Reactions of ion pairs

Electrostatic attraction causes formation of ion pairs. The lower is the solvent relative permittivity, and the smaller are ion radii, the larger is the ion pair formation. The degree of ion association can be very significant even in solvents of high relative permittivity, eg., in aqueous solution. Bjerrum calculated that univalent ions, having a diameter of 0.282 nm, are about 14% associated in aqueous solutions if the diameter is 0.176 nm, the ion association is about 29%. It may be concluded that ion association is important even in aqueous solutions, and of very great importance in solvents of low permittivity. 

Let us consider the substitution of a ligand X by a ligand Y in a metal complex in which a ligand A is very strongly bonded, thus not undergoing substitution (en = ethylenediamine)  

Nils Janniksen Bjerrum, 1879-1958, professor at the University of Copenhagen, signifi cantly contributed to the theory of strong electrolytes and to solution coordination chemistry. 

The equilibrium is quickly established. The rate of product formation is given by  

From (1.13.1) it follows that the equilibrium constant for ion pair formation is [IP] 

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Dynamics for the reactions of ion pair intermediates of solvolysis, 39, 1 Dynamics of guest binding to supramolecular systems techniques and selected examples, 42, 167... 

This selection of reactions, by way of example, is not meant to imply that, for instance, a second-order termination reaction of ion-pairs, say... 

Interest within the physical organic community on the mechanism for the formation and reaction of ion-pair and ion-dipole intermediates of solvolysis peaked sometime in the 1970s and has declined in recent years. The concepts developed during the heyday of this work have stood the test of time, but these reactions have not been fuUy characterized, even for relatively simple systems. Richard and coworkers have prepared a short chapter that summarizes their recent determinations of absolute rate constants for the reactions of these weak association complexes in water. This work provides a quantitative basis for the formerly largely qualitative discussions of competing carbocation-nucleophile addition and rearrangement reactions of ion and dipole pairs. 

A Global scheme for solvolysis 2 Clocks for reactions of ion pairs 3 Addition of solvent to carbocation-anion pairs i Protonation of a carbocation-anion pair 11 Isomerization of ion pair reaction intermediates Reorganization of ion pairs in water 13 Internal return of isotopically labeled ion pairs Racemization of ion pairs 22 Concluding remarks 24 Acknowledgements 24 References 24... 

In retrospect, it should have been clear to me - as I am sure it was to Bill Jencks -that the rate and equilibrium constants for addition of solvent to 1-phenylethyl carbocation intermediates of solvolysis of 1-phenylethyl derivatives would serve as the first step in the characterization of the dynamics of the reactions of their ion pair intermediates. Therefore, this earlier work has served as a point of departure for our experiments to determine relative and absolute barriers to the reactions of ion pair intermediates of solvolysis. 

Contact- and solvent-separated ion pairs form whenever solvolysis proceeds to the free carbocation. However, these intermediates are generally only thought of as significant when their formation can be detected by experiment. We have focused on several different reactions of ion pairs that leave detectable signatures. 

We have focused on determining partition rate constant ratios for a variety of reactions of ion pairs, and of absolute rate constants from these ratios. This has been accomplished by use of one of the rate constants from this product ratio as a clock for the second reaction. 

Because the diffusive flux is enhanced by this drift of a charge under the influence of the coulomb potential [as represented in eqn. (142)], the partially reflecting boundary condition (127) has to be modified to balance the rate of reaction of encounter pairs with the rate of formation of encounter pairs [eqn. (46)]. However, the rate of reaction of ion-pairs at encounter is usually extremely fast and the Smoluchowski condition, eqn. (5), is adequate. The initial and outer boundary conditions are the same as before [eqns. (131) and (128), respectively], representing on ion-pair absent until it is formed at time t0 and a negligibly small probability of finding the ion-pair with a separation r - ... 

Just as in the example mentioned above, CT excitation produces a geminate ion pair ( solvent caged species ) within few ps, which, however, decays in fast chemical reactions (more about reactions of ion pairs in Sect. 3.4 of this article). 

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