Reactive intermediate generation radical ions

Many anodic and cathodic conversions are not mediated by soluble redox couples or by redox coatings but are essentially initiated by a one-electron charge transfer between the electrode and the organic molecule-generating radical ions or radicals as first, reactive intermediates ... 

We will discuss briefly the reactive species such as an exciplex and radical ion species generated by the excitation of organic molecules in the electron-donor (D)-acceptor (A) system. An exciplex is produced usually in nonpolar solvents by an interaction of an electronically excited molecule D (or A ) with a ground-state molecule A (or D). It is often postulated as an important intermediate in the photocycloaddition between D and A. In the case of D = A, an excimer is formed as an excited reactive species to cause photodimerization. In some cases, a ter-molecular interaction of an exciplex with another D or A generates a triplex, which is also a reactive intermediate for photocycloaddition. The evidence for the formation of excimers, exciplexes, and triplexes are shown in the fluorescence quenching. Excimer and exciplex emission is, in some cases, observed and an emission of triplex rarely appears. 

Radical ions are, in the main, not very important as active centres of polymerizations. In media suitable for the existence both of radicals and of ions, the latter are usually more reactive. Moreover, the radicals decay by combination their contribution to chain propagation is usually negligible. Radical ions are more important as precursors of active centres, as intermediates generated from initiators and monomers through their radical ends they can combine (disproportionate) yielding active centres, frequently diions. Studies of radical ion behaviour contribute to our knowledge of the processes connected with electron transfer from molecule to molecule. These oxidation-reduction processes are very important in macromolecular chemistry. 

It is hoped that our guide for predicting the reactivity of radical ions, whether generated by electrolysis, or by chemical or photochemical ET processes, will encourage scientists to devise novel radical-ion reactions for synthetic applications. Because our analysis has aimed at covering synthetically relevant radical-ion transformations, it should be noted that less frequently used reactions, such as cis trans isomerizations, and ET oxidation or reduction of radical ions are not included. One should, moreover, bear in mind that the reactivity of radical ionic intermediates might be heavily influenced by counterion effects [388], a research area which still deserves major attention. 

Electrochemistry is the combination of a heterogeneous electron transfer at an electrode with a chemical reaction. The electron transfer leads to the reactive intermediates carbo-cations [1], carbanions [2], radicals [3], and radical ions [4]. The differences in comparison to chemical reactions are due to the way in which these reactive intermediates are generated. In electrochemistry, the electrode transfers the electron in chemical reactions, chemical reductants or oxidants are used. Furthermore, carbocations, radicals, and carbanions are chemically generated by dissociation, homolysis, or deprotonation. The reaction conditions, the solvents, and the reactants encountered by the electrochemically or chemically formed intermediates can differ substantially and thus can lead to diverse reaction pathways and products. 

Additionally, there are features of organic electrochemistry that cannot be compared with organic chemistry because they are unique to this field and have no counterpart elsewhere in common chemistry. They result from the combination of electron transfer with a chemical reaction. This way reactive intermediates can be generated, which in another way are not or only in a limited extent accessible, such as radical ions. Mechanisms can be studied in otherwise not accessible depth from time-resolved electrical signals of starting materials, products, or intermediates (see Chapter 2) [6]. 

By electron transfer radical ions, anions, cations, and radicals can be generated as reactive intermediates. Radical ions are mostly products of outer sphere electron transfer [Eq. (1)] ... 

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