Radical ions heterolytic cleavage

The carbon-carbon single bond can be broken down into either two ions (heterolytic cleavage) or into two radicals (homolytic cleavage). A bond in a molecule exists because its formation is energetically favorable. Consequently, the assemblage from either pair of precursors should be a thermodynamically allowed process. The charged species shown in Scheme 2.13 contain a large... 

In certain cases, e.g. with Z = tert-butyl, the experimental findings may better be rationalized by an ion-pair mechanism rather than a radical-pair mechanism. A heterolytic cleavage of the N-R bond will lead to the ion-pair 4b, held together in a solvent cage ... 

Recombination fluorescence has been used to study the decay of radical ion pahs generated photolytically.288 Simulation of quantum beats caused by hyperfine interaction in the R and R+ enable the values of hfc to be determined for very short-lived species. In the case of one R excellent agreement with the value of hfc as determined by ESR is reported. The primary reaction in the photolysis of 1-arylalkyl radicals (128) is the heterolytic cleavage of the -halogen (X), generating the radical cation (129).289... 

Cation radicals of weak acids may react either by heterolytic cleavage (loss of a proton to produce a radical) or by homolytic cleavage (loss of a hydrogen to form a carboca-tion). Alkane cation radicals in liquid hydrocarbon solution undergo ion-molecule reactions, such as proton transfer on a submillisecond scale (Werst et al. 1990). 

A much simpler situation is the anion loss from an a-alkoxyalkyl radical with a leaving group in the P-position, such as the phosphate release from the 2-phosphato-methoxyethyl radical (Behrens et al. 1978 for some further reactions of P-(phosphatoxy)alkyl radicals see Whitted et al. 1999 Crich et al. 2000). This reaction only proceeds by a heterolytic cleavage into a radical cation and a phosphate ion. The rate of this reaction strongly depends on the protonation state of the phosphate group [reactions (40)-(44)]. 

The displaced electron is generally assumed to be the electron with the lowest ionization energy. In order of probability, this will be a nonbonding electron followed by a 7t bond electron and then a a bond electron. Thus El yields, in the first instance, a molecular ion which is a radical cation with an unpaired electron. In principle, any remaining energy will then be dissipated by bond cleavages that result in the formation of the most stable cation with a paired electron (even-electron ion). These even-electron ions may be formed by homolytic or heterolytic cleavages. This whole process happens very rapidly (<10-8s) and is the reason for the close similarity of El spectra produced across all different instruments. It is important to remember that mass spectral reactions in the El source are unimolecular. This is because the pressure in the El source is too low for bimolecular (ion-molecule) reactions to occur. 

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