Quinoid resonance

Figure 7.27 Quinoid resonance forms activating the para ring position.

Quasi-prepolymers, 236, 237 Quinoid resonance forms, 402 Quinone methide reactions, with... 

Further evidence supporting this suggested interpretation of the X values of Table XXI is obtained in the lack of solvent dependence of X values for ionization equilibria which involve little or no para quinoidal resonance interaction of the type... 

Spiropyran merocyanine specffa shift markedly to the blue as the solvent polarity increases [4,25], as shown in Fig. 7a for 6,8-dinitro-BIPS merocyanine. This is generally accepted to imply that they have a zwitterionic character caused by the donation of electron density from the indoline nitrogen to the phenolic C9 oxygen [4,25]. The rational behind this assignment to the zwitterion is based simply on the fact that if the zwitterion is in the highest occupied molecular orbital (HOMO) state, then the corresponding quinoidal resonant form is in the lowest unoccupied molecular orbital (LUMO) state and changing the solvent to a more polar one... 

When the temperature was allowed to increase from —40 to about 10 C, no change of the H- and UC-NMR spectra was observed,16 indicating that 5 is also thermodynamically most favored. In n-adduct 5 there is a notable contribution of the para-para quinoid structure3 (5b) because it leads to a considerable negative charge on N-6. This para-para quinoid resonance structure has already been discussed as an important contribution in stabilization of adduct 2 (see Section II,A,1). It explains why adduct 6 is not formed even under conditions that are favorable for thermodynamic control. Apparently the azaallylic contribution (6b) is of less importance than the para-para quinoid contribution (5b) when a nitrogen is present at position 6. 

Again, remarkably high At) values are found for C-6 and C-8 of tr-adduct 816 (Table III). This indicates once more the importance of para-para and, to a lesser extent, ortho-para quinoid resonance structures.3 On heating the solution of 8 and 9 in KNH2/NH3 from -40 to 10 C the mixture irreversibly converts to 9. The important azaallylic resonance stabilization present in 9 and not in 8 accounts for the higher thermodynamic stability of species 9. 

Apparently a-adduct 12 is kinetically favored and is the thermodynamically more stable one as well. This result is surprising in the light of the results (discussed in Section 11, A. 1) that the C-4 adduct (3), formed from compound 1, is more stable than the isomeric C-2 adduct (2) because ofazaallylic stabilization. We assume that the ortho-para resonance contribution (12b) adds to the stability of rr-adduct 12.3 A similar contribution cannot play a role in a rr-adduct on position 4. The importance of ortho-para quinoid resonance structures has already been discussed in Sections II,A,1 and II,A,3. 

It is possible to draw Kekul6 structures as well as polar quinoid resonance structures for isomeric disiibstitiited benzenes with a donor and an acceptor... 

Dey and Dogra96 studied the solvatochromic properties of 2-(aminophenyl)benzthiazole (79), with the amine group in positions 2, 3 or 4, in 12 solvents ranging from cyclohexane to water in polarity. The largest shift was found for the meta isomer from 26320 cnr1 for cyclohexane to 19880 cm-1 for water. The 2- and 4-isomers have a quinoid resonance form and smaller spectral shifts, the least being for the 2-isomer that has an internal hydrogen bond. 

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