Cyclohexadienyl radicals structure

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

Scheme 1 Decomposition outline ofthe aromatic part of Apollofix-Red dye, modeled with a substituted benzene (lower part) and achieving its complete mineralization (upper part). A disproportionation reaction between two cyclohexadienylic radicals regenerates the aromatic ring structure In a phenol derivative. The R-group can be an alkyl group ora halogen atom.

Fig. 3. (a) The ESR spectra obtained upon UV irradiation of aqueous H2O2 in the presence of p-toluene sulfonic acid at pH 0.4 (al, identified as the benzyl radical), pH 4.8 (a2, cyclohexa-dienyl radical), and pH 11.5 (a3, a superposition of spectra from three phenoxyl type radicals). Note the different scales. Arrows marked d, t, and q indicate doublet, triplef and quartet spUtt-tings, respectively. The corresponding radical structures are shown in (b). The pH dependence of total signal amplitudes (obtained by integration of an isolated line and then scaled to represent the entire spectrum) are given in (c) for the benzyl radical ( ), the cyclohexadienyl radical (O), and the sum of the three phenoxyl radicals (A). (From Ref. 37 with permission.)... 

Obviously the structures and yields of Birch reduction products are determined at the two protonation stages. The ring positions at which both protonations occur are determined kinetically the first protonation or 7t-complex collapse is rate determining and irreversible, and the second protonation normally is irreversible under the reaction conditions. In theory, the radical-anion could protonate at any one of the six carbon atoms of the ring and each of the possible cyclohexadienyl carbanions formed subsequently could protonate at any one of three positions. Undoubtedly the steric and electronic factors discussed above determine the kinetically favored positions of protonation, but at present it is difficult to evaluate the importance of each factor in specific cases. A brief summary of some empirical and theoretical data regarding the favored positions of protonation follows. 

Thus, Hine (1966a) used PLNM successfully to rationalise the sites of attack on conjugated reactive intermediates (cations, radicals and anions). The data is puzzling since the thermodynamically less stable non-conjugated isomers predominate protonation of the cyclohexadienyl anion, for example, yields predominantly cyclohexa-1,4-diene. The PLNM rationalisation of this result is set out in Scheme 14 in terms of the resonance structures of the pentadienyl anion fragment. 

The thermal volatilization analysis of a mixture of polyvinylchloride and polystyrene is given in Fig. 81. The first peak corresponds to the elimination of HC1 and the second to that of styrene. Dehydrochlorination is retarded in the mixture. The production of styrene is also retarded styrene evolution, in fact, does not occur below 350°C. This contrasts with the behaviour of polyvinylchloride-polymethylmethacrylate mixtures for which methacrylate formation accompanies dehydrochlorination. The observed behaviour implies that, if chlorine radical attack on polystyrene occurs, the polystyrene radicals produced are unable to undergo depolymerization at 300° C. According to McNeill et al. [323], structural changes leading to increased stability in the polystyrene must take place. This could also occur by addition of Cl to the aromatic ring, yielding a cyclohexadienyl-type radical which is unable to induce depolymerization of the styrene chain. 

It seems reasonable that rapid rotation around the sixfold axis could occur more easily than any other reorientation (30). If the anisotropic hyperfine coupling for the protons is of the same form as that observed for other 7r-electron hydrocarbon radicals (20), then the anisotropy would be averaged out by a rapid rotation around this axis. However, the small asymmetry observed indicates an incomplete averaging. The additional structure of the two outermost lines on the high field side may be an anisotropy effect. Alternatively, an underlying spectrum—e.g., from cyclohexadienyl—may also distort the line shape. 

Radicals containing nitrogen atoms as 7c-centers are included only when, in terms of valence bond resonance structures, the unpaired electron is not located at nitrogen (e.g. 2-aza-allyl, 3-aza-cyclohexadienyl). 

For cationic 7r-ligands, the usual ionic suffix, for example, dienium, becomes -dienyl as in cyclohexadienium becoming 7i-cyclohexadienyl. Cationic ligands sometimes behave as anionic or radical species because of the ligand structure [1 -20-1 -22]. 

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