Benzenes , oxidation potentials

Addition of benzene to a solution of a Lewis acid (SbFs) in liquid SO2 at different temperatures produced immediately an orange colour which was attributed to the formation of a Ti-complex [140,141] between the aromatic nuclei and SbFs. The lowering of the benzene oxidation potential (0.8 V versus Ag/AgCl) instead of 1.9 V without SbFs was attributed to the easier... 

It seems more likely that the reduction of the benzene oxidation potential (about 1 V in superacid media [106], which in any case results from a strong interaction of benzene with the proton, is due to the formation of a 7r-complex, more easily oxidizable than the single monomer [106]. This hypothesis is supported by the fact that the same fall in potential was observed when benzene was polymerized in SO2... 

A.rrange the three compounds in order of increasing oxidation potential. Plot the experimental value of oxidation potential, 1,3, 1.5. and 2,0 V for naphthalene, biphenyl, and benzene, respectively, vs, [3 for the HOMO of each compound. 

The above strategy was tested [27] with a 3-layer LED consisting of a poly(2,5-thienylene vinylene) (PTV) layer, known to have particularly low oxidation potential [28], followed by a layer of l,4-fcrs-(4 -diphenylaminostyryl)-2,5-di-methoxy-benzene (DASMB) [29] and a layer of 2-(4-biphenyl)-5-(4-tcrt-butyl-pheenyl)-1,3,4-oxadiazol (PBD) dispersed in polystyrene (PS) in a 20 80 ratio. Films of poly-(2,5-thienylene-a-bromoethylcne) were obtained by vapor phase pyrolysis of 2,5-W.v-(bromomethyl)lhiophcne and subsequent vapor deposition of the quinoid monomers onto a cold substrate following a previously published procedure [30]. They were converted to PTV by temperature-induced elimination of HBr. 

Oxepin and its derivatives have attracted attention for several reasons. Oxepin is closely related to cycloheptatriene and its aza analog azepine and it is a potential antiaromatic system with 871-elcctrons. Oxepin can undergo valence isomerization to benzene oxide, and the isomeric benzene oxide is the first step in the metabolic oxidation of aromatic compounds by the enzyme monooxygenase. 

Hendrickson HP, Sahafayan M, Bell MA, et al. 1994. Relationship of flavonoid oxidation potential and effect on rat hepatic microsomal metabolism of benzene and phenol. J Pharm Biomed Anal 12 335-341. 

P. Pellack-Walker, J. K. Walker, H. H. Evans, J. L. Blumer, Relationship between the Oxidation Potential of Benzene Metabolites and Their Inhibitory Effect on DNA Synthesis in L5178YS Cells , Mol. Pharmacol. 1985, 28, 560 - 566. 

Anodic conversion of aromatics proceeds in most cases by le-transfer to the anode to form a radical cation (34) (Scheme 9). Oxidation is facilitated by extension of the 7T-system ( 1/2 vs. Ag/Ag+ benzene 2.08 V, pyrene 0.86 V) and by electron donating substituents ( 1/2 vs. Ag/Ag+p-phenylenediamine —0.15 V). Oxidation potentials of polycyclic aromatics and substituted benzenes are collected in Ref [140-142]. 

The pattern for outer-sphere oxidation by Co(NH3)5 compared with Co(en)j+ (usually it is —10 times slower) towards inorganic reductants can be used to support an estimate of the proportion of electron transfer (Marcus-dependent) and charge transfer which Ru(bpy) + displays towards these oxidants (45 and 11%, respectively). Sec. 2.2.1(b). Finally, Eqn. 5.35 can be used to determine K 2 for a reaction in which the other kinetic parameters are known. The value of A, 2 can be used, in turn, to estimate the oxidation potential of one couple, which is normally inaccessible. Thus the potentials of the o-, m- and /7-benzene diol radicals 1T2A4 were determined from kinetic data for the oxidation of the diols (H2A) by Fe(phen) + (5.45) ... 

The photoreaction of polysilanes with Ceo has also been investigated [35]. Reaction (8.16) shows an example in which the irradiation in benzene with a low-pressure mercury-arc lamp afforded a product that contains 14wt% of Ceo into the polysilane chain. The incorporation of Ceo into the polysilane backbone has not been observed upon irradiation with X > 300 nm, when the cleavage of Si—Si bond does not take place. The adduct obtained from Reaction (8.16) has a lower oxidation potential than C6o( + 0-77 vs + 1.21 V) and a lower reduction potential than polysilane (—1.24 vs — 2 V). 

