Ionic Reactions—Reactivity

More recently, a fundamental re-evaluation is in progress. The problem is that it seems inherently unlikely that pairs of electrons would act in concert a pair of electrons in a single orbital spend as much time as far away from each other as possible—the electrons are described as correlated.—so why would a pair of electrons act together to move from one bond into another It seems, somehow, more reasonable for them to move one at a time. The transfer of one electron from one molecule to another is well known—it is the basis for one-electron oxidation and one-electron reduction, with many examples in electrochemistry, in sodium-in-ammonia reductions, and in inorganic redox reactions—but is it a common pathway in ionic organic chemistry, or something that only happens in favourable circumstances  

Molecular Orbitals and Organic Chemical Reactions Reference Edition Ian Fleming 2010 John Wiley Sons, Ltd 

There is good evidence that some nucleophilic substitution reactions do involve a single electron transfer, but the best established use a slightly different mechanism. These are the SrnI reactions, with the subscript RN standing for radical nucleophilic. Examples are the reaction of the nitronate anion 4.14 with p-nitrobenzyl chloride 4.15, 251 and the reaction of the pinacolone enolate 4.16 with bromobenzene.252 The former might have been a straightforward SN2 reaction, but actually takes the S l pathway because the nitro groups make the electron transfer exceptionally easy. The latter cannot take place by a conventional Sn2 reaction, because aryl (and vinyl) halides are not susceptible to direct displacement, and the S l pathway overcomes this difficulty. 

The discussion in the rest of this chapter will be phrased using the conventional two-electron mechanism—that mechanism has yet to be displaced from most discourse about most reactions. If an illuminating point can be made with the SET mechanism it will be. 

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