Oxidation alkylaromatics

An additional curious feature of alkylaromatic oxidation is that, under conditions where the initial attack involves electron transfer, the relative rate of attack on different alkyl groups attached to the same aromatic ring is quite different from that observed in alkane oxidation. For example, the oxidation of -cymene can lead to high yields of -isopropylbenzoic acid (2,205,297,298). 

Table 1. Phase transfer catalysis in alkylaromatic oxidation. 

Zaidi (ref. 28) has reported the autoxidation of cyclohexane in acetic acid, at 60-80 °C and 1 bar, in the presence of a Co(OAc)2/NaBr catalyst (4). Adipic acid was obtained in 31% yield. Based on the results obtained in alkylaromatic oxidations it would be interesting to try the Co/Mn/Br /HOAc system in cyclohexane oxidation. It is, however, difficult to believe that this has not already been done. 

Apart from their proven activity in alkylaromatic oxidations [11] we have also found they possess some activity in the oxidation of tetralin (Figure 7). The oxidation of tetralin is typically carried out at very high temperatures using unstable metal forms of ion exchange resins. 

Studies in this field are just beginning, and the number of publications hardly exceeds a dozen. The most interesting results were obtained by the research groups of Yamada [160-162], Neumann [163,164] and Kozhevnikov [165, 166], Using various type catalysts (Ru porphyrene complexes, polyoxometalates, supported metals), the authors conducted selective oxidations of various types. These include epoxidation of alkenes, oxidation of alcohols, oxidation of alkylaromatics, oxidation and aromatiza-tion of dihydroanthracenes, and some other reactions. The experiments were typically conducted at 373—423 K under 1.0 MPa pressure of nitrous oxide. 

The rate of oxidation of cyclohexane by Co(III) acetate in acetic acid is enhanced in the presence of bromide ions.265 By analogy with alkylaromatic oxidations (see Section II.B.3.b), these reactions probably involve chain transfer by bromine atoms [cf. Eqs. (20l)-(204)]. 

Numerous reports of heterogeneous catalysis active for alkylaromatic oxidations have appeared. These include an encapsulation of metal ions by zeolites or polymers [92-95]. Non-Co, Pd-based heterogeneous catalysts have been discovered by BP researchers [96-98]. Very recently, nanocrystalline ceria (Ce02) has been discovered to be a highly active heterogeneous catalyst for oxidation of pX in water to TA [99,100]. 

The scope of oxidation chemistry is enormous and embraces a wide range of reactions and processes. This article provides a brief introduction to the homogeneous free-radical oxidations of paraffinic and alkylaromatic hydrocarbons. Heterogeneous catalysis, biochemical and hiomimetic oxidations, oxidations of unsaturates, anodic oxidations, etc, even if used to illustrate specific points, are arbitrarily outside the purview of this article. There are, even so, many unifying features among these areas. 

The simple hydroperoxide mechanism so far discussed is incomplete for representing reactions with significant products other than hydroperoxides. It can be adequate for oxidations of certain unsaturates, aldehydes, and alkylaromatics where the yield of the corresponding hydroperoxide can exceed 90%. 

The aromatic core or framework of many aromatic compounds is relatively resistant to alkylperoxy radicals and inert under the usual autoxidation conditions (2). Consequentiy, even somewhat exotic aromatic acids are resistant to further oxidation this makes it possible to consider alkylaromatic LPO as a selective means of producing fine chemicals (206). Such products may include multifimctional aromatic acids, acids with fused rings, acids with rings linked by carbon—carbon bonds, or through ether, carbonyl, or other linkages (279—287). The products may even be phenoUc if the phenoUc hydroxyl is first esterified (288,289). 

As was noted by Jones (ref. 12) the success of a metal bromide as a catalyst for alkylaromatic autoxidations depends on the ability of the metal to transfer rapidly and efficiently oxidizing power from various autoxidation intermediates onto bromide ion in a manner which generates Br-. The fact that no free bromine is observable in this system is consistent with rapid reaction of intermediate bromine atoms with the substrate. Inhibition of the reaction by cupric salts can be explained by the rapid removal of Br2 or ArCH2- via one-electron oxidation by Cu (Fig. 10). 

As noted earlier, the Amoco catalyst system has been applied to the autoxidation of a wide variety of, mainly methylaromatic, substrates (ref. 19). It has also been applied to the oxidation of other alkylaromatics, e.g. the oxidation of m-phenoxyethylbenzene to the pharmaceutical intermediate, m-phenoxyaceto-phenone (2). 

In the presence of anunonium bromide cobalt (ref. 22) and manganese (ref. 23) have been shown to catalyze the ammoxidation of methylaromatics to the corresponding aromatic nitriles (Fig. 20). It is interesting to compare this homogeneous, liquid phase system with the more well-known vapour phase ammoxidation of alkylaromatics over oxidic catalysts (ref. 4). 

