Para-dihydroxybenzene

Quaternary structure (Section 27 22) Description of the way in which two or more protein chains not connected by chemical bonds are organized in a larger protein Quinone (Section 24 14) The product of oxidation of an ortho or para dihydroxybenzene denvative Examples of quinones include... 

Quinone (Section 24.14) The product of oxidation of an ortho or para dihydroxybenzene derivative. Examples of quinones include... 

Phenols may have more than one hydroxy group attached to the same benzene ring - for example, the three isomeric dihydroxybenzenes catechol (ortho-dihydroxy-benzene or 1,2-benzenediol), resorcinol (meta-dihydroxy-benzene or 1,3-benzenediol) and hydroquinone (para-dihydroxybenzene or 1,4-benzenediol) see Figure 25.3. 

Phenol is photooxidized via its hydroxylated compounds into CO2 and H2O. Para-dihydroxybenzene (p-DHB), ortho-dihydroxybenzene (o-DHB) and 1,2,3-trihydroxybenzene (1,2,3-THB) are identified as primary and secondary hydroxylation products (Al-Ekabi Seipone, 1988). 1,2,4-trihydroxybenzene (1,2,4-THB) and 1,4-benzoquinone (1,4-SQ) are also documented as intermediate species. These species appear to be present prior to the total phenol mineralization (Trillaseta/., 1996 Winterbottom et al., 1997). Tseng and Huang (1990) reported two intermediate species,p-DHB and 1,4-BQ, during the photodegradation of 5.10 mole/L of phenol. Several other species are also suspected as possible inteimediates with their formation following the aromatic ring break up muconic acid, maleic acid, oxalic acid, formic acid, and acetic acid. Researchers have however, experienced difficulty until now to directly detect these species. 

Targeted experiments can be developed to investigate the phenol photocatalytic conversion mechanism (Salaices et al., 2004). In this context, four main intermediates of phenol photo-degradation are identified para-dihydroxybenzene (p — DHB), ortho-dihydroxybenzene (o — DHB), 1,2,4-trihydroxybenzene (1,2,4 — 77/6), and 1,4-Benzoquinone (I, 4 — BQ). 

The Dakin reaction proceeds by a mechanism analogous to that of the Baeyer-Villiger reaction. An aromatic aldehyde or ketone that is activated by a hydroxy group in the ortho or para position, e.g. salicylic aldehyde 12 (2-hydroxybenzaldehyde), reacts with hydroperoxides or alkaline hydrogen peroxide. Upon hydrolysis of the rearrangement product 13 a dihydroxybenzene, e.g. catechol 14, is obtained ... 

The hydroxylation of a phenol 1 upon treatment with a peroxodisulfate in alkaline solution, to yield a 1,2- or 1,4-dihydroxybenzene 3, is called the Elbs reaction The phenol is deprotonated by base to give a phenolate anion 4, that is stabilized by resonance, and which is activated at the ortho or the para position towards reaction with an electrophilic agent ... 

Reaction with the electrophilic peroxodisulfate occurs preferentially at the para position, leading to formation of a cyclohexadienone derivative 5, which loses a proton to give the aromatic compound 6. Subsequent hydrolysis of the sulfate 6 yields 1,4-dihydroxybenzene 3 ... 

The main product of the Elbs reaction is the 1,4-dihydroxybenzene (hydro-quinone). If the para position is already occupied by a substituent, the reaction occurs at an ortho position, leading to a catechol derivative although the yields are not as good as for a hydroquinone. Better yields of catechols 7 can be obtained by a copper-catalyzed oxidation of phenols with molecular oxygen ... 

A variety of electronic effects control the relative energies of the disubstituted benzenes. For the difluorobenzenes, the meta and para isomers are of comparable energy the ortho isomer is clearly less stable. This is most likely due to the electrostatic repulsions of the two C-F bond dipoles which are aligned in the same direction. This is just like the difluoroethylenes where the 1,2-cis isomer is much less stable than the 1,1-isomer.poj. he dihydroxybenzenes. 

Numerous further examples of protonation of variously substituted hydroxy- and alkoxy-benzenes in several acid solvent mixtures have been reported more recently and show a bewildering variety of behaviour. Several hydroxy- or alkoxy-groups favour C-protonation. Thus, 1,3,5-trihydroxybenzene and its ethers have been known for some time to be C-protonated at moderate acid concentrations (35-65% perchloric acid) (Kresge et al., 1962). In partial methyl and ethyl ethers of 1,3,5-trihydroxybenzene, there is a preference for protonation para to the hydroxy- rather than the alkoxy-group (Kresge et al., 1971a). 1,3-Dihydroxybenzene and its methyl ethers are also C-protonated in perchloric and sulphuric acid, whereas C-protonation of anisole would be expected only in >70%... 

The compounds are listed in alphabetical order, whereby numbers, prefixes such as para or O- have been ignored. For example, 7 -coumaric acid is listed under C , and 1,2-dihydroxybenzene is listed under D When applicable, the compound may be listed multiple times. In the example above, 1,2-dihydroxybenzene is also listed under B , as benzene, 1,2-dihydroxy . ... 

Similar reactions with 1,4-dihydroxybenzene and para-substituted methoxybenzenes did not produce the expected diaryl tellurium dichlorides1. 

Aryl tellurium trichlorides and tribromides condense with methoxybenzene4,5, 1,3-dihydroxybenzene, and dimethylaniline6 to give unsymmetrical diaryl tellurium dihalides. The condensations occur with the hydrogen atom in the para-position to the activating group in the aromatic compound. Aryl tellurium triiodides did not react6. 

ABSTRACT. Selective formylation of phenol at the 4-position is achieved by using 3-cyclodextrin as catalyst in the reaction of phenol with chloroform in aqueous alkali. The reactions of 1,3-dihydroxybenzene and indol, respectively, in the place of phenol give 2,4-dihydroxybenz-aldehyde and indole-3-aldehyde in virtually 100% selectivies and high yields. The reactions of para-substituted phenols, 4-methylphenol and 5,6,7,8-tetrahydro-2-naphthol, instead of phenol, effect the selective dichloromethylation at the para-positions. Selective carboxylation of phenol at the 4-position is achieved in the reaction of phenol with carbon tetrachloride in aqueous alkali by using 3-cyclodextrin and copper powder as catalyst. 

Scheme 9.5. A representation of the Dakin reaction, which converts an ortho- or para-hydroxyl-substituted benzenecarbaldehyde (benzaldehyde) into a bisphenol. The particular case shown generates 1,2-dihydroxybenzene (o-catechol).

The antioxidant activity of phenol is also increased by the presence of additional hydroxyl group in the ortho or para positions. An example of such an antioxidant is TBHQ. The effectiveness of 1,2-dihydroxybenzene derivatives is attributed to a phenoxyl radical stabilised by an intramolecular hydrogen bond (11-8). The activity of 2-methoxyphenol is lower, because the generated radical cannot be stabihsed by a hydrogen bond. The antioxidant activity of 1,2-and 1,4-dihydroxybenzene is partly caused by the fact that the semi-quinone radical can be further oxidised to the corresponding o-quinone orp-quinone, respectively, by reaction with another lipid radical (Figure 11.7) or may disproportionate to the corresponding quinone and hydroquinone. 

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