Aromatic oxidation toluene

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. 

The authors proposed that the peroxide is decomposed by the metal catalysts to the ketone and alcohol in a manner similar to that previously reported (30-34). This later system was also reactive toward adamantane (giving a high 3°/2° carbon activation ratio of 3.5) and other saturated alkanes. These catalysts also oxidize toluene at both the aliphatic and aromatic carbons (ratio benzylic/aromatic = 3.4 0.9) (Table IV). Activation of the aromatic ring was attributed to the formation of hydroxyl radicals. 

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. 

The oxidation of aromatic aldehydes to carboxylic acids with manganese dioxide is somewhat surprising because manganese dioxide is used to oxidize toluenes to benzaldehydes. The transformation of benzaldehyde into benzoic acid is achieved in 75% yield by refluxing with manganese dioxide in petroleum ether for 24 h [S75]. 

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. ... 

The degradation schemes of four aromatic hydrocarbons benzene, toluene, /7-xylene and 1,3,5-trimethylbenzene, have been updated on the basis of new kinetic and mechanistic data from current literature and conference proceedings and are available as part of the latest version of the Master Chemical Mechanism (MCMv3.1) via the MCM website thttn //mcm.leeds.ac.uk/MCM). The performance of these schemes concerning ozone formation from tropospheric aromatic oxidation has been evaluated using detailed environmental chamber datasets from the two EU EXACT measurement campaigns at EUPHORE (EXACT I - September 2001 and EXACT II - My 2002 (Pilling et al, 2003)). 

The detailed oxidation mechanisms of aromatic hydrocarbons in the MCM have been revised and updated based on the latest available experimental data and is available via the MCM website ( http //mcm.leeds.ac.uk/MCMl as MCMvS.l. A series of chamber experiments were carried out to investigate the details of key areas of aromatic oxidation mechanisms, and these were primarily focused on toluene oxidation (Bloss et al, 2005b). 

In these oxidations Co(III) is needed to oxidize toluene, whereas more easily oxidized aromatics, p-methoxytoluene and 1-methoxy-naphthalene, react with Mn(III). 

Hydroxylation by the Metal lon—Oxygen Systems. A monosubsti-tuted benzene was suspended in aqueous solution of a metal salt through which oxygen was bubbled. Two aromatic compounds (toluene and anisole) were treated this way with each of four metal salts (ferrous sulfate in the presence of EDTA, titanous chloride, cuprous chloride and stannous pyrophosphate) a third compound (fluorobenzene) was oxidized with the ferrous, titanous, and cuprous systems, and a fourth aromatic compound (nitiobenzene) was treated with ferrous ion with EDTA. The initial concentration of the metal ion was varied. 

Knowledge of the saturation concentrations of the organic condensable species remains incomplete. These concentrations are expected to vary significantly with temperature. The few available relevant measurements include the saturation vapor concentrations of mono-carboxylic and dicarboxylic acids (Tao and McMurry, 1989) and the )3-pinene aerosol products (Pandis et al., 1991). Saturation vapor concentrations of condensable products from the oxidation of some aromatic hydrocarbons (toluene, m-xylene, and 1,3,5-trimethylbenzene) were estimated to lie in the range of 3 to 30 ppt (Seinfeld et al., 1987). 

Fossil-fueled vehicles give rise to emissions of unburned fuel and partially oxidized hydrocarbons [102,106]. Prominent are the BTEX suite of aromatics - benzene, toluene, ethylbenzene, and xylenes. These compounds are ubiquitous in the environment, present in essentially every hive atmosphere we test and often among the most prominent peaks in the chromatogram. To date, it has not been possible to position a bee colony that avoids capture of significant amounts of BTEX. We also detect more biorefractive fuel components in hive air - polycyclic aromatics and biphenyls commonly associated with diesel products [114]. Incompletely burned fuel residuals [102] were also evident as noted in the Oxygenates portion of Table 2.5. These comprised aldehydes, ketones, alcohols, and oxides. 

The use and importance of aromatic compounds in fuels sharply contrasts the limited kinetic data available in the literature, regarding their combustion kinetics and reaction pathways. A number of experimental and modelling studies on benzene [153, 154, 155, 156, 157, 158], toluene [159, 160] and phenol [161] oxidation exist in the literature, but it would still be helpful to have more data on initial product and species concentration profiles to understand or evaluate important reaction paths and to validate detailed mechanisms. The above studies show that phenyl and phenoxy radicals are key intermediates in the gas phase thermal oxidation of aromatics. The formation of the phenyl radical usually involves abstraction of a strong (111 to 114 kcal mof ) aromatic—H bond by the radical pool. These abstraction reactions are often endothermic and usually involve a 6 - 8 kcal mol barrier above the endothermicity but they still occur readily under moderate or high temperature combustion or pyrolysis conditions. The phenoxy radical in aromatic oxidation can result from an exothermic process involving several steps, (i) formation of phenol by OH addition to the aromatic ring with subsequent H or R elimination from the addition site [162] (ii) the phenoxy radical is then easily formed via abstraction of the weak (ca. 86 kcal moT ) phenolic hydrogen atom. 

Gas-phase analysis reveals the evolution of CO2, H2O, and formic acid with recalcitrant benzoate spedes (1604, 1517, 1497, 1454, 1419, 1280, and 1180cm ) found to accumulate at the catalyst surface, which are expected to be the source of catalyst deactivation by blocking active sites (Figure 4.5). Conclusively, the aromaticity of toluene plays a key role in the deactivation. The high stabihty of benzyl radicals favors the photocatalytic oxidation of this volatile organic compound (VOC) and the formation of recaldtrant-oxygenated aromatic molecules that accumulate on the photoactive surface. 

