Methyl aromatics, oxidation

The purpose of the present paper is to offer a contribute to the understanding of the mechanisms of these reactions by using an IR spectroscopic method and well-characterized "monolayer" type vanadia-titania (anatase) as the catalyst. We will focus our paper in particular on the following subjects i) the nature of the activation step of the methyl-aromatic hydrocarbon ii) the mechanism of formation of maleic anhydride as a by-product of o-xylene synthesis iii) the main routes of formation of carbon oxides upon methyl-aromatic oxidation and ammoxidation iv) the nature of the first N-containing intermediates in the ammoxidation routes. 

Numerous diamines and aromatic dianhydrides have been investigated. WhoUy aromatic Pis have been stmctiirally modified by incorporating various functional groups, such as ether, carbonyl, sulfide, sulfone, methylene, isopropjlidene, perfluoroisopropyUdene, bipyridyls, sdoxane, methyl phosphine oxide, or various combinations of these, into the polymer backbone to achieve improved properties. The chemistry and apphcations of Pis have been described in several review articles (4). 

This ladical-geneiating reaction has been used in synthetic apphcations, eg, aioyloxylation of olefins and aromatics, oxidation of alcohols to aldehydes, etc (52,187). Only alkyl radicals, R-, are produced from aliphatic diacyl peroxides, since decarboxylation occurs during or very shortiy after oxygen—oxygen bond scission in the transition state (187,188,199). For example, diacetyl peroxide is well known as a source of methyl radicals (206). 

Photolysis of pyridazine IV-oxide and alkylated pyridazine IV-oxides results in deoxygenation. When this is carried out in the presence of aromatic or methylated aromatic solvents or cyclohexane, the corresponding phenols, hydroxymethyl derivatives or cyclohexanol are formed in addition to pyridazines. In the presence of cyclohexene, cyclohexene oxide and cyclohexanone are generated. 

Methylated aromatic heterocycles (HetCHj) form cation-radicals that are typical n acids and expel a proton. Methylene radicals are formed. These radicals give rise to the corresponding carbocations if an oxidant was taken in excess. Nucleophiles attack the ions, completing the reaction. If water is the reaction medium (the hydroxyl anion is a nucleophile), an alcohol is formed. The alcohol rapidly transforms into an aldehyde on the action of the same oxidant. 

Selective and Nonselective Pathways in Oxidation and Ammoxidation of Methyl-Aromatic Compounds over Vanadia—Titania... 

These features provide evidence for a reaction path, common to the four methyl-aromatics under study, that involves the following steps i) activation of the methyl-aromatic in the form of a benzyl species ii) reaction of this species with the catalyst surface giving the corresponding aldehyde, probably with the intermediacy of benzy-loxy- species, as discussed previously (13) iii) oxidation of the aldehyde by the catalyst surface to give the carboxylate species iv) decarboxylation of the carboxylate species giving COj, and, likely, a demethylated hydrocarbon (toluene from xylenes and benzene from toluene). All these successive processes occur both with and without gas phase oxygen, that, consequently, is not active in them. 

Methyl nitrite (CH3ONO) MNIT High-molecular-weight aromatic oxidation ... 

V-Methyl aromatic amines can suffer oxidation by PDC, giving an immonium ion that can be trapped intramolecularly by a neighbouring alcohol. 

The structure and numbering system for pyridine are given in Section 11.21, where we are also told that pyridine is aromatic. Oxidation of 3-methylpyridine is analogous to oxidation of toluene. The methyl side chain is oxidized to a carboxylic acid. 

Esters of 3-aryl-4,5-dihydroisothiazole-4- and -5-carboxylic acids are unstable and aromatize in acetone solution (78JOC3742). The 1-methyl 1-oxide compound (157) is converted into 1,2-benzisothiazole on heating at 155 °C (80MI41700). 1-Methyl-3H-2,1-benzisothiazole 2,2-dioxide (158) loses sulfur dioxide on pyrolysis to give compound (159) (80CC471), and condenses with aromatic aldehydes to give compounds (160 Scheme 25) <75SST(3)54l). 

The oxidation of / -xylene to terephthalic acid is by far the most important process based on the oxidation of methyl aromatics. However other similar processes are also operated industrially and oxidize toluene to benzoic acid or m-xylene to isophthalic acid. The latter is used as comonomer with terephthalic acid in bottles for carbonated drinks, and for special polyesters, and its production is roughly 2% of that of the terephthalic derivatives. 

In the presence of dioxygen, the carbon radical R- produced by reactions (201) and (202) ar transformed into alkylperoxy radicals ROO, reacts with Co or Mn species to regenerate th Co " or Mn " oxidants, and produce primary oxygenated products (alcohol, carbonyl compounds which can be further oxidized to carboxylic acids. This constitutes the basis of several Industrie processes such as the manganese-catalyzed oxidation of n-alkenes to fatty acids, and the cobal catalyzed oxidation of butane (or naphtha) to acetic acid, cyclohexane to cyclohexanol-on mixture, and methyl aromatic compounds (toluene, xylene) to the corresponding aromatic monc or di-carboxylic acids. ... 

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