Alkylbenzene, oxidation

The mechanistic details of the anodic alkylbenzene oxidation have been under intense investigation. In the early stages of this research the major issue was the order of the electron and proton transfer steps leading to the benzyl cations and whether the second electron-transfer reaction takes place at the anode (ECE) or in solution (DISP) see Eq. (7). By the application of a combination of voltammetry and specular reflectance spectroscopy [173,192-195], it has been possible to monitor the electrode surface by UV-visible spectroscopy during electrolysis at controlled potential or to monitor the absorption at a predetermined value as a function of potential. In this way absorption spectra, which were attributed to the radical cation, XIII, the benzyl radical, XIV, formed by deprotonation, Eq. (60), the benzyl cation XV, and the N-benzylnitrilium ions, XVI, have been recorded in MeCN. 

Alkylbenzene oxidation is usually carried out at 90-100 and 0.2-1 bar oxygen pressure in acetic acid [41]. Cobalt(III) acetate is added in relatively high concentrations (ca. 0.1 M). To avoid long induction periods, methyl ethyl ketone or ozone are used as activators to promote reoxidation of cobalt(II) to the active cobalt(III) [42]. The following reaction mechanism is generally accepted [3,43-48] ... 

Side chain oxidation of alkylbenzenes is important in certain metabolic processes One way m which the body rids itself of foreign substances is by oxidation m the liver to compounds that are more polar and hence more easily excreted m the urine Toluene for example is oxidized to benzoic acid by this process and is eliminated rather readily... 

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. 

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

Although amine oxides are also beyond the scope of this volume, they are included here because of their analogy to amphoterics. Amine oxides derived from alkylbenzenes were prepared as follows [21] ... 

Harms G, K Zengle, R Rabus, F Aeckersberg, D Minz, R Rossell6-Mora, F Widdel (1999) Anaerobic oxidation of o-xylene, m-xylene, and homologous alkylbenzenes by new types of sulfate-reducing bacteria. Appl Environ Microbiol 65 999-1004. 

Seddon, K.R. Stark, A. (2002) Selective Catalytic Oxidation of Benzyl Alcohol and Alkylbenzenes in Ionic Liquids. Green Chemistry, 4(2), 119-123. 

Alkyl-substituted benzenes are oxidized both on the benzene ring and on the side chain. Additionally, some dimerization occurs.36 Alkylbenzenes containing linear alkyl groups are oxidized preferentially at the side chain33 nearest the benzene ring for example, ethylbenzene oxidizes first to 1-phenyl ethanol and then to acetophenone.36... 

Fig. 4 (a) Correlation of the standard oxidation potentials E%r of various alkylbenzenes with the irreversible CV peak potentials Ep at scan rate v = 100 mV s. (b) Correlation of the standard oxidation potentials r of various alkylbenzenes with the vertical ionization potentials IP. Numbers refer to the aromatic hydrocarbons identified in Tables I and III in Ref. 8. Reproduced with permission from Ref. 8. 

Using dicyclohexyl-18-crown-6 it is possible to dissolve potassium hydroxide in benzene at a concentration which exceeds 0.15 mol dm-3 (Pedersen, 1967). The free OH- has been shown to be an excellent reagent for ester hydrolysis under such conditions. The related solubilization of potassium permanganate in benzene, to yield purple benzene , enables oxidations to be performed in this solvent (Hiraoka, 1982). Thus, it is possible to oxidize a range of alkenes, alcohols, aldehydes, and alkylbenzenes under mild conditions using this solubilized reagent. For example, purple benzene will oxidize many alkenes or alcohols virtually instantaneously at room temperature to yield the corresponding carboxylic acids in near-quantitative yields (Sam Simmons, 1972). 

Alkylator, side reactions in, 23 329 Alkylbenzene(s), 12 162, 163, 169 23 329 from benzene, 3 619t, 620 biodegradation, 3 763t by-products, 23 335 dispersant moieties, 8 706t formylation of, 12 178 oxidation of, 26 894-896 Alkylbenzenesulfonates, rat oral LD50 values, 3 445... 

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