Vapour-phase chemistry of arenes. Part 13. Reactivity and selectivity in the gas-phase reactions of hydroxyl radicals with monosubstituted benzenes at 563 K

Abstract

The reactions of hydroxyl radicals with benzene derivatives C₆H₅Z (Z = H, Me, F, Cl, Br, I, CF₃, or CN) have been studied in a flow reactor at 563 K in nitrogen, using the thermolysis of Bu^tOOH as a source of ˙OH. Under these conditions there are two product-forming pathways. The major one involves hydrogen abstraction to give aryl radicals ZC₆H₄˙(II) as the first step; depending on Z, its displacement to form phenol may also occur. Relative rates for hydrogen abstraction were determined in competition experiments using side-chain hydrogen abstraction from added toluene as a reference. This resulted in the order (for Z =): 1.8(Me), 1.0(H), 0.47(F), 0.29(Cl), 0.34(CF₃), 0.20(CN), consonant with the electrophilic nature of ˙OH. The site selectivity of hydrogen abstractions was determined by scavenging part of the aryl radicals (II) with iodine. A Hammett plot, using σ constants for meta and para positions, led to ρ–1.0. The features of hydrogen abstraction by ˙OH are discussed and compared with those for the analogous reaction of Cl. The formation of phenol was found to decrease in importance in the order F, Cl, Br, and I. This result is rationalized on a thermochemical kinetic basis.