Benzoquinone para-quinone

Quercetin (see also Bracken fern) para-Quinone (see also 1,4-Benzoquinone) Quintozene... 

Quinone and Phloroglucinol.—Benzoquinone or quinone (p. 636) is considered a di-ketone derivative of benzene because of its relation to hydroquinol or para-di-hydroxy benzene. It may also be considered as an oxygen derivative of di-hydro benzene. 

Conversion to ortho- and para-quinones is by far the most common oxidation of phenols. Mercuric oxide or mercuric trifluoroacetate [583], lead dioxide [430], chromium trioxide [559], bromine [732], 2,3 dichloro-5,6-dicyano-/> benzoquinone (DDQ) [977], Fremy salt [487, 488, 489], and hydrogen peroxide in the presence of horseradish peroxidase [7958] are the most widely used oxidants (equations 310 and 311). 

Since a2 + b2 > 2ab for all positive values of a and b, except a = b 0, we conclude that S, 3 (ortho) > S 3 (para). Consequently, the pi LUMO of ortho benzo-quinone will be lower in energy than the pi LUMO of para benzoquinone. Furthermore, the degeneracy of the oxygen lone pair AO is lifted more in the case of ortho benzoquinone. The final conclusion is that the mr transition is predicted to occur at longer wavelengths in the ortho isomer relative to the para isomer. 

Benzoquinone[Quinone o-(or)p-Dioxybenzene Benzenone or Dihydrodiketobenzene], CgH402 mw 108.09, O 29-60%. This compd exists as the ortho and para isomers l,2(or o)-Benzoquinone... 

Reaction XCIII. Oxidation of Primary Aromatic Amines and their para-substituted Derivatives to Quinones. (A., 27, 268 194, 202 211, 49 215, 125 B., 19, 1467 20, 2283 25, 982 36, 4390.)—Many primary aromatic amines, when oxidised with chromic acid, readily yield p-quinones, aniline, for example, giving p-benzoquinone. 

In 1987, Kita and co-workers first developed a general and high yielding (59% quant.) route to p-benzoquinone monoacetals (2a) and spirolactones from para-substituted phenols (la) with PIFA in MeCN in the presence of alcohols (R"OH) [27]. Similar methods for preparing quinone monoacetals and quinol ethers have been developed independently by Lewis et al. (PIDA/CH2C12-R"OH (11-65% yields)) [28] and Pelter et al. (PIDA/R"OH (65-99% yields)) [29] [Eq.(l)]. 

Though extension of the n electron system in a homologous series has, in general, a bathochromic effect, there are some exceptions e.g. the series para-benzoquinone, naphtha-1,4-qui-none and anthra-9,io-quinone, where a hypsochromic effect is observed. 

V.D). When the electron density in the ring is high (as in polyalkyl phenols) and the ortho- and/or para position (with respect to the OH group) is vacant, the formation of ortho- or para-benzoquinone also occurs. Indeed, in the hydroxylation of phenol to catechol and hydroquinone, one of the major side products (and the main cause of the tar formation) is the formation of benzo-quinones and products derived from them. The benzoquinones of polyalkyl-benzenes are starting materials for many products in the photographic and fine chemicals industries. Trukhan et al. 234) reported the oxidation of 2,3,-... 

A cobalt complex salcomine (7.39) oxidizes substituted phenols but unsubstituted at para position, such as 2,6-di-fert-butylphenol (7.40), to give the corresponding p-quinone, 2,6-di- tert-butyl-p-benzoquinone (7.41). 

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. ... 

Benzo-quinone.—Benzoquinone, or more commonly, simply quinone, is the most common and important of the quinones derived from benzene. Other important quinones will be met with when we study derivatives of the more complex hydrocarbons napthalene and anthracene. Benzoquinone is the one we have used as our example in the above discussion and it is the para-di-keto benzene. It was first obtained by the oxidation of quinic acid, which in turn was obtained from quinine, hence its name. It may also be prepared by oxidizing... 

Ortho-quinone.—Corresponding to the common benzoquinone which is the para compound, there are known derivatives of the ortho-benzoquinone. In this compound, as represented by the ketone formula, no oscillation of the double bonds of the benzene ring is necessary. 

Quinone Oximes.—The most interesting of the derivatives of quinones are the oximes. As stated in the discussion of the ketone structure for quinones one of the proofs for this constitution is the fact that benzoquinone forms both a mono-and a di-oxime when treated with hydroxyl-amine. The mono- oxime of benzoquinone would have the structure as written below and as given on page 638. Now as previously mentioned, (p. 628), para-nitroso phenol, which is made by the action of nitrous acid upon phenol and the constitution of which is established by other methods of synthesis, (p. 627), proves to be one and the same compound with this mono-oxine of para-benzoquinone, the constitution of which is likewise established by the above reaction of hydroxyl amine upon quinone. This is explained by a rearrangement as shown in the following ,... 

It will be recalled that when para-di-hydroxy benzene, hydro-quinol, and also other para di-substitution products of benzene are oxidized there is obtained the compound known as quinone or benzo-quinone (p. 636). In a similar way para di-suhstituied naphthalenesy especially hydroxyl and amino compounds, and in fact naphthalene itself when oxidized with chromic acid, yield a quinone analogous to benzoquinone and which is known as alpha-naphthoquinone. 

Simple phenols (574, 575, 576 and 247) were treated with Fremy s salt in acetone and buffer solution to afford the corresponding benzoquinones (577, 578, 579 and 248) in 53, 60, 58 and 87% yields, respectively. In the cases of both 576 and 247, the initially formed phenoxy radicals were attacked by the second Fremy s salt at the ortho- and para-positions, respectively. The resulting radical coupled products were further converted into the corresponding quinones (Scheme 107). From these data, the product selectivity seems to be due to the finely balanced situation of electronic and steric factors - . ... 

Hydroquinone, a /tara-benzenediol, is easily oxidized to para-benzoquinone. Although a wide variety of oxidizing agents can be used, Fremy s salt (dipotassium ni-trosodisulfonate) is the preferred oxidizing agent. The quinone can easily be reduced back to hydroquinone. 

The main absorption band of benzoquinones appears around 260 nm in nonpolar solvents and at 280 nm iu water. Extinction coefficients are 1.3-1.5 x 10 M Upon reduction to hydroquinones, a four times smaller band at 290 nm is found. The most important property of quinones and related molecules is the relative stability of their one-electron reduction products, the semiquinone radicals. The parent compound 1,4-benzoquinone is reduced by FeCl, ascorbic acid, and many other reductants to the semiquinone anion radical which becomes protonated in aqueous media (pk = 5.1). Comparisons of the benzaldehyde reduction potential with some of the model quinones given below show that carbonyl anion radicals are much stronger reductants than semiquinone radicals and that ortho- and para-benzoquinones themselves are even relatively strong oxidants comparable to iron(III) ions in water (Table 7.2.1). This is presumably caused by the repulsive interactions between two electropositive keto oxygen atms, which are separated only by a carbon-carbon double bond. When this positive charge can be distributed into neighboring n systems, the oxidation potential drops significantly (Lenaz, 1985). 

In the presence of para-benzoquinone the reaction does not begin until a certain time h after which the rate has its normal value. Table 2 gives values of the time ti for several values of w the mass of quinone added to 13 cm of vinyl acetate. 

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