 1) can only be in physical contact (tight or contact ion pair) or infinitely for apart In other words, the critical approach distance is synonomous with the sum of the ionic radii, and the association energy is numerically far in excess of thermal energy k T. Hence the change in the electrostatic free energy of the system resulting from the dissociation... 

Since Bjerrum s [7] introduction of the concept of ion pairing in 1926, a variety of analytical methods has been employed to study the structure and energetics of ion pairs. Szwarc s Book deals with the development of the ideas up to 1972 [3a]. It is also a guiding reference for the different spectroscopic methods that have been employed in the examination of ion pairs. 

The concept of ion pairing refers to the ionic association between ions added to the mobile phase and oppositely charged ionized groups of the sample molecule, i.e.. 

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. 

Triple Ions.—The concept of ion-pairs has been extended to include the possibility of the presence in solution of groups of three ions, viz.,... 

Ion-Association and lon-Hvdration. Aqueous solutions of electrolytes have been chemically described using a variety of theories. The original theoretical approach used by geochemists to model aqueous systems was based on the concept of ion-pairing or ion-association. The ion association approach as described by Carrels and Thompson (1) accurately depicted the speciation of seawater and later many other aqueous solutions. This approach was subsequently found to be inadequate for defining the chemistry of more complex and more concentrated aqueous solutions or those solutions near the critical point of water. This deficiency led to the use of other theoretical approaches to describe these systems, such as the ion-interaction, mean salt, and ion-hydration theories. 

Bob published a book Mineral Equilibria in 1960 (later expanded to Solutions, Minerals, and Equilibria in 1965 and coauthored with Charles Christ), which brought about a quantum jump in our understanding and approach to the physical chemistiy of natural waters. At about the same time, he conducted a series of outstanding studies with Maty Thompson and Ray Siever at Harvard Universitj especially noteworthy is the work on a chemical model for seawater. In this study, Garrels and Thompson laid the foundation for the calculation of activities in seawater, based primarily on the concept of ion pairing. (This paper has been so influential that it has been designated a classic by Science Citation Index.)... 

One of the most important lessons taken from this body of work has been the excellence of carbohydrates as a teaching ground for many of the fundamental concepts of modem organic chemistry, including the concepts of ion pairs, of transient intermediates, of neighboring group participation and of its sister anchi-meric assistance, of torsional strain and interactions, of so-called stereoelectronic effects, and of double diastereoselection. 

More recently, a number of new ideas for asymmetric induction in cyclopropanation have been reported. The concept of ion pairing of a chiral anion 13 with a copper cation was implemented in the cyclopropanation of styrene and EDA with moderate enantioselectivity as a result (73). Supramolecular-catalyzed cyclopropanation with a double helix based on ligand 14 (R = raBu) and copper(I) ions gave high enantioselectivity (74). 

From their study of the reactions of organic halides with organosodium complexes in dioxane solution, Morantz and Warhurst (97) concluded that the measurements could be reconciled only with the concept of ion pairing. They concluded that the solutions contained exclusively the monomeric form of the radical anion paired with the alkali metal cation. The influence of the alkali ion on the electronic spectra was investigated by Carter et al. (100). They measured the absorption spectra in the region 10,000-25,000 cm of a number of alkali metal complexes in dioxane solution and found that for a given radical anion the frequency of the maximum of an absorption peak shifted towards the red as the radius of the alkali metal cation increased. A relation was found between this frequency and the radius of the cation... 

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