Chromic acid oxidation phenols

Phenols undergo oxidation, but they give different types of products from those seen with aliphatic alcohols. Chromic acid oxidation of a phenol gives a conjugated 1,4-diketone called a quinone. In the presence of air, many phenols slowly autoxidize to dark mixtures containing quinones. 

In small-scale syntheses, a wide variety of oxidants have been employed in the preparation of quinones from phenols. Of these reagents, chromic acid, ferric ion, and silver oxide show outstanding usefulness in the oxidation of hydroquinones. Thallium (ITT) triduoroacetate converts 4-halo- or 4-/ f2 -butylphenols to l,4-ben2oquinones in high yield (110). For example, 2-bromo-3-methyl-5-/-butyl-l,4-ben2oquinone [25441-20-3] (107) has been made by this route. 

The phenanthrene 1,2- and 3,4-diones are synthetically accessible from the related 8 phenols. Oxidation of 2-phenanthrol with either Fremy s salt ((KS0 )2N0) or phenylseleninic anhydride gave phenanthrene 1,2-dione directly (55). Unexpectedly, oxidation of 3-phenanthrol with (KSOg NO yielded 2,2-dihydroxybenz(e)indan-l,3-d-ione (Figure 10). However, phenanthrene 3,4-dione was readily obtained from 3-phenanthrol by Fieser s method entailing diazonium coupling, reduction, and oxidation of the resulting 4-amino-3-phen-anthrol with chromic acid (56). 

Oxidation of phenol with chromic acid produces a conjugated dlketone known as benzoquinone. In the presence of air, phenols are slowly oxidised to dark coloured mixtures containing quinones. 

Creating acidic groups (carboxylic acids and phenol groups) on the internal soot surface by chemical oxidation with, e.g., chromic acid, fixation of cationic complexes of the catalyst (e.g., amino complexes of Pt(II) or Pt(IV) at the inner surface of the soot particles by ion exchange, followed by chemical or electrochemical reduction of these complexes... 

Phenols are more easily oxidized than alcohols, and a large number of inorganic oxidizing agents have been used for this purpose. The phenol oxidations that are of the most use to the organic chemist are those involving derivatives of 1,2-benzenediol (pyrocate-chol) and 1,4-benzenediol (hydroquinone). Oxidation of compounds of this type with silver oxide or with chromic acid yields conjugated dicarbonyl compounds called quinones. 

Chromic acid test. This test is able to distinguish primary and secondary alcohols from tertiary alcohols. Using acidified dichromate solution, primary alcohols are oxidized to carboxylic acids secondary alcohols are oxidized to ketones tertiary alcohols are not oxidized. (Note that in those alcohols which are oxidized, the carbon that has the hydroxyl group loses a hydrogen.) In the oxidation, the brown-red color of the chromic acid changes to a blue-green solution. Phenols are oxidized to nondescript brown tarry masses. (Aldehydes are also oxidized under these conditions to carboxylic acids, but ketones remain intact see Experiment 31 for further discussion.)... 

Various adhesives can be used to bond polyphenylene oxide to itself or to other substrates. Parts must be prepared by sanding or by chromic acid etching at elevated temperature. Methyl alcohol is a suitable solvent for surface cleaning. The prime adhesive candidates are epoxies, modified epoxies, nitrile phenolics, and polyurethanes. Epoxy adhesive will provide tensile shear strength on abraded polyphenylene oxide substrates of 600 to 1300 psi and 1300 to 2200 psi on etched (chromic acid) substrates.71... 

Phenols are rather easily oxidized despite the absence of a hydrogen atom on the hydroxylbearing carbon. Among the coloured products from the oxidation of phenol by chromic acid is the dicarbonyl compound p-benzoquinone (also known as 1,4-benzoquinone or simply quinone). Dihydroxybenzenes, hydroquinone (7.30) and catechol (7.32) are oxidized to p-benzoquinone (7.31) and o-benzoquinone (7.33), respectively, by milder oxidants such as Jones reagent. Fremy s radical (7.34) is an excellent and very specific oxidizing agent for the oxidation of phenols to o- or p-benzoquinones. (m-Quinones do not exist.)... 

Oxidation of aromatic systems containing alkyl side-chains results in the formation of a carboxylic acid, irrespective of the length of the side-chain. The usual oxidizing agents are potassium permanganate [potassium manganate(VII)] or chromic acid [chromium(VI) acid]. For example, 1,4-dimethylbenzene is oxidized to benzene-1,4-dicarboxylic acid (tereph-thalic acid, 9), an important building block for polyesters. The oxidation of isopropylbenzene (cumene) to phenol is an important industrial process and is discussed in Chapter 4. 

Quinones can he prepared hy the oxidation of phenols, dihydroxy-henzenes, dimethoxyhenzenes and anilines. For example, 1,4-dihydroxy-henzene (hydroquinone) can he oxidized in good yield using sodium chlorate in dilute sulfuric acid in the presence of vanadium pentoxide and also hy manganese dioxide and sulfuric acid and hy chromic acid. Other reagents which convert hydroquinones to quinones include Fremy s salt [potassium nitrosodisulfonate, (KS03)2N0] and cerium(IV) ammonium nitrate [CAN, Ce(NH4)2(N03)J. 

Phenols are quite sensitive to oxidation. On the one hand, they are easily oxidized to quinols and on further oxidation with, for example, iron(III) chloride, chromic acid or silver(I) oxide give p-quinones. However, under one-electron transfer conditions the phenoxide anion is oxidized to the phenoxyl radical. This shows free radical reactivity on the oxygen atom and at the ortho and para positions (Scheme 4.20a). The phenoxyl radical may readily dimerize. This is exemplified by the formation of Pummerer s ketone from p-cresol (Scheme 4.20b). 

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