Violet-quinone

The occurrence of an additional two-membered bridge as intermediate in a [2.2]paracyclophane system has been postulated by Forrester and Ramasseul 47b>. During the synthesis of the bishydroxylamine 82, a precursor of the diradical 19 these authors were able to isolate as byproduct the pseudo-para hydroxy compound 83 which is converted into the violet quinone 84 through oxidation by atmospheric oxygen or reac-... 

Phenolphthalein behaves as a dibasic indicator in a titration with alkali. First the lactone group is transformed into the carboxylic group anion (the monovalent ions are colorless), and then the second equivalent of base changes the indicator into the red-violet quinone phenolate which acts like a bivalent anion. The first and second dissociation constants lie very close to each other, and therefore the relation between pH and color intensity of phenolphthalein (as well as other phthaleins) is rather complex. 

Many hydrazones and azines are colored and useful as dyestuffs. Examples are 2-hydroxynaphthazine, a yellow fluorescent dye (Lumogen LT Bright Yellow), and the pyridon—azino—quinone class of red-violet dyes. Numerous hydrazine derivatives are antioxidants and stabilizers by virtue of their reducing and chelating powers. 

Diaminoanthraquinone [128-95-0] M 238.3, m 268°. Purified by thin-layer chromatography on silica gel using toluene/acetone (9 1) as eluent. The main band was scraped off and extracted with MeOH. The solvent was evaporated and the quinone was dried in a drying pistol [Land, McAlpine, Sinclair and Truscott J Chem Soc, Faraday Trans 1 72 2091 1976]. Crystd from EtOH in dark violet crystals. 

Although treated as separate classes in the Colour Index, these structural types are closely related and the few diphenylmethane dyes such as auramine (1.28 Cl Basic Yellow 2) are now of little practical interest. Commercial usage of the triarylmethane dyes and pigments has also declined considerably in favour of the major chemical classes. They were formerly noteworthy contributors to the acid, basic, mordant and solvent ranges, primarily in the violet, blue and green sectors. Numerous structural examples are recorded in the Colour Index. The terminal groupings can be amine/quinonimine, as in auramine and crystal violet (1.29 Cl Basic Violet 3), hydroxy/quinone, or both. The aryl nuclei are not always benzenoid (section 6.5). 

Galinowski and T. Urbanski [74] prepared this substance by nitration of crystal violet with a mixture of nitric and sulphuric acids. On heating with 2% KOH two products were formed a semi-quinone (II) and a phenol (III). 

It has already been pointed out that this compound may be prepared by hydrolysis of the selenocyanate it may also be obtained by the interaction of sodium selenide and 1-chloroanthraquinone. By the latter process orange-red plates, M.pt. 212° C., result. If 2-chloroanthra-quinone be used in the reaction, 2-anihraquinone selenophenol is obtained as an orange-yellow product, which gives a violet-red solution in sulphuric acid. Concentrated nitric acid causes oxidation to the seleninic acid. 

Quinone Colors depend on mordant and pH 1. benzoquinone 2. naphthaquinone 3. anthraquinone Carthamine Juglone Alizarin Purpurin Walnut or butternut (Juglans sp.) Rubiaceae family Sorrel (Rumex sp.) Poligonaceae family Forms lake with mordant For 3. pH >7 -> violet-blue pH < 7-> yellow-red AL rose-red Ca bluish-red Zn red-violet Fe black-violet Cr=red brown... 

The corresponding reactions in the benzoquinone field were discovered by Cameron and Giles128 in 1965, who reported rapid decoloration of the violet-black quinones (242) on exposure to sunlight. The product was the benzoxazoline (243) which with acid treatment followed by oxidation, gave the quinone (245).129 A second irradiation process yielded the bis(monomethylamino)benzoquinone (246). Analogous benzoxazolines (248) were isolated from the pyrroli-dino-, the piperidino-, and the perhydroazepinobenzoquinones (247). 

DPPH Free Radical Blois (53) showed that a,a-diphenyl-p-picrylhydra-zyl radical (DPPH ) can be used for determining antioxidant activity of ascorbic acid, tocopherol, and quinones (Figure 8a and b). DPPH in ethanol shows a strong absorption band at 517 nm (independent of pH from 5.0 to 6.5), and the solution appears to be deep violet in color. As the DPPH radical is scavenged by the donated hydrogen from the antioxidant, the absorbance is diminished according to the stoichiometry. 

This series of dyestuffs, which were first prepared by Witt and Koechlin, by simultaneous oxidation of paradiamines or para-amido phenols with phenols, exhibit, both as regards constitution and general behaviour, a close relationship to the indamines. Like the latter, they are decomposed by acids with formation of a quinone. In general they possess a weak basic nature, but, unlike the indamines, form colourless salts, and have mostly a blue or violet colour in the free state. 

These dyestuffs are far more stable than the indamines and indophenols, and, unlike these, do not give quinone when treated with acid. For this reason they are capable of practical application, one of them, methylene blue, discovered by Caro, being used on a very large scale. The dyestuffs of this class have a violet or blue shade. 

The Phthaleins. All phthaleins are rather insoluble in water but very soluble in alcohol. Most of them are colorless in acid medium (lactone form), while the alkaline form is colored (red, violet, blue, green) and possesses the quinone phenolate structure. In strongly alkaline medium the color gradually fades due to the transformation of the quinone phenolate into the colorless carbi-nol form. These structural changes will be discussed in detail in Chapter Seven. The velocity with which the colors of various phthaleins fade in alkaline medium has been measured by A. Thiel and coworkers, and the velocity constants determined. 

It is peculiar that the ionization of the quinone group should influence the color so profoundly. The free quinone compound is yellow while the color of its corresponding cation is red to red-violet. The alkaline form (quinone phenolate) on the con-... 

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