Quinonoid resonance structures

The practical route for oxidizing leuco diphenylmethanes 15 demands inital conversion to an imine salt 16. The imine salt is obtained by heating a mixture of diphenylmethane, sulfur, ammonium chloride, and sodium chloride at 175°C in a current of ammonia or by heating a mixture of diphenylmethane, urea, sulfamic acid, sulfur, and ammonia at 175°C (Scheme 3). Dyes 16 can be represented as the quinonoid resonance structure 17. Dyes of this class, known as auramines, are all yellow, with the only commercial representative being auramine O 16a. Due to its poor lightfastness and instability to hot acids and bases, its use has been restricted to dyeing and printing cotton, paper, silk, leather, and jute. 

For example, we recall discussions of o- and p-directing vs m-directing groups for electrophilic aromatic substitution, the importance of o- andp-quinonoid resonance structures vs the irrelevance of m-quinonoid long bonded resonance structures for stabilization of push-pull disubstituted benzenes. 

Thus, treatment of 6-hydroxy-substituted salts 30 (R4 = H) with aqueous or alcoholic alkaline solutions results in stable betaines 241 (vinylo-gous pyrones) having a para-quinonoid resonance structure, whereas 7-hydroxy-substituted salts 30 (R5 = H) give rise to diketones 244 [66ACH(50)381 75ACH(85)79]. 

Benzo[c]pyrylium-3-oxides 240 having an orf/io-quinonoid resonance structure act as dienes in reactions with dienophiles (84TL3659). 

Phenol can be considered as the enol of cyclohexadienone. While the tautomeric keto-enol equilibrium lies far to the ketone side in the case of aliphatic ketones, for phenol it is shifted almost completely to the enol side. The reason of such stabilization is the formation of the aromatic system. The resonance stabilization is very high due to the contribution of the ortho- and / ara-quinonoid resonance structures. In the formation of the phenolate anion, the contribution of quinonoid resonance structures can stabilize the negative charge. 

In contrast to aliphatic alcohols, which are mostly less acidic than phenol, phenol forms salts with aqueous alkali hydroxide solutions. At room temperature, phenol can be liberated from the salts even with carbon dioxide. At temperatures near the boiling point of phenol, it can displace carboxylic acids, e.g. acetic acid, from their salts, and then phenolates are formed. The contribution of ortho- and -quinonoid resonance structures allows electrophilic substitution reactions such as chlorination, sulphonation, nitration, nitrosation and mercuration. The introduction of two or three nitro groups into the benzene ring can only be achieved indirectly because of the sensitivity of phenol towards oxidation. Nitrosation in the para position can be carried out even at ice bath temperature. Phenol readily reacts with carbonyl compounds in the presence of acid or basic catalysts. Formaldehyde reacts with phenol to yield hydroxybenzyl alcohols, and synthetic resins on further reaction. Reaction of acetone with phenol yields bisphenol A [2,2-bis(4-hydroxyphenyl)propane]. 

This character results from quinonoid resonance structures in addition to the more important Kekul6-type structures and tends to cause the hydrogen atom to be placed in the molecular plane. This leads to two equivalent configurations with the hydrogen of the OH group being on one side of the other of the C—O bond . It implies the existence of the activation barrier K of the OH torsion motion around the C—O bond estimated in the mid-thirties as equal to 14 kJmoU . 

As was the case for dinitrobenzene, the meta and para nitroaniline isomers have essentially the same gaseous enthalpy of formation. In the gaseous phase, it is surprising to find that despite the more attractive quinonoid resonance structures for the para isomer (58) than for the meta (59) the meta and para nitroaniline have essentially the same gas-phase enthalpy of formation. In the solid and liquid states the intemolecular stabilization lowers the enthalpy of formation of the para isomer relative to the meta. Interestingly, the gas-phase intramolecularly hydrogen-bonded ortho isoma- is of comparable stability to its isomers. In contrast, it is considerably less stable than its isomers in the solid state because it can form fewer intermolecular hydrogen bonds. All isomers of nitroaniline are more stable than calculated by additivity. 

An early attempt to test the disjoint hypothesis compared the magnetic properties of two isomeric tricyclic m-quinonoid non-Kekule molecules 17, formally a biradical with tetraradical resonance structures, and 18, formally a tetraradical (Section 2.3). These molecules belong to the point groups C2 and C2v, respectively, and it will be mnemonically convenient to use those descriptors in what follows. The test derives from the recognition that the connectivities of the two molecules... 

Note that this conclusion is based on the assumption that no 4n-mem-bered rings are present. If they are, resonance theory fails because the parallel between numbers of resonance structures and NBMO coefficients no longer holds. Consider, for example, styrene (48) and cyclooctatetraene (49). There are in each case two possible classical structures yet styrene is aromatic, while cyclooctatetraene is antiaromatic. This situation is quite general. Thus one can write four classical structures for biphenyl (50), corresponding to the Kekule structures for each benzene ring, but for biphenylene (51), where there is an additional quinonoid structure (52). This should imply that the central ring in biphenylene is aromatic in fact, it is antiaromatic. Thus resonance theory should not be allowed to survive even as a poor substitute for the PMO method since there are cases where it leads to qualitatively incorrect results. The reason for the failure of resonance theory in these cases stems from the fact that resonance theory has no firm foundation in the wave properties of matter. 

Loss of the a-hydrogen (Group a). Dissociation of the a-hydrogen from the Schiff base leads to a quinonoid-carbanionic intermediate whose structure in depicted in Fig. 14-5. The name reflects the characteristics of the two resonance forms drawn. 

Depending on the reaction conditions, the product can be isolated in either the lactoid form A [2321-07-5] (2) or the quinonoid form B [56503-30-1] (3). These 9-phenylxanthenes are closely related structurally to the triphenyl methane dyes (4) and, like them, are cationic resonance hybrids. 

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