Oxidation of alkylbenzenes

The term oxidation includes so many reaction types that space allows only a few to be included here, and those only briefly. Of these, one is a large-scale industrial synthesis, a second is a laboratory method, the third is biological. 

Cumene is the common name for isopropylbenzene its oxidation provides two high-volume industrial chemicals, phenol and acetone, by the reaction sequence shown. 

In the first step, oxygen abstracts a hydrogen atom from the benzylic carbon, setting the stage for a chain reaction that begins when the cumene hydroperoxy radical shown abstracts a benzylic hydrogen from a second molecule of cumene. 

The reaction of cumyl radical with oxygen is a propagation step in the free-radical chain reaction that converts cumene to cumene hydroperoxide. Write an equation for the next step in the chain. Hint What is the hydrogen atom donor ) 

The combination of sodium dichromate and suifuric acid is equivaient to chromic acid. 

On the other hand, an alkyl side chain on a benzene ring is oxidized on being heated with chromic acid. The product is benzoic acid or a substituted derivative of benzoic acid. 

Potassium permanganate (KMn04) is also a strong oxidizing agent and reacts similarly. Under the usual conditions of oxidation, the carboxylic acid product is formed as its potassium salt. A subsequent acidification step converts the salt to the desired acid. 

Potassium o-chlorobenzoate 6 -Chlorobenzoic acid (76-78%) When two alkyl groups are present on the ring, both are oxidized. 

Chromic acid oxidation of 4-ferf-butyi-l,2-dimethyibenzene yielded a single compound having the molecular formula C12H14O4. What was this compound  

What product is expected from chromic acid oxidation of 2,3-dihydroindene  

An alternative oxidizing agent, similar to chromic acid in its reactions with organic compounds, is potassium permanganate (KMn04). 

When two alkyl groups are present on the ring, both are oxidized. 

Side-chain oxidation of alkylbenzenes is important in certain metabohc processes. One way in which the body rids itself of foreign substances is by oxidation in the hver to compounds more easily excreted in the urine. Toluene, for example, is oxidized to benzoic acid by this process and is eliminated rather readily. 

Benzene, with no alkyl side chain, undergoes a different reaction in the presence of these enzymes, which convert it to a substance capable of inducing mutations in DNA. This difference in chemical behavior seems to be responsible for the fact that benzene is carcinogenic but toluene is not. 

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Oxidation (Section 11.13) Oxidation of alkylbenzenes occurs at the benzylic position of the alkyl group and gives a benzoic acid derivative. Oxidizing agents include sodium or potassium dichromate in aqueous sulfuric acid. Potassium permanganate (KMn04) is also an effective oxidant. 

Kopc nski, S. L. Photo-oxidation of alkylbenzene-nitrogen dioxide mixtures in air. Int. J. Air Water Pollut. 8 107-120, 1964. 

In 2003, Velusamy and Punniyamurthy reported on a copper(II)-catalyzed C—H oxidation of alkylbenzenes and cyclohexane to the corresponding ketones with 30% hydrogen peroxide (Scheme 131). The reaction was catalyzed by the copper complex 192a depicted in Scheme 131 and yields were high in the case of alkylbenzenes (82-89%) whereas cyclohexanone was obtained with a low yield of 18%. Chemoselectivity was very high in every case neither aromatic oxidation nor oxidation at another position of the alkyl chain was observed. 

This classification of reactivity can be compared with the values of kp obtained from the apparent oxidizabilities and from the termination constants (Table I). For ethers, the order of reactivity above is the same as the order of kp. Similarly, the rates of oxidation of alkylbenzenes (11, 12) lead to the following classification. 

Toland, W. G., Oxidation of Alkylbenzenes with Sulfur and Water, ... 

Oxidation of alkylbenzene by Co(OAc)3 mainly occurs at the side-chain benzylic positions, without formation of nuclear adducts. In nitrogen, benzylic acetates predominate, whereas aromatic acids are favored when oxygen is present.512 Thus toluene is transformed into benzyl acetate and benzaldehyde by Co(OAc)3 in AcOH under anerobic conditions.553 When this reaction is... 

The photocatalytic oxidation of alkylbenzenes and alkenylbenzenes has been widely reported. The data concerning alkylbenzenes have shown that the reactivity of toluenes is low when compared to other monosubstituted benzenes (Somarani et al., 1995). The effect of adding a zeolite, which is an acid solid catalyst, by Ti02/UV on various 4-substituted toluenes was studied by Somarani et al. (1995). The compounds of interest were prepared at 0.03 M in solutions containing TiOz. The effect of a zeolite was also studied by adding HY-type zeolites with various Si/Al ratios. The solutions were irradiated with a 125-W mercury lamp emitting light at 330 nm. Samples were taken at 48 hours and analyzed by GC/mass spectroscopy (MS) to determine percent conversions of the toluenes to the desired products. 

Lead tetraalkyl derivatives are used in catalytic systems to polymerise olefines, as catalysts of re-etherification and polycondensation, to speed up the alkylation of lateral chains of alkylbenzenes with ethylene and its derivatives. An addition of lead tetraalkyl derivatives (0.05-2% of alkylben-zene quantity) to catalysts of the liquid-phase oxidation of alkylbenzenes speeds up the oxidation. Tetraethyllead proved to be a good initiator for Diels-Alder reactions to join polymers with alkenylsiloxy chains and can be used as an additive to reduce the attrition and wear of rubbing metal parts. Tetrabutyllead is an active cross-linking agent for polyethylene and modifying agent for plastics. 

In contrast to oxidations with Mn(III) acetate, the oxidation of alkylbenzenes by the stronger oxidant, Co(III) acetate, appears to involve only electron transfer. No competition from classical free radical pathways is apparent. Waters and co-workers,239,240 studied the oxidation of a series of alkylbenzenes by Co(III) perchlorate in aqueous acetonitrile. They observed a correlation between the reactivity of the arene and the ionization potential of the hydrocarbon which was compatible with the formation of radical cations in an electron transfer process. 

In the presence of bromide ion there is apparently no direct reaction of Co(III) with the hydrocarbon substrate, in contrast to cobalt-catalyzed autoxidations carried out in the absence of bromide. That different mechanisms are operating is illustrated by the relative rates of oxidation of alkylbenzenes catalyzed by cobalt acetate alone compared to those obtained in the presence of added bromide ion (Table VIII). In the presence of bromide ion, the relative reactivities are consistent with a mechanism involving attack by bromine atoms but not one involving electron transfer. Individual discrepancies in selectivities between bromine atom and the species active in the Co(0 Ac)2-NaBr system (Table VIII) were attributed to a bromine complex,... 

If Stern s and Eberson s mechanisms are combined, the following tentative scheme can be formulated for competing nuclear acetoxylation, side-chain acetoxylation, and oxidative coupling in Pd(II) oxidations of alkylbenzenes (initial attack is shown only for the para position a similar scheme can be written with initial attack at the ortho position) ... 

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