Isotope effect alkylaromatics

Oxidation of various alkylaromatics, including toluene, ethylbenzene, and cumene, by trans-[Ru (0)2(N202)] in MeCN also has large kinetic isotope effects k-alk-o = 16 for ethylbenzene), indicating C—bond cleavage in the transition state. The second-order rate constants for ethylbenzene and cumene are similar but are substantially higher than that for toluene. " Representative kinetic data for the oxidation of ethylbenzene, cumene, and toluene are collected in Table 10. 

The reactions with paraffins established that the r-system is not required. The reactions with paraffins and alkylaromatics do differ in two significant respects the reactions of alkylaromatics are faster and exhibit a deuterium isotope effect. This implies that reaction (26) is faster than reaction (25), and the final step of eq. (26) is rate-limiting. The rate with paraffins is primarily governed by the ratio kfk2 [67]. 

The primary motivation for these studies is the analysis of the reactivity patterns of organic compounds, when Ce(IV) is used as an oxidant. These patterns are determined for the most part by product analysis of selected series of organic compounds. The results obtained in two studies that bear more directly on the chemical behavior of Ce(IV) as an oxidant for hydrocarbons have been interpreted to indicate different mechanistic behavior of Ce(IV). In a product study of the oxidation of isodurene (1,2,3,5-tetramethyl benzene) by ceric ammonium nitrate compared to anodic oxidation, Eberson and Oberrauch (1979) concluded that the oxidation by Ce(IV) occurs via a H atom transfer from the alkylaromatic compound to Ce(IV). Badocchi et al. (1980) measured the variation of second-order rate constants for the oxidation of a series of alkylaromatic compounds with added Ce(III). These results along with those from the determination of kinetic deuterium isotope effect were dted to support a mechanism involving radical cations. The Ce(IV)/Ce(III) functions as an electron acceptor/donor in such a mechanism. 

The reaction of alkyl- and halo-substituted phenols with Cr02Cl2 results mainly in the formation of quinones and diphenoquinones. Phenoxyl radicals are involved as intermediates. The mechanism of oxidation of a-hydroxycar-boxylic acids by pyridinium chlorochromate involves a rate-limiting hydride transfer. In the reaction of HOCD2CO2H, a kinetic isotope effect (W d) = 5.80 has been determined. Spectroscopic evidence for Cr(IV) and Cr(V) species has been obtained in the oxidation of alkylaromatics by chromyl acetate in acetic anhydride.Stopped-flow and esr studies show two stages (equations 22, 23) in the reactions of RCH2Ph, with both rates being decreased on deuteration at... 

---