Electron transfer reactions concurrent

This reaction also produces Ph Ph and [Fe(CO)2(> -C5H5)]2, indicating the occurrence of a concurrent electron-transfer process . 

The distinction between addition and electron transfer chemistry is clear in cases for which the product alkene radical cation can be observed directly. For example, the reaction of the styrene radical cations with 2,5-dimethyl-2,4-hexadiene results in the concurrent formation of the diene radical cation (A. = 360 nm). Observation of an electron transfer reaction in this case is consistent with the low oxidation potential of 2,5-dimethyl-2,4-hexadiene = 1.3 V) relative to the oxidation potentials of substituted styrenes such as 4-methoxystyrene, 4-methylstyrene, and styrene ( = 1.5, 1.9, and 2.05 V, respectively, vs. SCE in acetonitrile). 

The recognition that corrosion occurs by a mechanism involving two concurrent electron transfer processes at the same surface has implications elsewhere in chemistry. Perhaps this is a mechanism which should be considered for other chemical reactions. 

As with any other physical methods, the CIDNP method is not universal and not immune to misinterpretation. It has certain drawbacks The polarization is weak and hardly detected in reactions involving extremely short-lived radicals and, if so, the polarization disappears quickly. It is often difficult to attribute the polarization to products of the main conversion, rather than the side or reverse conversions. The latter threat is most serious for the reactions with participation of ion-radicals—the formation of end products often proceeds concurrently with the restoration of the initial neutral molecules, due to a reverse electron transfer as in Scheme 4.29. 

Under oxygen in the absence of water, toluene will transfer an electron to the positive hole, concurrently with electron transfer from the conduction band to oxygen, to give a toluene radical cation. On the other hand, in the presence of water, both toluene and water will transfer an electron to the positive holes. The resulting toluene radical cation may subsequently lose a proton affording a benzyl radical, which will be oxidized with oxygen or the superoxide anion to benzyl alcohol and benzaldehyde, as proposed for the reactions of Fenton s reagent with toluene (57). 

Under a nitrogen atmosphere, cobalt carbonyl probably experiences disproportionation by base (Scheme 5) to give Co(CO)4. One of the disproportionation by-products is the Co(II) ion which gives a blue color in aqueous NaOH due to the presence of small amounts of the Co(OH)42-ion. The subsequent reaction of the cobalt tetracarbonyl anion with 14 is probably a displacement (path a), giving the cr-allyl complex 16. However, the possibility of an electron-transfer pathway (path b), either as an alternative to, or concurrent with, the displacement pathway cannot be dismissed at this time. 

The photocatalytic cycles shown in Figure 3 are based on oxidative (left) and reductive (right) quenching of electronically excited polypyridine complexes. Such cycles can operate in a homogeneous solution, provided that the excited state electron transfer is much faster than concurrent, unproductive decay to the ground state / et 1/to, where tq is the inherent excited state lifetime at given experimental conditions, but in the absence of the excited state chemical reaction 1 /tq =... 

In polar solvenis such as acetonitrile, electron transfer occurs followed by proton transfer from the radical cation to the radical anion with concurrent loss of COj. The radicals collapse to addition products such as 16 or 17. Alternatively, the radical pair may escape the solvent cage to give, after hydrogen abstraction from a suitable hydrogen source, the reduction product 18. In nonpolar solvents such as benzene, however, electron transfer is not possible only exciplex emission and no chemical reaction are observed (Libman, 1975). 

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