Hydroperoxides were proved to be the only primary molecular product of the oxidation of aliphatic and alkylaromatic hydrocarbons [79-84], When the hydrocarbon is oxidized under mild conditions, in which the formed hydroperoxide is a stable product, the amount of produced ROOH was found to be nearly equal to the amount of consumed dioxygen [45,80,82],... 

VF Tsepalov. The Study of Elementary Reactions of Liquid-Phase Oxidation of Alkylaromatic Hydrocarbons. Doctoral Thesis Dissertation, Institute of Chemical Physics, Chernogolovka, 1975, pp 3-40 [in Russian],... 

When alkylaromatic hydrocarbon is oxidized, acids catalyze the decomposition of hydroperoxide with production of phenolic compounds [57,58],... 

In addition, phenols are formed by the reaction of hydroxyl radical addition to the aromatic ring of oxidized alkylaromatic hydrocarbon [56]. 

The addition of hydroxyl radicals to benzene ring of alkylaromatic hydrocarbon gives phenolic compounds. Phenols retard oxidation, terminating the chains (see Part II). 

Catalysis by nitroxyl radicals in hydrocarbon oxidation was discovered and studied recently [82-89], The introduction of /V-hydroxyphthalimide into oxidized alkylaromatic hydrocarbon was found to accelerate the oxidation. The formation of the stable phthalimide-/V-oxyl (PINO) radical was evidenced by the EPR method [90]. The following kinetic scheme was put forward to explain the accelerating effect of PINO on the chain oxidation of hydrocarbons [82-84]. 

Aldehydes do not co-oxidize alkanes due to a huge difference in the reactivity of these two classes of organic compounds. Alkanes are almost inert to oxidation at room temperature and can be treated as inert solvents toward oxidized aldehydes [35]. Olefins and alkylaromatic hydrocarbons are co-oxidized with aldehydes. The addition of alkylaromatic hydrocarbon (R2H) to benzaldehyde (R1H) retards the rate of the initiated oxidation [36-39]. The rate of co-oxidation obeys the equation [37] ... 

Alkylaromatic hydrocarbons, such as tetralin, ethylbenzene, and cumene, are oxidized in a solution of acetic acid in the presence of cobalt acetate by a different mechanism. In these... 

As alkylaromatic hydrocarbon (toluene, p-xylene, etc.) is oxidized, aldehydes appear radicals and peracids formed from them play an important role. First, aldehydes react rapidly with the Co3+ and Mn3+ ions, which intensifies oxidation. Second, acylperoxyl radicals formed from aldehydes are very reactive and rapidly react with the initial hydrocarbon. Third, aldehydes form an adduct with primary hydroperoxide, which decomposes to form aldehyde and acid. 

This catalyst makes the increase in the oxidation rate of alkylaromatic hydrocarbons possible due to the intense participation of the catalyst itself (Co2+, Co3+, Br, and Br ) in chain propagation. 

Finally, it makes possible the oxidation of hydrocarbon to a significant depth, and when the RH molecule contains several methyl groups, the catalyst allows all these groups to be transformed into carboxyls. This last specific feature is insufficiently studied so far. Perhaps, it is associated with the following specific features of oxidation of alkylaromatic hydrocarbons. The thermal decomposition of formed hydroperoxide affords hydroxyl radicals, which give phenols after their addition at the aromatic ring... 

In addition, alkylaromatic hydroperoxide ArCH2OOH under the action of acid is heterolytically transformed into phenol and formaldehyde. Phenols are accumulated and retard the oxidation process at early stages when the amount of methylcarboxylic acids (intermediate products) is low and they have no time to be oxidized further. In the... 

E0 = 40 kJ mol-1 at AH=0) is substituted by a few consecutive fast reactions with electron transfer. Russel [284-291] studied a few reactions of oxidation of alkylaromatic hydrocarbons in the presence of strong bases. He proved the chain mechanisms of these reactions. One of them includes a few stages with addition of dioxygen to carbanion. Another includes the electron transfer from carbanion to dioxygen. 

Emulsion oxidation of alkylaromatic compounds appeared to be more efficient for the production of hydroperoxides. The first paper devoted to emulsion oxidation of cumene appeared in 1950 [1], The kinetics of emulsion oxidation of cumene was intensely studied by Kucher et al. [2-16], Autoxidation of cumene in the bulk and emulsion occurs with an induction period and autoacceleration. The simple addition of water inhibits the reaction [6], However, the addition of an aqueous solution of Na2C03 or NaOH in combination with vigorous agitation of this system accelerates the oxidation process [1-17]. The addition of an aqueous phase accelerates the oxidation and withdrawal of water retards it [6]. The addition of surfactants such as salts of fatty acids accelerates the oxidation of cumene in emulsion [3], The higher the surfactant concentration the faster the cumene autoxidation in emulsion [17]. The rates of cumene emulsion oxidation after an induction period are given below (T = 353 K, [RH] [H20] = 2 3 (v/v), p02 = 98 kPa [17]). 

Hence, the peculiarities of emulsion oxidation of alkylaromatic hydrocarbons can be formulated as follows. 

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