Simple Quinoids

There are, however, arguments against the simple quinoid hypothesis. Thus, it has not been possible to establish by either x-ray or infrared measurements any difference between starting anil and photo-converted anil. Tet the two molecules involved in the equilibrium (1) should behave very differently in these respects. The lack of difference is probably not due to insufficient conversion, for in the... 

Polcin and Rapson [12] have shown that hydrosulfite predominantly attacks simple quinoid, a,p-unsaturated aldehyde and anthocyanidine structures found in groundwood pulps prepared from western hemlock (Tsuga heterophylla) and eastern spruce (Picea glauca). A few years later, de Vries et al. [13,14] have demonstrated that hydrosulfite can reduce many types of aldehydes and ketones in solution according to the mechanism shown in Figure 13.1. Ketones were found to react sluggishly in water, esters are hydrolyzed, while carboxylic acids and amines are not reduced. 

Depending on the electronic state of azafulvalene and the reaction conditions, simple nucleophiles such as amines or alcohols show a different behavior. Upon heating methanol reacted with azafulvalenes as electron-rich olefins by addition to the central double bond (64BSF2857 67LA155). Using the TAF 77 (Ar = Ph), the addition reaction in a neutral benzene-ethanol solution required several days to obtain a minor amount of 147, while the reaction proceeded rapidly in the presence of a catalytic amount of potassium hydroxide (79JOC1241). Tlie yellow-colored adduct 147 can be reconverted to the quinoid starting material by irradiation (Scheme 58). 

The chemistry of quinone dyes has been discussed in a series of books entitled The Chemistry of Synthetic Dyes, edited by Venkataraman.1 The general chemistry of quinoid compounds has been discussed by Patai.2 There have been many books that cover quinoid compounds as dyes and pigments but very few discuss the chemistry of the corresponding leuco dyes. Traditional vat dyes are applied to cellulosic fiber in the leuco form. The chemistry of the leuco form of vat dyes is rather simple. Some leuco quinones are quite stable in the solid state and can be stored for a year. Other leuco dyes are unstable in solution and gradually undergo aerial oxidation. 

A simple, fast and specific color test for urea nitrate was reported recently by Almog et al. It is based on the reaction between urea nitrate and ethanolic solution ofp-dimethylaminocinnamaldehyde (p-DMAC) (9) under neutral conditions [91]. A red pigment is formed within 1 min from contact. Its structure has also been determined by the same group, by X-ray crystallography [92]. It appears to be a resonance hybrid between a protonated Schiffbase (10) and a quinoid system (10a) (Eq. (14)). The limit of detection on filter paper is 0.1 mg/cm. Urea itself, which is the starting material for urea nitrate, does not react with p-DMAC under the same conditions. Other potential sources of false-positive response such as common fertilizers, medications containing the urea moiety and various amines, do not produce the red pigment with p-DMAC. 

In flavylium compounds that bear OH substituents in their 4 - and/or 7-positions, deprotonation of the OH group can result in other forms being obtained, not seen in the case of the 4 -methoxyflavylium compound discussed above. Figure 8 illustrates this for the 4 -hydroxyflavylium ion.1171 The new species are the quinoidal base A, obtained by simple deprotonation of the AH+ flavylium cation, and the dianionic Cc2- and Ct2- forms, obtained by second deprotonations of Cc and Ct. The roles played by these forms depend on the specific compound and the pH conditions. For... 

Simple lignin-like para- and ort/to-quinoid structures were impregnated onto cellulose and mechanical pulp and photolyzed with a xenon lamp. The photo-reversion properties of these impregnated pulp samples were monitored as a function of time. The results of these studies indicate that in general not all quinoid type structures contribute to further photo-yellowing of mechanical pulp. 

The authors conclude that the emitting state created after excitation of the phenol form is predominantly quinoid but not identical with the excited cls-qulnone. It is speculated that the proton transfer process may not be a simple transfer between two minima of a hydrogen bond but rather involve another minimum In which the proton can be trapped. The same suggestion has been made previously by Shlgorln (84) on the basis of theoretical arguments. 

OH group may be assumed to form the anion/molecule complex 139 which, however, is non-reactive with respect to further tautomerization. Rather, this complex loses the entire carboxylate residue as the fragment ion m/z 255), leaving the phenolic unit as a quinoid neutral fragment (Scheme 37) °. In a further work, 3,4-dihydroxybenzyl carboxylates derived from stearic acid (cf. 140), dihydrocinnamic acid and phenylacetic acid were studied under NC1(NH3)-MS/MS conditions. In these cases, deprotonation was found to take place exclusively at the phenolic sites, owing to the increased acidity of hydroxyl groups in a catechol nucleus, in contrast to simple phenols. Heterolysis of the benzylic C—O bond, e.g. in ions [140 — gives rise to reactive anion/molecule complexes,... 

For the angular condensed-ring systems there is no simple rule for m, but Uq is still the product of the number of possible forms in each of the two parts of the molecule on either side of the quinoidal ring. 

Phenolics. The most widely used antioxidants in plastics are pheno-lics. The products generally resist staining or discoloration. However, they may form quinoid (colored) structures upon oxidation. Phenolic antioxidants include simple phenolics, bisphenolics, polyphenolics, and thiobisphenoUcs. 

Quinones represent a group of about 200 yellow, red, brown and almost black pigments with variable structure. They include simple quinones, dimers, trimers and condensation products that mutually differ in the number of hydroxyl groups and other substituents. The naturally occurring quinoid pigments are mostly derived from ... 

Benzo-l,4-quinoid structures occur in nature as the final oxidation products ofvarious mono- and polycyclic compounds. Most simple benzo-1,4-quin ones occur in microorganisms (moulds), higher fungi and lichens, and less frequently in higher plants and some insects. Common substances are glycosides occurring in colourless reduced forms (such as derivatives of hydroquinone known by the systematic name of benzene-l,4-diol). Coloured quinones are formed from these precursors by hydrolysis catalysed by saccharases and by enzymatic oxidation or autoxidation of aglycones. 

Quinones themselves are coloured compounds. Simple quinones are usually red when they contain an o-quinoid structure (such as o-benzoquinone) in the molecule, while compounds with the p-chinoid arrangement are yellow (e.g. p-benzoquinone). The catechin oxidation product is bright yellow, quinone formed of chlorogenic acid has a yeUow-orange colour and quinone derived from amino acid dopa (dopaquinone) is pink. 

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