Cation-anion recombination

This is the reverse of the first step in the SnI mechanism. As written here, this reaction is called cation-anion recombination, or an electrophile-nucleophile reaction. This type of reaction lacks the symmetry of a group transfer reaction, and we should therefore not expect Marcus theory to be applicable, as Ritchie et al. have emphasized. Nevertheless, the electrophile-nucleophile reaction possesses the simplifying feature that bond formation occurs in the absence of bond cleavage. 

Table 7-16. Nucleophile Parameters for Cation-Anion Recombinations ...

Turning to cation-anion recombination reactions we find that most of the quantitative studies have been by Ritchie,who defined a nucleophilic constant by Eq. (7-71),... 

Rates and equilibria within these cation-anion recombination reactions are not correlated. Ritchie considers that extensive desolvation of the reactant ions... 

It is probably inappropriate that the RSP has been called a principle, which implies a statement of wide generality, because many examples of its failure are known. For example, Ritchies cation-anion recombination reactions follow Eq. (7-71), so they are LFER with the same slope this is an instance of constant selectivity. Anti-RSP behavior is also known. As a consequence, the validity of the RSP is currently a controversial matter. There are several aspects of this problem. 

With the advent of picosecond-pulse radiolysis and laser technologies, it has been possible to study geminate-ion recombination (Jonah et al, 1979 Sauer and Jonah, 1980 Tagawa et al 1982a, b) and subsequently electron-ion recombination (Katsumura et al, 1982 Tagawa et al, 1983 Jonah, 1983) in hydrocarbon liquids. Using cyclohexane solutions of 9,10-diphenylanthracene (DPA) and p-terphenyl (PT), Jonah et al. (1979) observed light emission from the first excited state of the solutes, interpreted in terms of solute cation-anion recombination. In the early work of Sauer and Jonah (1980), the kinetics of solute excited state formation was studied in cyclohexane solutions of DPA and PT, and some inconsistency with respect to the solution of the diffusion equation was noted.1... 

S. S. Shaik, J. Org. Chem. 52, 1563 (1987). Nucleophilicity and Vertical Ionization Potentials in Cation-Anion Recombinations. 

FIGURE 6.Ans.3 Qualitative dissociation energy curves for the R—X bond in a polar solvent, along with the reactivity parameters of the VBSCD for the cation—anion recombination process. 

An interesting observation is that in the presence of a large excess of Br, the reaction of AcO- with 15, Ar = An, is of a second order (71). This result is a rare example of an SN2(C+) route where the cation-anion recombination is rate-determining in a solvolysis reaction, and in principle, such a process could be used for obtaining directly rate constants for capture of vinyl cations. 

Several chemical reactions verify the RSP, the more reactive species tend to be less selective in their reactivity. Nevertheless, we have shown [10] that this is valid when the reactivity is controlled by changes in AG otherwise exceptions occur and more reactive species can also be more selective than the less reactive ones. Table 8 illustrates the effect of n on the selectivity of radicals towards CH bonds. None the less, a is always a good parameter to assess selectivity, for reactions which obey RSP and even for reactions which have a behaviour opposed to such a principle. But literature reports also reaction families where no free-energy relations are observed as, for example, in cation-anion recombinations [57]... 

Structure-reactivity relationships in chemistry have been dominated by LFER. However, the kinetics of several cation-anion recombinations,... 

A rational for such a relationship can be provided by ISM in terms of a curve-crossing model. One would wish to find the conditions for which the effect of AG° on rate constants is minimised and that of m or n is maximised. Table 7.3 studies such conditions for harmonic oscillators and reveals that they are verified when /pUnd correlated changes are occuring in the bond lengths. This is precisely the situation with cation-anion recombinations, where the force constants of the cation and anion bonds with the solvent molecules are much weaker and have lower force constants than the normal chemical X R bond. The nucleophile parameter is directly proportional to the electronic transition-state bond order, n (see Figure 7.12). 

Figure 7.11 Correlation of cation-anion recombinations by eq. (7.32). Circles Malachite Green squares p-tolyldiazonium. The reaction of Malachite Green with water in pure water was taken as the reference and the scale is defined as the difference between the logarithms of the rate constants of the other Malachite Green reactions and this reference reaction.

Figure 7.12 Correlation between Ritchie parameter for nucleophiles and the transition state bond order in cation-anion recombinations. These recombinations were studied in water (circles),...

---