The Benzylperoxyl Radical as a Source of Hydroxyl and Phenyl Radicals

Abstract

The benzyl radical (1) is a key intermediate in the combustion and tropospheric oxidation of toluene. Because of its relevance, the reaction of 1 with molecular oxygen was investigated by matrix-isolation IR and EPR spectroscopy as well as computational methods. The primary reaction product of 1 and O₂ is the benzylperoxyl radical (2), which exists in several conformers that can easily interconvert even at cryogenic temperatures. Photolysis of radical 2 at 365 nm results in a formal [1,3]-H migration and subsequent cleavage of the O—O bond to produce a hydrogen-bonded complex between the hydroxyl radical and benzaldehyde (4). Prolonged photolysis produces the benzoyl radical (5) and water, which finally yield the phenyl radical (7), CO, and H₂O. Thus, via a sequence of exothermic reactions 1 is transformed into radicals of even higher reactivity, such as OH and 7. Our results have implications for the development of models for the highly complicated process of combustion of aromatic compounds

Reinvestigation of the Deceptively Simple Reaction of Toluene with OH and the Fate of the Benzyl Radical: The “Hidden” Routes to Cresols and Benzaldehyde

“…It is well known that alkyl radicals can associate with molecular oxygen to give the peroxyl radical, both in combustion and in the atmosphere. Thus, one of the reaction paths we explored is the association of the benzyl radical with molecular oxygen, which has been studied experimentally by Fenter et al, Nelson et al, and Elmaimouni et al and theoretically by several authors. − Although the reaction of the benzyl radical with O 2 is believed to occur without a barrier, Zhang et al obtained a very small transition state at high levels of theory …”

“…Our theoretical result, published together with others of the intermediates and products in this PES, is also in agreement with the experimental value. The radical and the isomerization reaction to benzylhydroperoxy radical (BHP), which will be discussed in the following section, were studied extensively by Canneaux et al. , The most recent papers on BP were produced by Sander et al, who used matrix-isolation IR and electron paramagnetic resonance (EPR) spectroscopy and theoretical methods to follow the photolysis of BP to benzaldehyde and the benzoyl radical; Zhang et al, who used CASPT2 to study the recombination reaction of several aromatic radicals with molecular oxygen; and Pelucchi et al, who used DFT and high-level ab initio calculations.…”

“…62 It is known that benzaldehyde is one of the major products obtained from the reaction of benzyl radical with O 2 . 58 The obvious reaction path seems to be the direct abstraction of a hydrogen atom from the CH 2 group, as studied at the semiempirical level by Clothier et al, 56 at the CBS-QB3 level by Murakami et al, 57 and at several DFT and ab initio levels by Canneaux, 58,59 Sander, 60 and Pelucchi et al 61 In the present work, moreover, after reanalyzing the reaction mechanisms proposed by Murakami et al (using the chemical models discussed above), we further explored the possibility of water participation, i.e., when the reactions occur in solution or in atmospheric conditions. We will show that, contrary to the case studied by Murakami et al, in those conditions the most stable product is not benzaldehyde per se but the benzoyl radical, which can proceed further to other products.…”

“…Our own results give a quite high barrier of 40.5 kcal mol −1 at the G4 level and 39.4 kcal mol −1 at the M06/cc-pVQZ level. The most sophisticated calculations by Sander et al 60 at the CCSD(T)/ 6-311++G(2d,2p)//M06-2X-6-311++G(2d,2p) level provided a value of 45.2 kcal mol −1 . The presumably most accurate method employed by Canneaux et al 58 (CCSD(T)/cc-pVTZ//MPW1K/cc-pVZ) afforded a value of 40.2 kcal mol −1 .…”

In vitro Antioxidant and Antibacterial Activities of Quinoa Flavonoids Extracted by Ethanol-Ammonium Sulfate Aqueous Two-Phase System

“…Scavenging of hydroxyl radical: The hydroxyl radical belongs to a kind of strong oxidant, and the most active oxygen molecule in the organism. Thus, it is highly destructive (Sander et al 2014). It can react with almost any biological molecule in a living cell to cause damage.…”