1,4-Benzoquinones from hydroquinone

The same comparison was made for hydroquinone for a given conversion of 80%, which is exhibited in Fig. 12.8. Unlike catechol and 3-methycatechol, products resulting from hydroquinone cracking in the presence and absence of iron oxide are identical. A peak found at m/z 110 is probably hydroquinone and its fragment ions are at m/z 39, 55, and 81. The identities of some of the products (Fig. 12.8a and b) are likely to be as follow acetylene (m/z 26), vinyl acetylene (m/z 52), butadiene (m/z 54), cyclopentadienone (m/z 80), and 1,4-benzoquinone (m/z 108). To confirm the differences in chemistry between catechols and hydroquinones, 2,3-dimethyhydroquinone was subjected to the same comparison. 

Problem 19.33 From phenol prepare (a) p-benzoquinone, (b) p-benzoquinone dioxime, (c) quinhydrone (a 1 1 complex of p-benzoquinone and hydroquinone). 4... 

Equation 9 indicates the addition of benzoquinone to CoH to form a new complex which cannot react further with CoH. Equation 10 defines the role of excess alkali in effecting the catalytic reduction of benzoquinone. As shown in previous examples, the hydroxo complex may then undergo the reverse aging process, leading to hydrogen absorption. The over-all result is reduction of benzoquinone to hydroquinone when limited amounts of substrate are available, and to quinhydrone when excess substrate is available. Equation 11 is an attempt to explain the lowered amount of hydrogen absorption noted when cyanocobaltate(II) is prepared in the presence of excess benzoquinone. Displacement of reduced substrate from this binuclear complex by alkali is assumed, since quinone was catalyti-cally reduced when the above procedure was carried out in the presence of added alkali. 

The effect of the halogen substituent (fluoro, chloro, bromo and iodo) on the yield and mechanism of 4-halophenol photolysis was investigated by Durand et al. [24], Transient spectroscopy in aerated aqueous solutions indicated the formation of p-benzoquinone O-oxide from each derivative except 4-iodophenol for which no transients were detected p-benzoquinone and hydroquinone were found as photoproducts for all four compounds. It was concluded that the carbene mechanism was valid for the whole series. Under continuous irradiation, the 4-halophenol degradation quantum yields were determined to be

fluorescence lifetimes decreased in the same order, from 2.1 ns for 4-fluorophenol to 0.4 ns for 4-chlorophenol and < 0.1 ns for 4-bromophenol. 

Derivatives of phenol or aniline can be oxidized to quinones, the yield and ease of oxidation depending on the substituents. If an amino or hydroxyl group is in the para position, the reaction proceeds readily, as illustrated by the synthesis of quinone from hydroquinone by oxidation with a sodium chlorate-vanadium pentoxide mixture (5>6%) or with chromic-sulfuric acid mixture (92%). A para halogen atom usually has a favorable effect. Any group in the para position is eliminated or oxidized. o-Quinones are usually prepared from the corresponding catechols. A survey of procedures for the synthesis of benzoquinones by oxidation has been made. ... 

Macrocyclic metal complexes have recently attracted attention as dioxygen activating catalysts in oxidation reactions. A triple catalytic procedure [1,2] involving three redox systems Pd(II)/Pd(0) - benzoquinone/hydroquinone - ML° /ML was developed for the aerobic oxidation reactions. The multistep electron transfer occurs in the following way electron transfer occurs from the substrate to Pd (II), giving Pd (0), followed by another electron transfer from Pd (0) to benzoquinone. The hydroquinone thus formed, transfers electrons to the oxidized form of the metal macrocycle, which is reduced. The latter is reoxidized by electron transfer to molecular oxygen. 

The quinones may be regarded as unsaturated diketones derived from cycloolefins, and closely related to aromatic compounds. They exhibit great reactivity with a variety of reagents. They are readily reduced and oxidized, and undergo both olefinic and carbonyl reactions. Equimolecular proportions of p-benzoquinone and hydroquinone yield a crystalline molecular compound, quinhydrone. 

Reaction of Aniline with Organic Functional Groups. In the absence of catalysis by enzymes or metals, aniline undergoes nucleophilic addition reactions to quinone and other carbonyl groups in humic substances to form both heterocyclic and nonheterocyclic condensation products (9). In aqueous solution, aniline undergoes 1,4-addition (Michael addition) to both 1,2- and 1,4-quinones (10-14). The reaction of aniline with 1,4-benzoquinone, from the oxidation of hydroquinone, and with 4-methyl-1,2-quinone, from the oxidation of 4-methylcatechol, are illustrated here. 

