Phenoxyl radicals dissociation

The phenoxyl radical has an increased electron density in the ortho- and pura-positions and adds dioxygen similar to alkyl radicals. However, the C—00 bond is weak in this peroxyl radical and back dissociation occurs rapidly. Therefore, the formation of quinolide peroxide occurs in two steps, which was studied for the 2,4,6-tris(l,l-dimethylethyl)phenoxyl radical [100,101],... 

In an attempt to establish unequivocally the spectroscopic features of coordinated (vs uncoordinated) phenoxyl radicals a series of phenolato precursor complexes containing a spectroscopically and redox-innocent Ga(III), Sc(III), or Zn(II) central metal ion were synthesized (142-148). In order to avoid metal-ligand bond dissociation in solution, the phenolate or, after one-electron oxidation, phenoxyl moieties were covalently attached to the strongly metal ion binding 1,4,7-triazacy-clononane (149) backbone. Thus a series of phenolate pendent-arm macrocyclic... 

In the proposed mechanism, Craq002+ attacks the phenyl ring of ArO + and produces a peroxochromium intermediate I. This step is in agreement with literature precedents (133-137) and with the recent discussion of delocalization of electron density in phenoxyl radicals (138). The peroxochromium intermediate was not observed, implying that the dissociation of CraqOH2+ in the next step is fast. Finally, the loss of teri-butanol was written in analogy with the known reactions in organic solvents (139,140). 

Many aliphatic carbonyl compounds show the same dissociation reaction, even acetone (Figure 4.33b). When the carbonyl group is separated from the benzene ring by a suitable substituent such as O, the dissociation takes a different course, as shown in Figure 4.33(c). The resonance stabilization of the phenoxyl radical is much greater than that of the aliphatic radical R, so that splitting of the O-CO bond is favoured. 

To be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

Fig. 21. Proposed catalytic mechanism for substrate oxidation by galactose oxidase. (A) Substrate binding displaces Tyr-495 phenolate which serves as a general base for abstracting the hydroxylic proton. (B) Stererospecihc pro- hydrogen abstraction by the Tyr-Cys phenoxyl radical. (C) Inner sphere electron transfer reducing Cu(II) to Cu(I). (D) Dissociation of the aldehyde product.

A very useful method in transforming phenols, even unstable ones, into phenoxyl radicals is flash photolysis 176,177>. When 2,4,6-triphenylphenol (la) in benzene was photolysed the radical 3a as well as its dimer 4a were observed. The K-value 3a 4a, determined by flash photolysis, agrees well with the K-value of the colorimetric determination, mentioned below. Flash photolysis can also be used to evaluate the rates of dimerization or dissociation of 3a -> 4a (k being in the order of 0.3 sec mole-1), as well as determining the activation parameters 177). 

Reaction of eaq , produced by pulse radiolysis, with bromophenols in alkaline solutions exhibited completely different pathways . When the hydroxyl group of the hydroxyphenyl radical is dissociated, the negative charge is partly delocalized from to the site of the radical on the aromatic ring and this site then undergoes very rapid protonation by water to form a phenoxyl radical (equation 8). 

TABLE 12. Reduction potentials of phenoxyl radicals and O—H bond dissociation energies of phenols... 

The reduction potential changes with pH if either the radical or the molecule undergoes protonation or deprotonation upon pH change. For example, for dihydroxy compounds, where the two OH groups have dissociation constants K and Ki, and the phenoxyl radical has a dissociation constant for the second OH group, the potential at any pH, E, is related to the potential at pH 0, Eq, according to equation 38. 

Acetylsugars 674 Acid-base dissociation 992 Acid-base equihbria, of phenoxyl radicals... 

Oxidation of the [Pd(tbu-iepp-cX l] and [Pd(p-iepp-c)Cl] complexes shown in Fig. 20 with 1 equiv of Ce(TV) afforded the corresponding one-electron oxidized species (76). (Htb-iepp = 3-[iV-2-pyridylmethyl-iV-2-hydroxy-3,5-di(-tcrt-butyl)benzylamina] ethylindole H p-iepp = 3-(iV-2-pyridylmethyl-iV-4-hy-droxybenzylamine] ethylindole H denotes a dissociable proton and tbu-iepp-c p-iepp-c and tbu-iepp-c denote p-iepp and tbu-iepp bound to Pd(ll) through a carbon atom respectively (76). The EPR spectroscopy (one sharp signal at g = 2.004) and absorption spectroscopy (peak at 550 nm) are consistent with an indole n radical rather than a Pd(lll) or a phenoxyl radical. 

