Benzoid structure

Quinone methides are the key intermediates in both resole resin syntheses and crosslinking reactions. They form by the dehydration of hydroxymethylphenols or dimethylether linkages (Fig. 7.24). Resonance forms for quinone methides include both quinoid and benzoid structures (Fig. 7.25). The oligomerization or crosslinking reaction proceeds by nucleophilic attack on the quinone methide carbon. 

The polaron-bipolaron topic has been treated repeatedly from a theoretical point of view. Results may be used to interpret experimental evidence gathered with spectroelectrochemical methods. Aleman [985] studied the spatial extension of bipolaronic defects in various polymers based on five-membered aromatic rings including thiophene benzoid structure was found for neutral oligomers, whereas a quinoid structure extending over six rings was deduced for dications. 

Oligomer molecules exhibit a benzoid structure with a single bond between the rings. This structure allows various conformers in solution at room temperature which show deviations from planarity but only small energy differences [9]. 

Ouyang et al. have interpreted the changes observed in the Raman spectra as a change from a benzoid structure toward a quinoid structure of the PEDOT molecules due to a move from a coil structure toward an extended coil or linear structure of PEDOT chains. ... 

All the known porphyrin isomers are typical benzoid aromatic compounds which show distinctly porphyrin-like characteristic electronic absorption spectra.13 Also the complexation properties for metal ions, NH tautomerism and the NMR spectra are quite similar to the parent porphyrin structure. 

Fig. 10. Formation of the bipolaron (= diion) state in poly-p-phenylene upon reduction In the model it is assumed that the ionized states are stabilized by a local geometric distortion from a benzoid-like to a chinoid-Iike structure. Hereby one bipolaron should thermodynamically become more stable than two polarons despite the coulomb repulsion between two similar charges...

As we have already seen these lone pairs can form part of the system of n electrons. The difference between chromo-phoric and auxochromic groups is in this way of secondary importance. Also the much discussed question whether a ben-zoid (benzene-like) or a quinonoid (quinone-like) structure should be attributed to dyestuffs becomes, in the light of the resonance theory, an incorrectly chosen alternative. It is the possibility of resonance which is reflected in the multiplicity of the valence structure that forms the true basis for light absorption. An isolated benzoid configuration is just as little a colouring matter as a quinonoid structure compare the uncoloured hydroquinone and the very weakly coloured quinone. 

Benzoid Chromophobe. The spectrum of the aromatic parent, benzene (Figure 2), displays considerable fine structure, a property which is not shared to the same extent with many of its derivatives. The three-bonded spectrum (248, 254, and 260 /x) of benzene will be considered as one chromophore. Benzene absorbs at 184 fi (Om 60,000), 203.5 fi (Om 7400), and 254 fi (Om 204) in hexane (4). These maxima are considered as the 7T 7T bands of the benzene chromophore. Increasing alkyl substitution causes a bathochromic shift of the 254-/i band, an eflFect which reaches its maximum at tetrasubstitution. New intense bands appear in the spectrum of benzoid compounds upon introduction of a substituent... 

The stabilization of ortAo-fused benzoxete (32) occurred by valence isomerization into a quinoid isomer (33) (84MI1363). Such benzoid-quinoid processes are impossible for naphth[l,8-Z c]oxete because of structural reasons. Therefore, the biradicals isomerization (A B) seems likely (Scheme 5). 

Non-benzoid phanes have always played an important role in cyclophane chemistry and many structures have been prepared with other Hiickel aromatic rings such as azulene or tropone [49]. A cyclophane with two bridged annulene units was recently synthesized by Mitchell [50,51 ]. Dimethyldihydropyrene (76), an excellent NMR probe first introduced by Boekelheide [52], was converted into the dialcohol 77 in three steps. Reaction with adipoyl chloride afforded the large [10.10]cyclophane 78. Unfortunately the conversion of the dialcohol into the corresponding dibromide 79, an obvious precursor to the interesting phane 80, has failed so far. 

In ideal semiquinone-radical structure, all rings and nitrogen should be chemically equivalent. As a result, at the potentials of the electrochemical stability of semiquinone radical structures, the observed bands should be intermediate between those expected for the benzoid and quinoid structures. During the second oxidation process which is accompanied by de-protonation, quick spectroscopic differentiation between the... 

Cumulene structures also undergo the Myers-Saito reaction. Cyclization of acyclic enyne[3]cumulenes, on the activation of Z-configured dienediyne 38 via acid solvolysis, has been described by Bruckner et al. It has been found that 38 dissolved in /-BuSH/dichloro-methane and treated with a catalytic amount of triflic acid forms the monocyclic cumulene 39. Storage of the mixture for 4 days at room temperature gave the corresponding styrene derivatives 40 and 41 these products form as a result of cycloaromatization via path A (benzoid radical). Independently, after... 

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