Photochemistry of halogenated benzene derivatives. Part VIII. Photoformation of 2-methyl-4,5,6,7-tetrachlorobenzoxazole from pentachlorophenol in water–acetonitrile

Abstract

Photolysis of a solution of pentachlorophenol (1) in H2O–CH3CN (2:3 v/v) was conducted on a synthetic scale using wavelengths ≥290 nm. After changing the pH of the photolyzate of 1 to ≥12, the photoproduct 2-methyl-4,5,6,7-tetrachloro-benzoxazole (2) was extracted with n-hexane. The product 2 was crystallized from methanolic solution. Ultraviolet, IR, FT-1H-NMR, proton-decoupled normal FT-13C-NMR, and proton-decoupled DEPT (distortionless enhancement by polarization transfer) sequence FT-13C-NMR spectroscopy were used to identify photoproduct 2 as 2-methyl-4,5,6,7-tetrachlorobenzoxazole. A mechanism for the photoformation of benzoxazole 2 from 1 is proposed. 3,4,5,6-Tetrachlorocyclohexa-3,5-diene-1,2-ketocarbene formed from 2-hydroxy-3,4,5,6-tetrachlorophenyl radical is suggested as key intermediate involved in the generation of oxazole 2 from phenol 1. The chemical yield of 2 on the basis of 1 was ca. 1%. A mechanism is proposed to explain the mass spectral fragmentation of 2 via the losses of Ċl,:C=O, Cl—Ċ=O, CH3—Ċ=O, CH3CN, and C3H3ClNO species from the parent molecular ion.

Reviews of Environmental Contamination and Toxicology

“…It is noteworthy that Lemaire and his co-workers (1985a) have frequently invoked the involvement of 1,2-ketocarbene (structure 27 without Cl substituents) in the photolysis of 2-chlorophenolate in dilute aqueous solutions. Finally, structures similar to 28 can be attributed to the products such as C s H 4 Cl 3 NO (from CI 4 -Pns, Table 2) (Choudhry et al 1987b).…”

“…Therefore, the scheme showing the mechanism of formation of 2,3-dichlorodibenzofuran during the irradiation of solution of 1,2,4,5-tetrachlorobenzene in H 2 0:CH 3 CN (4:6 vJv) containing phenol, previously postulated by Choudhry et al (1983), can be modified by including phenoxy radicals bearing full, instead of partial, radical character on the oxygen atom which interact with photochemically generated 2,4,5trichlorophenyl radicals (Choudhry et al 1985). Very recently, Choudhry et al (1987b) have conducted photolysis (A> 285 nm) of ca.O.l mM solution of pentachlorophenol in H 2 0:CH 3 CN (4: 6 vJv) on synthetic scale (other conditions are given in Table 2) and have isolated pure crystals of the photoproduct having molecular formula, C s H 3 Cl 4 NO described above (rnJe for M+· being 269). The IR, UV, lH_ NMR, 13C-NMR, and mass spectrometric (MS) data assisted in ascribing a structure, namely, 2-methyl-4,5,6,7-tetrachlorobenzoxazole (28) to this product.…”

“…The IR, UV, lH_ NMR, 13C-NMR, and mass spectrometric (MS) data assisted in ascribing a structure, namely, 2-methyl-4,5,6,7-tetrachlorobenzoxazole (28) to this product. Oxazole 28 appears to be formed through the nucleophilic attack of acetonitrile on the electron deficient carbene position of tetrachloro-I,2ketocarbene 27, presumably generated photolytically from pentachlorophenol, and the eventual ring closure (Scheme 2) (Choudhry et al 1987b). It is noteworthy that Lemaire and his co-workers (1985a) have frequently invoked the involvement of 1,2-ketocarbene (structure 27 without Cl substituents) in the photolysis of 2-chlorophenolate in dilute aqueous solutions.…”

Environmental photochemistry of PCDDs. 2. Quantum yields of the direct phototransformation of 1,2,3,7-tetra-, 1,3,6,8-tetra-, 1,2,3,4,6,7,8-hepta-, and 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin in aqueous acetonitrile and their sunlight half-lives

“…In view of our continuing interest in the photochemical fate of chlorinated aromatic pollutants present in the environment (see Choudhry et al (1987) and preceding publications), we decided to further investigate the environmental solution-phase photochemistry of some model isomers of PCDDs, viz., 1,2,3,7-T4CDD (1), 1,3,6,8-T4CDD (2), 1,2,3,4,6,7,8-H7CDD (6), and OgCDD (7) using our previously reported methodology (Choudhry and Webster 1985b). This paper presents the values of laboratory at 313 nm and the estimated sunlight photolysis half-lives ((ti/2)sp) of nonlabeled PCDDs 1, 2, 6, and 7 along with experimentally determined absolute natural sunlight photolysis first-order rate constant and the corresponding half-life of the 14C-labeled T4CDD (2) ([14C]-2) in autoclaved pond water.…”