In Table 7.2 the correlation equations are presented for the steps (11)-(13), (16) and (17) that describe the rate constant dependency for reactions, with the participation of para-substituted phenol and with the corresponding phenoxyl radical on the dissociation energy of OH-bond in ora-substituted phenol. 

Table 7.2. Dependence of logA (IVT s ) for steps with/jara-substituted phenols and appropriate phenoxyl radicals vs. dissociation energy of 0-H bond of phenols,...

Numerical determination of molecular structure of the efficient inhibitor based on the reaction kinetic model The molecular stmcture of /> ra-substituted phenol having a maximum inhibiting ability in the ethylbenzene oxidation was determined by the method described above. The dissociation energy of the 0-H bond in the phenols (Don) served as the characteristic reactivity indices for the molecular stmcture of the inhibitor. By this value it has been possible to describe the reactivity of appropriate phenoxyl radicals (see Table 7.2)... 

As an example a model of die liquid-phase oxidation of the ethylbenzene in the presence of inhibitors, the iora-substituted phenols and the butylated hydroxytoluene, was selected. The identified dynamics of die value contribution of steps in the reaction mechanism is complicated. The dominant steps for die different time intervals of the inhibited reaction were determined. The inhibition mechanism of die ethylbenzene oxidation by sterically unhindered phenols is conditioned by establishing equilibrium (7.24) in the reaction of the chain carrier, the peroxyl radical, with the inhibitor s molecule (within sufficiently wide interval of the inhibitor s initial concentration), followed by the reaction radical s quadratic termination with the participation of the phenoxyl radical. The value analysis has established that the efficient inhibitor with low dissociation energy of the phenolic 0-H bond promotes shifting the mentioned equilibrium from the chain carrier to the direction of the phenoxyl radical formation. 

Conventional free radical chain oxidation (referred to as autoxidation) involves chain initiation, propagation, and termination steps [15-17]. Thepossibility of hydrogen atom abstraction by radical species in arenes without alkyl side chains is small because of the strong sp C—H bonds (the dissociation energies of the sp Ar—H bonds and sp ArCR —H are 112 and ca. 90kcal/mol, respectively [18]). Phenols readily react by direct abstraction of H from the hydroxyl gronp to form phenoxyl radical ArO, and this pathway is implicated in their antioxidant properties [16, 19]. Numerous literature addresses structure-reactivity relationships in the chemistry of phenols and phenoxyl radicals (see Refs. 1, 16, 19 and references therein). 

Alkylphenols can be converted to p-BQ using dioxygen and HPA- ( =2-6) [121]. Oxidation of TMP in AcOH-HjO (95/5) produced TMBQ with a selectivity of 86% at complete TMP conversion [121a]. The main reaction by-product was 2,2, 3,3, 5,5 -hexamethyl-4,4 -biphenol. Based on kinetic and spectroscopic studies, a stepwise mechanism that involves dissociation of HPA-w in the acidic medium generating the active species VOj, oxidation of TMP by VOj to produce phenoxyl radical and VCP+ followed by reoxidation of V(fV) to V( V) with in the coordination... 

Phenols are known to quench DBO with higher rate constants (ca. 10 M s" ), which can be traced back to their low OH bond dissociation energies [183], resulting from the resonance stabilization of phenoxyl radicals [174]. The observation of substantial deuterium isotope effects for the fluorescence quenching of DBO by phenols corroborates, however, that hydrogen transfer operates for phenols as well (cf. Table 3.5). 

By using Ziegler s colorimetric method 178) to determine the dissociation constant of 2-triphenylmethyl and its dimer, the dissociation constant of the 2,4,6-triphenyl-phenoxyl dimer under various conditions has been measured The values presented in Table 9 were found by evaluating the extinction coefficients of the maximum at 530 nm of the radical depending on the concentrations 133 ... 

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