1.4- Dimethylbenzene oxide

Dihydrodiols have not been obtained from hydrolysis reactions of benzene oxides however, evidence for the formation of 1,4-dihydrodiols has been obtained from acid-catalysis studies on 1,4-dimethylbenzene oxide 63 ° and 8,9-indan oxide 7. The mechanism of acid-catalyzed rearrangement and hydrolysis of benzene oxide has been investigated theoretically using the semiempirical allvalence electron MINDO/3 method and the perturbational MO method, both from the viewpoint of product stabilities and reaction pathways. The aromatization reaction was found to be much more exothermic than the hydrolysis and thus would be the preferred reaction, as was found experimentally. 

Based on the add-catalyzed addition of methanol to 1,4-dimethylbenzene oxide which gave 4-methoxy-l,4-dimethyl-2,5-cyclohexadienol (173), Anderson and Faulkner (5) proposed that an analogous 1,4-addition of solvent to an arene oxide 286 could result in the formation of dienone 1 or the corresponding ketal 3 (5). Although arene oxides have been proposed as intermediates in the biosynthetic oxidation of aromatic compounds, there is no evidence of their existence as natural products. The coexistence of the lactone aeroplysinin-2 (18) with dienone 1 and the ketal 3 suggested that the imino-ether 287 should also be considered as a possible precursor of these alkaloids. 

The same trend is continued for the 1,2-dimethylbenzene oxide (156)-2,7-dimethyloxepin (157) system.8 The oxepin is so strongly stabilized that the... 

Dimethoxycarbonylbenzene oxide (98) exists in equilibrium with the oxepin form (159), the latter predominating at equilibrium at ambient temperature.8 The NMR spectrum shows temperature dependence, indicating that a considerable amount of the oxide tautomer exists at equilibrium. 4,5-Dimethylbenzene oxide (160) also exists in equilibrium with its corresponding oxepin tautomer (161). ... 

Chromic acid oxidation of 4 tert butyl 1 2 dimethylbenzene... 

Anthraquinone itself is traditionally available from the anthracene of coal tar by oxidation, often with chromic acid or nitric acid a more modern alternative method is that of air oxidation using vanadium(V) oxide as catalyst. Anthraquinone is also produced in the reaction of benzene with benzene-1,2-dicarboxylic anhydride (6.4 phthalic anhydride) using a Lewis acid catalyst, typically aluminium chloride. This Friedel-Crafts acylation gives o-benzoylbenzoic acid (6.5) which undergoes cyclodehydration when heated in concentrated sulphuric acid (Scheme 6.2). Phthalic anhydride is readily available from naphthalene or from 1,2-dimethylbenzene (o-xylene) by catalytic air oxidation. 

Benzenetetracarboxylic dianhydride (pyromellitic dianhydride) is a typical bifunctional acid anhydride, and it is a useful raw material for preparing many useful chemicals. Polyimides and polyimidazopyrrolons prepared from this dianhydride have excellent heat and chemical resistance, as well as excellent mechanical and electrical properties. Pyromellitic dianhydride is produced by the oxidation of 1,2,4,5-tetraalkylbenzenes such as 1,2,4,5-tetramethylbenzene (commonly known as durene) and 4,6-diisopro-pyl-l,3-dimethylbenzene. Durene, in particular, is a fundamental raw material for the production of the dianhydride 1-8). 

Aromatic hydrocarbons which have methyl side chains mainly behave like toluene and form aldehydes, while combustion is stimulated and selective oxidation of the nucleus is repressed. The oxidation of methyl-naphthalene, for example, exhibits a low selectivity with respect to phtha-lic anhydride formation, combustion and maleic acid formation being the dominating reactions. Durene is a special case because it resembles o-xy-lene. The oxidation of durene over a V—W—O catalyst at 420° C is reported to produce pyromellitic dianhydride, phthalic and maleic anhydride, although combustion dominates (Geiman et al. [122]). 1,2,4-Trimethyl-benzene yields dimethylbenzene and trimellitic acid if oxidized on a Sn— V—O catalyst. Kinetic data have been measured by Balsubramanian and Viswanath [37]. 

Both anhydrides are prepared in this manner on a large scale for use in the production of ester polymers (Section 29-5A). Phthalic anhydride also is prepared by the oxidation of 1,2-dimethylbenzene ... 

In the case of 1,4-dimethylbenzene epoxide (181), incursion of intermediates like l,4-dimethyl-l,4-dihydroxydihydrobenzene (182)91,92 or intermediates like 4,8- and 4,5-indane oxides (183 and 184), respectively, by oxygen walk8,93-96 does not allow prediction of the product of epoxide opening on... 

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. 

Carbonium ions can be generated at a variety of oxidation levels. The alkyl carbocation can be generated from alkyl halides by reaction with a Lewis acid (RCl + AICI3) or by protonation of alcohols or alkenes. The reaction of an alkyl halide and aluminium trichloride with an aromatic ring is known as the Friedel-Crafts alkylation. The order of stability of a carbocation is tertiary > secondary > primary. Since many alkylation processes are slower than rearrangements, a secondary or tertiary carbocation may be formed before aromatic substitution occurs. Alkylation of benzene with 1-chloropropane in the presence of aluminium trichloride at 35 °C for 5 hours gave a 2 3 mixture of n- and isopropylbenzene (Scheme 4.5). Since the alkylbenzenes such as toluene and the xylenes (dimethylbenzenes) are more electron rich than benzene itself, it is difficult to prevent polysubsiitution and consequently mixtures of polyalkylated benzenes may be obtained. On the other hand, nitro compounds are sufficiently deactivated for the reaction to be unsuccessful. 

Solvothermal reaction (thermal decomposition) of metal aUcoxides in toluene usually yields the corresponding metal oxides (see Section III.B.l). Aromatic hydrocarbons are favored for this reaction over aliphatic hydrocarbons because of the higher solubility of the precursors in the former than those in the latter. Xylenes (dimethylbenzenes) are also suitable solvents for the solvothermal synthesis. 

The results of pyrolysis of polypropylene in air depends on the pyrolysis heating rate because the pyrolysis process competes with the oxidation [108], By heating between 120° C and 280° C in air, polypropylene is reported to generate ethene, ethane, propene, propane, isobutene, butane, isobutane, pentadiene, 2-methyl-1-pentene, 2,4-dimethyl-1-pentene, 5-methyl-1-heptene, dimethylbenzene, methanol, ethanol, 2-methyl-2-propene-1-ol, 2-methylfuran, 2,5-dimethylfuran, formaldehyde, acetaldehyde, acrolein, propanal, methacrolein, 2-methylpropanal, butanal, 2-vinylcrotonaldehyde, 3-methylpentanal, 3-methylhexanal, octanal, nonanal, decanal, ethenone, acetone, 3-buten-2-one, 2-butanone, 1-hydroxy-2-propanone, 1-cyclopropylethanone, 3-methyl-2-buten-2-one, 3-penten-2-one, 2-pentanone, 2,3-butanedione [109]. 

PROBLEM 11.7 Chromic acid oxidation of 4-tert-butyl-1,2-dimethylbenzene yielded a single compound having the molecular formula C12H14O4. What was this compound J... 

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