Quinonoid group

It has been proposed that quinonoid structures 8) contribute to the color of kraft lignin. If quinonoid groups are present, they must have been formed in an oxidative process whose nature is unknown. 

On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically are converted to o- and p-quinonoid structures and oxirane derivatives of quinols (E, D, C, resp., Fig. 1.4). It should be noted that the conversion of aromatic nuclei to o-quinonoid rings is accompanied by loss of the methoxyl group as methanol and that conversion to p-quinonoid groups in many cases leads to displacement of the side chain. 

The liberated phenol condenses with 4-aminoantipyrine and, in the presence of oxidizing conditions (alkaline ferricyanide), gives rise to an organic product in which the phenol residue contains the chromophoric quinonoid group. 

Argyropoulos, D. S., Heitner, C., and Schmidt, J. A., Observation of quinonoid groups during the light-induced yellowing of softwood mechanical pulp, Res. Chem. Intermed. 21(3-5), 263-274 (1995). 

While most pigments are immediately obvious on the plate, the use of longwave ultraviolet light may improve detection in some cases. Spray reagents have found extensive use in the flavonoid and quinonoid groups and a silver nitrate spray has been used to distinguish certain carotenoid end groups. 

Dissolve ca. 0 2 g. of product (I) in cold ethanol, and add with shaking 1-2 drops of dilute sulphuric acid. A deep purple coloration appears at once. This shows that salt formation has occurred on the quinoline nitrogen atom to form the cation (Ha), which will form a resonance hybrid with the quinonoid form tils). [Note that the forms (IIa) and (11b) differ only in electron position, and they are not therefore tautomeric.] If, hoAvever, salt formation had occurred on the dimethylaniino group to give the cation (III), thrs charge separiition could not occur, and the deep colour would be absent. 

Diketones and tetraketones derived from aromatic compounds by conversion of two or four SCH groups into keto groups, with any necessary rearrangement of double bonds to a quinonoid structure, are named by adding the suffix -quinone and any necessary affixes. 

On more severe thionation, a third thiamine ring is formed to give a sulfur black. However, if hydroxyl groups instead of amino groups are attached at positions 2 and 2, no ring closure would take place and the blue dye would be stable to heat. These formulas are general expressions for the nuclear stmctures of the blue-to-black sulfur dyes they do not take into consideration the quinonoid formation of each dye and other aspects. 

Polycyclic Aromatic Carbonyl Dyes. StmcturaHy, these dyes contain one or more carbonyl groups linked by a quinonoid system. They tend to be relatively large molecules built up from smaller units, typically anthraquinones. Since they are appHed to the substrate (usually cellulose) by a vatting process, the polycycHc aromatic carbonyl dyes are often called the anthraquinonoid vat dyes. 

Hydroxypyrimidine (89a) can tautomerize to two alternative 0X0 forms, 90 (R = H) and 91 (R = H). The infrared solution spectra of pyrimidin-2- and -4-one clearly indicate the presence of both C=0 and NH groups, and by using these spectra Mason demonstrated that pyrimidin-4-one existed predominantly in the o-quinonoid... 

Between 1951 and 1953 investigations by three English groups clearly demonstrated the preponderance of the oxo forms of pyrimidin-2- and -4-ones by comparing the ultraviolet spectra of these compounds with those of the N and 0-alkylated derivatives, The o-quinonoid form 91 (R = H) is favored by the evidence that A -methylation of the 6-methyl derivative of 89a does not cause a bathochromic shift in the ultraviolet spectrum (A -methylation of pyrid-4-one causes a bathochromic shift, but this is not observed for pyrid-2-one) The isomeric A -methyl derivatives of pyrimidin-4-ones [e.g., 91 (R = Me) and 90 (R = Me)] form similar cations (e.g., 92 and 93), and hence the equilibrium constant between... 

As a result of various side reactions, the yields are relatively low. However, in no case was ring fission found during the oxidations. Specially noteworthy is the ease with which the two methine groups in the 5-position of the 2-hydrazino-selenazoles are coupled together. Reference to models indicates that the quinonoid dyes exist in the trans form. 

In conclusion, with regard to the structure of benzenediazonium compounds with electron donor substituents in the 2- or 4-position, the most recent experimental data, mainly X-ray analyses and 13C and 15N NMR data, are consistent with 4.4 as the dominant mesomeric structure of quinone diazides, as proposed by Lowe-Ma et al. (1988). For benzenediazonium salts with a tertiary amino group in the 4-position the data are consistent with the quinonoid structure 4.20 as the dominant mesomeric form. 

Doubt has been expressed as to the validity of the above mechanism by the observation that in the bromination of 2-hydroxy-4,6-methoxyacetophenone, bromine enters the 3 position and replaces the acyl group at a rate which is increased by acetylation of the hydroxy group, which should not be the case if a quinonoid intermediate is formed, as required above734. However, since the hydroxy group becomes acetylated during the course of the reaction, thereby partly changing the medium to bromine in acetic acid, this result is ambiguous... 

Synthetic pyrethroids with a-cyano ester group react with sodium hydroxide to yield hydrogen cyanide, which reacts with 4-nitrobenzaldehyde and 1,2-dinitrobenzene to yield a pink-colored derivative (o-quinonoid di-anion) according to the following scheme ... 

---