Hydroxymethyl cyclohexadienyl radical

The first step of the addition reactimi of OH to the benzene ring is the formation of hydroxymethyl cyclohexadienyl radical (A) as shown in Reaction Scheme 7.10 (see Sect. 5.2.10). 

The direct meastirements of the rate constants of the reaction of hydroxymethyl cyclohexadienyl radical with O2 has been made using the UV absorption method (Sect. 5.2.10), and the values of 2.5 x 10 cm molecule for benzene (Bohn and Zetzsch 1999 Grebenkin and Krasnoperov 2004 Raoult et al. 2004 Nehr et al. 2011), 6.0 x 10 cm molecule s for toluene (Knispel et al. 1990 Bohn 2001) are reported. Therefore, most hydroxymethyl cyclohexadienyl radicals are thought to react solely with O2. Also, the unimolecular decomposition rate of cyclohexadienyl radical from benzene back to CgHg + OH has been reported as (3.9 1.3) s at 298 K (Nehr et al. 2011). 

In the case of toluene, as shown in Reaction Scheme 7.10, three reaction pathways of hydroxymethyl cyclohexadienyl radical with O2 are conceived the formation of (1) cresol (B) by the H-atom abstraction from the OH-added carbon (pathway a), (2) toluene 1,2-epoxide (C) and the 2-methyl oxepin (D) equilibrated with it, and (3) hydroxymethyl cyclohexadienyl peroxy radical (E) formed by the O2 addition to the benzene ring (Suh et al. 2003 Cartas-Rosado and Castro 2007 Baltaretu et al. 2009). The formation of cresol by pathway (a) is well known, and o-cresol is produced predominantly followed by p- and m-cresol reflecting the ortho- and para-orientation of the OH addition to benzene ring (see Sect. 5.2.10). However, the ratio of the H-atom abstraction from the benzene ring of hydroxy... 

Reaction Scheme 7.10 Formation of hydroxymethyl cyclohexadienyl radical in the addition reaction of OH to toluene and its pathway for the reaction with O2... 

The formation of hydroxymethyl cyclohexadienyl peroxy radical (E) by pathway (c) in Reaction Scheme 7.10 is thought to be the most important as a process to produce ring-opened compounds, which are well known reaction products of OH and aromatic hydrocarbons. The O2 addition reaction to the OH-added benzene ring radical is reversible, and in the case of benzene, the equilibrium constant and the enthalpy change is given as K(298 K) = (8.0 0.6) x 10 ° cm molecule" (Johnson et al. 2002), and = ( 44 2) kJ mol (Grebenkin and Krasnoperov 2004) for the hydroxycyclohexadienyl radical. Therefore, hydroxycyclohexadienyl radicals formed in pathway (c) are in general stable in the atmosphere, and play an important role as intermediates in the oxidation reactiOTi of aromatic compounds. 

Two reaction pathways to form ring-opened compounds in the oxidation reaction of toluene and OH are thought to occur, one through hydroxymethyl cyclohexadienyl peroxy radical (E) and the other through toluene-1,2-epoxide (C) or 2-metyl oxepin (D) in Reaction Scheme 7.10. Reaction Scheme 7.11 summarizes former pathways for peroxy radical (E) shown in Reaction Scheme in 7.10 based on recent findings. 

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