Benzylperoxy radicals

Table 4 Comparison of reaction pathway energetics (kcal/mol) for benzylperoxy radical and 2-picolinylperoxy radical at 298 K, via B3LYP/6-311+G //B3LYP/6-31G. Numbers refer to pathways depicted in Fig. 18. All enthalpies and free energies are relative to the peroxy radical (2) when preceded by TS, the relative data are the enthalpy and free energies of activation...

Under the reaction conditions, the benzyl radical is trapped by dioxygen and transformed into a benzylperoxy radical which eventually reacts with cobalt(II) affording an aromatic aldehyde and regenerating cobalt(III) (Equations 5-6) ... 

Studies of benzene and toluene oxidation in the turbulent flow reactor at Princeton University have provided valuable information on the mechanisms of oxidation at temperatures in excess of 1000 K [213]. The first extensive study of benzene and toluene at temperatures of about 750 K was made by Burgoyne [35]. Apart from CO and CO2, the major products of benzene oxidation that were detected were phenols and acids. An autocatalytic reaction was observed by Burgoyne [35] presumably driven by H2O2 formation and decomposition. Amongst the main products of toluene slow oxidation were benzyl alcohol, benzaldehyde and benzoic acid. Phenolic compounds were also reported. This reaction also showed an autocatalytic development. An equilibrium constant for the equilibrium between benzyl and benzylperoxy radicals has been measured by Fenter et al. [214], but this cannot be followed by an isomerization in the way that is possible in alkanes. 

A kinetic and mechanistic study of the self-reaction and reaction with HO2 of the benzylperoxy radical,... 

The high recombination rates of benzylperoxy radicals (Table of VII. 1), resulting in the formation of benzyl alcohol and benzaldehyde ( ) [24, 25], and low stability of benzyl hydroperoxide, BH, do not permit significant selectivity in BH formation already at low extents of oxidation. Benzaldehyde, formed at decomposition of BH and in chain termination reaction ( ), is then oxidized at high rates to benzoic acid. 

Fig. 6.20 Difference IR spectra in the O-H and C=0 stretching regions showing the photochemistry of benzylperoxy radical matrix isolated in argon. Bands pointing upward appeared and bands pointing downward disappeared during the irradiation, (a) 10 min irradiation at 365 nm at 3 K. (b) Same matrix as in (a) warmed at 25 K. (c) Reference spectrum of benzaldehyde, matrix isolated in 1 % H20-doped argon. The difference spectra, taken at 3 K and after warming at 30 K, show the formation of the 4-H2O complex, (d) Same matrix as (b) after additional 10 min irradiation at 320 nm [39]...

The photochemistry of the thus formed benzylperoxy radical was then studied in the matrix. Irradiation at 365 nm caused a formal 1,3 hydrogen migration followed by cleavage of the peroxyl bond and prolonged irradiation finally yielded phenyl radical 31, CO, and water. This result shows that the benzyl radical is transformed via a series of exothermic steps into even more reactive radicals, such as OH and phenyl radicals [39] (see Scheme 6.13, Fig. 6.20). 

Rate Constant Calculation of Benzylperoxy Radical Isomerization... 

The internal transfer is easier when the transferred hydrogen atom is benzylic. In the case of toluene, the only possible transfer is a l,3sb transfer involving one of the two secondary benzylic hydrogen atoms of benzylperoxy radical ... 

It is worth noticing that, in all these aforementioned studies, no structure for the hydroperoxyalkyl radicals was characterized. Clothier et al. worked on the simulation of diesel fuel ignition by benzyl radicals and showed, using ab initio molecular orbital calculations, that there is a plausible mechanism by which benzylperoxy radical thermal decomposition could lead to the production of OH radicals. The calculations were performed at the low level of theory ROMP2/3-21G//ROHF/3-21G. The energy barrier for the benzylperoxy radical isomerization was calculated to be 121 kJ mol . ... 

At all levels of theory, the global minimum for the benzylperoxy radical is found to be in a conformation in which the dihedral angles C3C2C1O2 and C2C1O2O1 are equal to about —90° and 180°, respectively. The same results were obtained by Garcia et using the UHF approximation combined with the 6-31G(d,p) and 6-31+G(d,p) basis sets. 

Table 2.1 Selected bond lengths in A for the benzylperoxy radical (reactant R), the transition state (TS), and the 1-hydroperoxybenzyl radical (product P) at diiferent levels of theory. 

Figure 26. Possible selective transformations of benzylperoxy radicals to stable oxidation products in acetic acid.

Nozibre, B., R. Lesclaux, M.D. Hurley, M.A. Dearth, and T.J. Wallington (1994), A kinetic and mechanistic study of the self-reaction and reaction with HO2 of the benzylperoxy radical, J. Phys. Chem., 98, 2864-2873. 

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