title: Theoretical and experimental evidence of the photonitration pathway of phenol and 4-chlorophenol: a mechanistic study of environmental significance.
abstract: Light-induced nitration pathways of phenols are important processes for the transformation of pesticide-derived secondary pollutants into toxic derivatives in surface waters and for the formation of phytotoxic compounds in the atmosphere. Moreover, phenols can be used as ˙NO(2) probes in irradiated aqueous solutions. This paper shows that the nitration of 4-chlorophenol (4CP) into 2-nitro-4-chlorophenol (NCP) in the presence of irradiated nitrate and nitrite in aqueous solution involves the radical ˙NO(2). The experimental data allow exclusion of an alternative nitration pathway by ˙OH + ˙NO(2). Quantum mechanical calculations suggest that the nitration of both phenol and 4CP involves, as a first pathway, the abstraction of the phenolic hydrogen by ˙NO(2), which yields HNO(2) and the corresponding phenoxy radical. Reaction of phenoxyl with another ˙NO(2) follows to finally produce the corresponding nitrated phenol. Such a pathway also correctly predicts that 4CP undergoes nitration more easily than phenol, because the ring Cl atom increases the acidity of the phenolic hydrogen of 4CP. This favours the H-abstraction process to give the corresponding phenoxy radical. In contrast, an alternative nitration pathway that involves ˙NO(2) addition to the ring followed by H-abstraction by oxygen (or by ˙NO(2) or ˙OH) is energetically unfavoured and erroneously predicts faster nitration for phenol than for 4CP.
authors: Dipartimento di Chimica Analitica, Università di Torino, Via P. Giuria 5, 10125, Torino, Italy.
authors: Andrea
authors: A
authors: Bedini
authors: Valter
authors: V
authors: Maurino
authors: Claudio
authors: C
authors: Minero
authors: Davide
authors: D
authors: Vione
mesh_terms: D002733:Chlorophenols / Q000737:chemistry*; D004785:Environmental Pollutants / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D009566:Nitrates / Q000737:chemistry*; D009573:Nitrites / Q000737:chemistry*; D055668:Photochemical Processes; D011789:Quantum Theory
chemical_list: D002733:Chlorophenols; D004785:Environmental Pollutants; D009566:Nitrates; D009573:Nitrites; C029107:4-chlorophenol