Oxidation of alkyl benzenes

Methods of synthesis for carboxylic acids include (1) oxidation of alkyl-benzenes, (2) oxidative cleavage of alkenes, (3) oxidation of primary alcohols or aldehydes, (4) hydrolysis of nitriles, and (5) reaction of Grignard reagents with CO2 (carboxylation). General reactions of carboxylic acids include (1) loss of the acidic proton, (2) nucleophilic acyl substitution at the carbonyl group, (3) substitution on the a carbon, and (4) reduction. 

Quite different kinetics are exhibited by the anaerobic oxidation of alkyl-benzenes by cobaltic acetate in a 95 % acetic acid medium , viz. 

Although the ability of microwaves (MW) to heat water and other polar materials has been known for half a century or more, it was not until 1986 that two groups of researchers independently reported the application of MW heating to organic synthesis. Gedye et al. [1] found that several organic reactions in polar solvents could be performed rapidly and conveniently in closed Teflon vessels in a domestic MW oven. These reactions included the hydrolysis of amides and esters to carboxylic acids, esterification of carboxylic acids with alcohols, oxidation of alkyl benzenes to aromatic carboxylic acids and the conversion of alkyl halides to ethers. 

The oxidation of alkyl benzenes to benzoic acids [reaction (7.3)] is still carried out occasionally, and this oxidation is most likely the only one where potassium permanganate is the reagent of choice. Any carbon group attached to the aromatic ring is degraded to the carboxylic acid group under the very vigorous conditions of this oxidation. 

A different mechanistic picture has been proposed for the oxidation of alkyl-benzenes by N03 generated by LFP in MeCN this involves reversible formation of a weak adduct between the aromatic compound and N03 (Scheme 40) [161]. 

Benzoic acid is prepared by the oxidation of alkyl benzenes. For the oxidizing agent, a hot, acidic solution of KMn04 or K2Cr207 is used. The number of carbon atoms attached to the aromatic structure, doesn t change the product. 

Mechanism of the reaction. As in the case of the oxidation of benzene, accompanied by ring cleavage, the mechanism of the oxidation of alkylated benzene compounds may best be followed by considering the results which have been obtained in the older method of oxidation in the liquid phase. 

In the vapor phase oxidation of benzene to maleic anhydride an active catalyst is necessary to force oxidation to rupture the ring without leading to complete destruction. Vanadium pentoxide or vanadium compounds such as tin vanadate have been successfully used for this purpose.26 In the oxidation of alkylated benzene compounds to benzaldehyde, benzoic acid, or phthalic anhydride, a milder form of catalyst is effective. The oxidation of naphthalene to naphthaquinone would also require a mild form of catalyst to prevent ring rupture caused by too severe oxidation. However, oxidation to phthalic anhydride may be realized under ordinary conditions by the use of such catalysts as have been found effective in benzene oxidation, i.e., oxides of the metals of the fifth and sixth groups of the periodic system, especially the oxides of vanadium and molybdenum. 

Oxidation of alkyl benzene using heteropolycompounds are effective in presence of oxidants like hydrogen peroxide and t-butylhydroperoxide. Vanadium substituted heteropolymolybdates are more effective than unsubstituted heteropoly compounds. Both side chain and products with oxidation in the aromatic ring are observed in presence of heteropoly compound-hydrogen peroxide system whereas only side chain oxidized products were observed in presence of heteropolycompound-t-butyl hydroperoxide system. This difference in activity can be due to the formation of different active intermediate species. 

Table 1. Oxidation of alkyl benzenes using H5PV2Moio040 - H202 system at 303 K for 3 hrs.

This difference in the nature of the active species generated can be one of the reasons for the differences observed in the oxidation of alkyl benzenes. 

Oxidation of alkyl benzenes give benzoic acid, irrespective of the length of the side chain. 

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