Becker has performed detailed product studies on the OH reaction with toluene and p-xylene. At present, product formation can best be explained by the mechanism shown in Fig. 10 where muconaldehydes (hexa-2,4-diene-1,6-dial) are proposed as direct products of the reaction of the aromatic-OH adduct with O2. Several of these muconaldehydes have been synthesised and their reactions with OH radicals investigated (Becker). These reactions result in the formation of unsaturated 1,4-dicarbonyl compounds, glyoxal, methylglyoxal and maleic anhydride, which have also been observed in the reaction of OH with toluene and p-xylene. It has been demonstrated that the unsaturated 1,4-dicarbonyl species react very rapidly with OH and photolyse, yielding products which possibly accelerate O3 formation in smog chamber type studies. Many of the aromatic oxidation products especially the unsaturated 1,6-dicarbonyl species are known either to be toxic or are potentially toxic with both carcinogenic and mutagenic properties [8]. 

A range of aromatic oxidations involve direct SET from an organic substrate to the oxidant (catalyst, anode), leading to a radical cation [35]. Radical cations are much stronger acids than the parent hydrocarbon molecules [35a, b]. For example, the of toluene drops from 41 to ca. -13 with the removal of one electron, which makes deprotonation the predominant process in the transformation of the radical cation. Benzyl radicals formed in this way dimerize and participate in the side-chain oxidation (Scheme 14.5). On the other hand, radical cation can undergo attack by nucleophiles (H O, AcOH, etc.) followed by the second FT leading to the ring oxidation products [36]. 

Aerobic oxidation of aromatics (benzene, toluene, chlorobenzene) at room temperature in the presence of zinc powder as reducing agent and a p-oxo binuclear iron complex as catalyst in CH Clj led to phenols as major products [63]. Co-oxidation of benzene and crotonaldehyde catalyzed by VO(dpm)j [dmp = l,3-bis(p-methoxyphenyl)-l,3-propanedionato] gave phenol in a 21% yield (Eq. 14.19) [64]. 

Other molecules present in the gas mixtures like alcohols or ketones may have a moderate effect on aromatic oxidation but the reverse (inhibition of alcohol oxidation by aromatics) is most often observed. Different supports of Pt were used for toluene oxidation Al203, Al203/Al, Zn0/Al203, Ti02, mesoporous fibrous silica or monoliths. Zeolites, generally promoted by platinum, were shown to give excellent catalysts for aromatic oxidation. Basic zeolites showed excellent performances in oxidation of m-xylene even in the absence of platinum. Palladium catalysts, either supported on alumina or ceria-alumina, were also investigated for oxidation of benzene and several alkylbenzenes. ... 

Dinitroresorcine (DNR) forms two isomers 2,4 and 4,6. Unlike in the nitration of some other aromatic molecules (toluene, phenol), it is possible to prepare practically pure dinitro isomers. The position of nitro groups in the ring depends on the reaction conditimis. The 2,4-isomer of DNR can be easily prepared by dinitrosatimi of resorcinol followed by alkaline oxidation of 2,4-dinitrosoresorcinol [8,14]. 2,4-DNR cannot be prepared by sulfonation of resorcinol followed by reaction with nitric acid (method used for phenol) because this method yields the trinitro compound. The 4,6-DNR isomer can be prepared in two ways (a) by nitration of 4,6-diacetylresorcinol and (b) directly by nitration of resorcinol using 98 % nitric acid at low temperatures (between —20 and —15 °C) [8],... 

In the absence of propane, the interaction between methane and zeolite Zn/HBEA yields methylzinc (ZnCHj) and methoxide (ZnOCHj) species and formate fragments, which undergo further conversion into acetaldehyde and acetic acid (Fig. 29D). In the presence of benzene, only the formation of the methoxide ZnOCH is observed, which is apparently not oxidized by oxygen of the defected ZnO structure (Fig. 29E). At 823 K, benzene is methylated by ZnOCHj, yielding methyl-substituted aromatics, namely, toluene and xylenes (Fig. 29F). It was thereby found that methane participated in the methane-propane co-aromatization reaction hy alkylating the aromatic compounds that resulted from propane, as is illustrated hy Scheme 7. 

Aromatic hydrocarbons Toluene 0 Hw-CrHt- -4H, Low 1020 Chromic, molybdic, etc., oxides... 

Oxidation of a side chain by alkaline permanganate. Aromatic hydrocarbons containing side chains may be oxidised to the corresponding acids the results are generally satisfactory for compounds with one side chain e.g., toluene or ethylbenzene -> benzoic acid nitrotoluene -> nitrobenzoic acid) or with two side chains e.g., o-xylene -> phthalic acid). 

Oxidation of side chains. The oxidation of halogenated toluenes and similar compounds and of compounds with side chains of the type —CHjCl and —CH OH proceeds comparatively smoothly with alkaline permanganate solution (for experimental details, see under AromcUic Hydrocarbons, Section IV.9,6 or under Aromatic Ethers, Section IV,106). The resulting acid may be identified by a m.p. determination and by other teats (see Section IV,175). 

Nitrations can be performed in homogeneous media, using tetramethylene sulfone or nitromethane (nitroethane) as solvent. A large variety of aromatic compounds have been nitrated with nitronium salts in excellent yields in nonaqueous media. Sensitive compounds, otherwise easily hydroly2ed or oxidized by nitric acid, can be nitrated without secondary effects. Nitration of aromatic compounds is considered an irreversible reaction. However, the reversibihty of the reaction has been demonstrated in some cases, eg, 9-nitroanthracene, as well as pentamethylnitrobenzene transnitrate benzene, toluene, and mesitylene in the presence of superacids (158) (see Nitration). 

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