The work of Frost et al. has focused on the aromatic pathway with respect to the biosynthesis of quinoid organics such as benzoquinone and hydroquinone, as well as adipic acid production [194-197]. The same principles have been used whereby a cloned gene is overexpressed, which allows the recombinant organism to funnel carbon down a specific, selected metabolic pathway. Although currently tlrese compounds are synthesized from petroleum-based feed stocks primarily for economic reasons, environmental concerns and the shortage of these starting... 

Using neutral (pH 7) hydrogen peroxide Oki et al. [44] isolated methoxy hydroquinone and methoxy benzoquinone from the reaction of vanillin and acetovanillone with hydrogen peroxide. Other p-hydroxybenzaldehydes were degraded in a similar way. Hirashima and Sumimoto [45] obtained evidence for the presence of quinones in mechanical pulps, using a photochemical reaction with acetaldehyde. 

Similarly, p-benzoquinone 6 was obtained from hydroquinone 5 (2.00 mmol) and DDQ (2.00 mmol). 

Reaction with benzoquinone. Apinol tetramethyl ether (1,2,3,4-tetra-methoxybenzene (3) can be prepared readily from hydroquinone (1) as formulated. 

Vogna et al. (2004) showed that UV/H2O2 (as well as molecular ozone) was effective at oxidizing transformation products of the pharmaceutical, diclofenac. Lau et al. (2007) studied the formation of a suite of ten transformation products of butylated hydroxyanisole by including 1,4-benzoquinone, f-butyl-l,4-benzoquinone, and hydroquinone. Some of the degradates were precipitated from solutions as an orange-colored solid that could be removed by filtration. The study showed that UV/ozone and ozonation were more effective than UV alone at removal of the parent and transformation products. 

Oxidation of hydroquinone (1,4-benzenediol) produces a compound known as /)-benzoquinone. The oxidation can be brought about by mild oxidizing agents, and, overall, the oxidation amounts to the removal of a pair of electrons (2 e") and two protons from hydroquinone. (Another way of visualizing the oxidation is as the loss of a hydrogen molecule, H H, making it a dehydrogenation.)... 

The quinone-hydroquinone system represents a classical example of a fast, reversible redox system. This type of reversible redox reaction is characteristic of many inorganic systems, such as the interchange between oxidation states in transition metal ions, but it is relatively uncommon in organic chemistry. The reduction of benzoquinone to hydroquinone formally involves the transfer of two hydrogen atoms. In practice, the reversible reaction involves the stepwise transfer of two electrons from the reducing agent as shown in Scheme 4.1. 

Fig. 189. Fatigue strength of rubbers made from SKS-30A after the introduction of various additives, a) At 20 b) at 100 C. I) 10 parts by weight of the additive II) three parts by weight III) one part by weight IV) 0.3 parts by weight V) without additives. 1) Benzoyl peroxide 2) dihydroxydiphenyl sulfide 3) azoisobutyrodinitrile 4) di-tert-butylhydroquinone 5) tri-tert-butylphenol 6) polyethylene-polyamine 7) diproxide 8) santovar 0 9) chlorinated paraffin 10) benzoquinone 11) hydroquinone 12) quinhydrone 13) acetoneanil ...

The reactions of aromatic ketones with radical anions lead to reduction of the ketone and yield an alcohol as the major product. For example, benzophenone produces benzhydrol in 90% yield and p-benzoquinone yields hydroquinone in 92% yield. The reactions of aliphatic ketones, on the other hand, produce alcohol derivatives resulting from addition to the anion. 

To a cold aqueous solution of benzoquinone, add 1 drop of sulphurous acid solution (SOj-water) the solution turns deep green-brown owing to the intermediate formation of quinhydrone, CeH402,CeIl4(0H)2. Now add excess of sulphurous acid the solution becomes colourless owing to the formation of hydroquinone. Add a few drops of FeClj solution the reaction is reversed and the deep yellow colour (distinct from that of FeCl ) is restored. 

Other Methods. A variety of other methods have been studied, including phenol hydroxylation by N2O with HZSM-5 as catalyst (69), selective access to resorcinol from 5-methyloxohexanoate in the presence of Pd/C (70), cyclotrimerization of carbon monoxide and ethylene to form hydroquinone in the presence of rhodium catalysts (71), the electrochemical oxidation of benzene to hydroquinone and -benzoquinone (72), the air oxidation of phenol to catechol in the presence of a stoichiometric CuCl and Cu(0) catalyst (73), and the isomerization of dihydroxybenzenes on HZSM-5 catalysts (74). 

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