Reaction 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. 

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. 

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]... 

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.

Employing the lactone 279, the trimethylsilyl cation generates a positive charge at the anomeric carbon position of 280. The corresponding triflate anion recombines with one of the silyl ethers in 281 and yields the alk oxide 282 and a molecule of catalyst. After nucleophilic attack, 283 is formed, and by a subsequent intramolecular reaction, the orthoester 284 is obtained simultaneously releasing a molecule of hexamethyldisil-oxane. 

Following stages of the transformation are dissociation of the radical-ions and recombination of reaction product (anion from radical-anion, or proton from radical-cation) with surface sites. These reactions may be first steps in catalytic transformations of organic compounds with participation of the oxygen atom of oxides lattice. 

If the electron affinity of the additive is much larger than that of the polymer or of the physical traps, a protective effect can be observed, since the energy released by the recombination may be insufficient to induce bond scission in the polymer. Of course, any abnormal link in the polymer can play the same role as a low molecular weight additive. Ionisation potential and electron affinity are the only determining parameters. Reactions of anions and cations of additives with the matrix may also... 

In cycloaddition reactions of tetra-t-butylcyclobutadiene with either dicyanoacetylene or tetracyanoethylene it is suggested that the initial step may be oxidation of the cyclobutadiene by the cyano compound to a radical cation which recombines with the radical anion of the cyano compound to provide the adduct [53],... 

The speed of reaetion of cationic Au clusters with neutral and anionic electron-pair donor bases is amazingly fast. Such reactions occur via complicated fragmentation and recombination reactions and new Au clusters are formed within minutes. Four types of reaction can be discerned (as follows), and examples of each can be found in the scheme. 

The simplest systems where electron-transfer chemiluminescence occurs on interaction of radical ions are radical-anion and radical-cation recombination reactions in which the radical ions are produced from the same aromatic hydrocarbon (see D, p. 128) by electrolysis this type of chemiluminescence is also called electro-chemiluminescence. The systems consisting of e.g. a radical anion of an aromatic hydrocarbon and some other electron acceptor such as Wurster s red are more complicated. Recent investigations have concentrated mainly on the energetic requirements for light production and on the primary excited species. 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

The efficiency and specificity of this method depends on the irreversibility of the whole process due to a high rate constant and favorable thermodynamics of Reaction (10) [4] and a high rate of subsequent Reaction (11) (which is the recombination of a free radical anion and a free radical cation with the diffusion rate constant of about 109 1 mol-1 s ). 

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