Photochemical reaction of 4-chlorobiphenyl with nitrous acid in atmospheric aqueous solution

The photochemical reaction of 4-chlorobiphenyl (4-PCB) and HONO in atmospheric aqueous phase was studied by 355 nm laser flash photolysis combined with 365 nm UV steady-state irradiation technique. The steady-state study showed that the conversion rate of 4-PCB was affected by the initial concentration of 4-PCB, pH value and HONO concentration, while chloride ions had little effect on the conversion of 4-PCB. HONO produces an HO˙ attack on 4-PCB to form a 4-PCB-OH adduct with the second-order reaction rate constant of (9.0 ± 1) × 10⁹ L mol^-1 s^-1. The 4-PCB-OH adduct continues to react with HONO and O₂ with second-order rate constants of (5.3 ± 0.1) × 10⁶ L mol^-1 s^-1 and (4.9 ± 0.2) × 10⁶ L mol^-1 s^-1, respectively. The main transient intermediates of 4-PCB-OH adducts had a variety of decay pathways, and the final products mainly included 4-hydroxybiphenyl, 4-chlorobenzyl-4-ol, 4-chlorobenzyl-4-nitrobiphenyl, and 4-(4-chlorophenyl-2-nitrophenol). The formation mechanism of these products was also discussed.