Oxygen atoms radical stability

In contrast to phenols (Section 5,3.4), phenothiazine (39) is reported to be an excellent scavenger of both carbon-centered and oxygen-centered radicals by hydrogen atom transfer and is also used to stabilize monomers in storage.198... 

The BDE of the a-C—H bonds of ethers R1R2C HOC HR1R2 are weaker in comparison with the C—H bonds of the parent hydrocarbons [2], When a-C—H of ether is cleaved, the formed a-alkoxyalkyl radical is stabilized by the interaction of an unpaired electron with p-electrons of the oxygen atom. Therefore, the attack of the peroxyl radical on the ether molecule occurs more rapidly than on hydrocarbon. Most of the ethers are easily oxidized by dioxygen at moderate temperatures. Table 7.11 contains a list of BDEs of ethers, as well as BDEs of hydrocarbons, and values of enthalpies of peroxyl radical abstraction with ether. 

M s 1). Consideration of reaction (6) suggests that these reactions fail, despite the stability of the 0-P< >2 an< SO3 products, because the primary product cobalt species is an unstable cobalt(III)-radical complex, Co-(0 ) This, in turn, suggests that oxygen atom transfer could succeed if accompanied by elec-... 

If the dibenzoyl ketyl ion is not included in the ion-pair complex with the alkali cation, it exists in the trans form. The potassium salt exists in the cis form, in which both charged oxygen atoms are in the vicinity of the metal cation (Bauld 1965). On benzoylation, the cis ion-radical pair gives rise mainly to cw-dibenzoyl stilbene. Meanwhile, the reduction of dibenzoyl under conditions, which do not stabilize the contact ion pair, is not stereospecific (Bauld 1965, Thiele 1899). Scheme 3.49 compares the results of these two different reactions of the same substrate. 

The second example depicted in Scheme 3.64 is the trioxotriphenylamine cation-radical. Kuratsu et al. (2005) compared structures of the cation-radical and its neutral counterpart. The neutral compound has a shallow bowl structure, whereas the cation-radical has a planar structure. In the latter, spin delocalization embraces a whole molecular contour, involving the oxygen atoms. This contribntes to the cation-radical stability. (The solid species is easily formed after oxidation of the neutral parent compound with tris(p-bromophenyl)aminiumyl hexafluorophosphate in methylene... 

Hydrogen abstraction from a position a to the oxygen of alcohols and ethers provides a simple route to a-oxyalkyl radicals. Resonance stabilization and polar factors have been used to explain the ease of radical attack on these substrates. Recent studies appear to exclude the possibility that the oxygen atom in position a to the free C-radical may cause stabilization by resonance. The ease of hydrogen abstraction Avould be determined only by polar factors, arising with electrophilic radicals (X ) in contributions from the polar forms 13-15 to the transition state. 

This stabilization may also be interpreted in terms of oxygen anions, which, due, to the vacancy, are initially double bonded to Mo. One electron is transferred to the catalyst in this reaction step. To form acrolein, a second hydrogen atom is transferred (to form water) and an oxygen atom is bonded to the allyl radical. In this (rather complex) process, another three electrons are transferred to the catalysts and doubtless distributed over several Mo ions. Reoxidation takes place at the bismuth cations, where oxygen molecules are attracted by the free electron pair. The intermediate result is a surface bismuth with an oxygen coordination similar to that in the bulk, viz. 

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