However, because of the very similar ionisation potentials (and electrochemical oxidation potentials) of the precursors and products (e. g., benzene, 9.25 eV fluorobenzene, 9.21 eV p-difluorobenzene, 9.15 eV) selective mono-fluorination is often difficult to achieve. 

The course of hydrocarbon photocatalyzed oxidations seems to depend significantly on the relative positions of the valence band edge of the active photocatalyst and the oxidation potential of the substrate. For example, in contrast to the clean oxidation of toluene described above, lower activity was observed in neat benzene, a substrate whose oxidation potential lies at or slightly below the valence band edge This observation implies the importance of radical cation formation (via photoinduced electron transfer across the irradiated interface) as a preliminary step to hydrocarbon radical formation. If beitzene-saturated aqueous semiconductor suspensions are... 

FIG. 31. (a) Current voltage curve of the PbOi anode in electrolytes without and saturated with benzene and (b) potential dependence of current efficiency of benzene oxidation. 

Some nickel(II) tetraaza macrocycles have been proved to act as efficient catalysts for the electrochemical reduction of C02 in H20/MeCN medium. This indirect electroreduction occurs at potentials in the range -1.3 to -1.6 V vs. SCE and mainly produces either CO or a CO/H2 mixture, depending upon the type of complex.2854 The five-coordinate complexes [NiL] (394) formed by some deprotonated dioxopentamine macrocycles have been found to display very low oxidation potentials Nin/Nira in aqueous solution (about 0.24-0.25 V vs. SCE at 25 °C and 0.5 M Na2S04). Air oxidation of the same complexes in aqueous solution yields 1 1 NiL-02 adducts (5 = 1) which are better formulated as superoxo complexes, NimL-02 (Scheme 56). The activation of Ni-bound oxygen is such that it attacks benzene to give phenol.2855... 

Iron complexes with organic ligands are widely used as such reservoirs. For example, the stable 19-electron complex cyclopentadienyl benzene iron(I) is a clean electron reservoir that has a sufficiently negative oxidation potential and can be easily made, stored, and handled and can be weighed accurately (Alonso Astruc 2000). 

Similar results were obtained [139] with the three dimethoxybenzenes and acrylonitrile, methacrylonitrile, and crotonitrile. The amounts of substitution products decrease in the order acrylonitrile (49%) > methacrylonitrile (45%) > crotonitrile (6%), which agrees with the electron affinities of these compounds. Simultaneously, the amount of addition product increases acrylonitrile, 0% methacrylonitrile, 38% crotonitrile, 67%. In the series of anisole and the dimethoxybenzenes with crotonitrile, the amount of substitution products decrease in the order ortho- and para-dim ethoxy benzene > meta-dimethoxyben-zene > anisole, which is just the reverse of the order of their oxidation potentials. Ohashi et al. [139] have attempted to relate the photochemical behavior of these systems to the free enthalpy of electron transfer in the excited state as calculated with the Rehm-Weller equation, AG = E(D/D+) - E(A /A) - el/eR - AE00. 

TABLE 12.4 Voltammetric Oxidation Potentials and Peak Currents (pA/mM) of Alkoxy-Substituted Benzenes, Phenols, and Benzyl Alcohols, and Their Derivatives in MeCN (0.1 M TEAP) at a Glassy-Carbon Electrode... 

Table 12.4 summarizes the voltammetric oxidation potentials and peak currents for l,4-(MeO)2Ph and other alkoxy-substituted benzenes, phenols, and benzyl alcohols. Only the 1,4-(MeO)2PhX members of the series exhibit an initial irreversible anodic cyclic voltammogram via the sequence of Eq. (12.37). These plus the l,2-(MeO)2Ph isomer yield a metastable product from the second oxidation [species A, Eq. (12.37)] that undergoes a reversible reduction. Thus, the two-electron oxidation of dimethoxy benzenes yields the corresponding quinone. 

Figure 12.6 Voltammetric oxidation potentials ( p a) of substituted benzenes versus their substituent constants (apara, Ref. 16) slope, 2.43 V a. Conditions 1 mM substrate in MeCN [0.1 M (Et4N)C104] scan rate 0.1 V s 1 GCE (0.09 cm2) (Ref. 18).

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