Phenols electron-donor-acceptor complex

A reasonable model has been proposed to accommodate these results (2/y 23). The presence of quinoid functions in lignin would give rise to electron donor-acceptor complexes with existing phenolic groups. These complexes, like quinhydrone, would form stable radical anions (semiquinone anions) on basification, according to the scheme shown below. Both biological and chemical oxidation would create more quinone moieties, which in turn would increase the contribution of Reactions 1 and 2. Alternately, enzymatic (< ) and/or alkaline demethylation 16) would produce... 

Three main mechanisms have been proposed to explain this behavior the TT-TT dispersion interaction mechanism, the H-bonding formation mechanism, and the electron donor—acceptor complex mechanism. The first two mechanisms were proposed by Coughlin and Ezra [16] in 1968, and the third mechanism was proposed by Mattson and coworkers [20] in 1969. At that time, phenol was known to be adsorbed in a flat position on the graphene layers, and in this situation the adsorption driving forces would be due to tt-tt dispersion interactions between the aromatic ring of phenol and the aromatic structure of the graphene layers. 

Abuzaid and Nakhla, and Vidic et al. found that the adsorption of phenol by activated carbons from aqueous solutions in the presence of molecular oxygen in the test environment resulted in a threefold increase in the adsorption capacity of the carbon. This has been attributed to the oxygen induced polymerization reactions on the surface of the carbon. Juang et al. studied liquid-phase adsorption of eight phenohc compounds on a PAN-based activated carbon Fiber in the concentration range of 40 to 500 mg/L and observed that the chlorinated phenols showed better adsorption than methyl substituted phenols. Moreno-CastiUa et al. studied the adsorption of several phenols from aqueous solutions on activated carbons prepared from original and deminerahzed bituminous coal and found that the adsorption capacity depended upon the surface area and the porosity of the carbon, the solubility of the phenolic compound, and the hydrophobicity of the substituent. The adsorption was attributed to the electron donor-acceptor complexes formed between the basic sites on the surface of the carbon and the aromatic ring of the phenol. 

In addition to the supporting self-citations [742,6,457,330], there was much early support for the formation of donor-acceptor complexes. Radke and Praus-nitz [743] interpreted the extensive loadings of phenols even at very low concentrations, in comparison with lower uptakes of several aliphatic adsorbates, as evidence for specific interaction with the activated carbon surface. Barton and Harrison [744] studied the effect of graphite outgassing temperature on the heat of immersion of benzene and attributed a shallow minimum at ca. 800°C to the effect of CO desorption, thus implicitly supporting the donor-acceptor complex proposal in terms of a reduction in the interaction between the partial charge on the carbonyl carbon atom and the 7t-electron cloud of the benzene molecule. ... 

Quinoid compounds are excellent acceptors of electrons and form electron donor-acceptor (EDA) complexes as a consequence of low-lying unoccupied electronic energy levels205. The EDA complexes may be easily formed in interactions with phenolic or amine components of a stabilizing mixture, with other additives which have reactive H atoms, with RO 2 radicals, or with some metallic impurities in polymers via rr-orbital interactions. Quinones efficiently participate in oxidation of polymers by virtue of these processes. 

Some complications arise from the presence of proton donor-acceptor interactions134 when the donor is a protic amine. The separate evaluation of the two kinds of interactions may be a difficult problem. Similarly, if the electron acceptor is also a proton donor, the overlapping of salification and complexation processes makes the separate investigation of the interactions very difficult. This is the case in the complexes between amines and picric acid or other related phenols. For complexes of 2,4,6-trinitro-3-hydroxypyridine135 and... 

The frequency shift Av = v(OH) — v(OH O), where v(OH) is the frequency of the stretching mode of the O—H bond of the isolated phenol and v(OH O) in the presence of the electron donor 0(X,)2, is very informative for this series. The quantity Av is linearly correlated with the change of enthalpy (energy of donor-acceptor bond in the H-complex) and free energy (stability of the H-complex) , as well as with the value of effective charge q on the donor centre B, which was calculated by quantum-chemical... 

In considering the distribution of points for phenol and its meta and para substituted derivatives, arguments on metal-ligand tt bonding can be applied (2) to explain some of the scatter. In this way the stabilization of the iron complexes for the tt electron donor P"CH, p-F and p-I and the destabilization of the complex for the tt electron acceptor p-NO could be accomodated. But the positions of p-Cl, p-Br and m-F would still leave some questions. 

The most striking visual feature of DDQ-mediated reactions is the vibrant color produced, often attributed to charge-transfer complexes arising from an interaction between electron donors and acceptors that lowers the activation barrier for subsequent reaction. For example in an early report of dehydrogenation reactions of phenols by Becker proposed that... 

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