The photochemistry of chloroaromatic compounds. Is ".pi.-chlorobenzene" an intermediate?

Arnold, Donald R.; Wong, P. C.

doi:10.1021/ja00452a029

Cited by 33 publications

(12 citation statements)

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“…Electron transfer mediated dehalogenation of haloaromatic molecules in polar solvents normally proceeds via loss of halide from the anion radical followed by hydrogen abstraction by the resulting aryl radical (lower path in Scheme 4). [38][39][40][41][42][43][44][45][46] Nearly thirty years ago we reported that in the case of the DMHinduced photoconversion of DCA to DMA the hydrogen at the 10 position of MCA is provided by an alternative pathway involving protonation of the anion radical followed by loss of chlorine or disproportionation of the resulting radical (upper path in Scheme 4 or Scheme 5). This full account of our work confirms definitively that conclusion, which was disputed in a recent kinetics study.…”

Section: Discussionmentioning

confidence: 99%

“…Prior to the preliminary report of our work, 8 photochemical reductive halogen displacement from haloaromatics (ArX) was thought to occur via the intermediacy of aryl radicals. 38-46 Formation of aryl radicals was attributed to excited state carbon-halogen bond homolysis [38][39][40] in the absence of electron donors, eqn (5), and to electron transfer followed by loss of halide ion from the haloaromatic radical anion, eqn (6) and (7), in the presence of electron donors (D). 41- 47 The sequence of electron-transfer in the singlet exciplex to form the radical anion and its subsequent dissociation to aryl radical and halide was proposed by several workers to account for the catalysis of photodehalogenation by amines.…”

Section: Photodehalogenation Mechanismmentioning

confidence: 99%

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2,5-Dimethyl-2,4-hexadiene induced photodechlorination of 9,10-dichloroanthracene

Saltiel

Smothers

Schanze

et al. 2009

Photochem Photobiol Sci

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Photochemical formation of 9-chloroanthracene (MCA) from 9,10-dichloroanthracene (DCA) is observed in the presence of 2,5-dimethyl-2,4-hexadiene (DMH) in acetonitrile (AN). The mechanism of the reaction was investigated using kinetics, deuterium labeling, and quenching techniques. Contrary to conclusions in a recent publication, our work supports the salient features of the mechanism we had proposed earlier. DCA is photostable in degassed AN in the absence of DMH. When DMH is added, irradiation of DCA at 365 or 404 nm converts it quantitatively to MCA. The photoreaction is strongly inhibited when low concentrations of molecular oxygen or 1,2,4,5-tetracyanobenzene are also present. Results from fluorescence quenching studies along with kinetics parameters from the dependence of DCA loss and MCA formation quantum yields on [DMH] implicate participation of the DCA/DMH singlet exciplex, the DCA/(DMH) 2 triplex and the DCA radical anion (DCA ∑ -) as intermediates in the photodechlorination. Results from experiments using deuterated DMH, deuterated AN, and AN containing D 2 O or H 2 O show that the 10-H of MCA is introduced by protonation of DCA ∑ -. Contrary to a recent report, there is no radical pathway to MCA via dissociation of DCA ∑to chloride and MCA ∑ radical. Changes in the absorption spectrum of DCA in AN with increasing [DMH] suggest that the static quenching of DCA fluorescence at high [DMH] is due primarily to nearest neighbour quenching instead of DCA/DMH ground state complex formation.

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Section: Discussionmentioning

confidence: 99%

Section: Photodehalogenation Mechanismmentioning

confidence: 99%

2,5-Dimethyl-2,4-hexadiene induced photodechlorination of 9,10-dichloroanthracene

Saltiel

Smothers

Schanze

et al. 2009

Photochem Photobiol Sci

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“…Halogen atoms are electron deficient species with high electron affinities (22) and their complexation by aromatic molecules is well known from photohalogenation (23) and flash photolysis (24) studies. Such complexation has been considered important in several photochemical problems (25), though these reactions were subsequently reinterpreted along different lines (26). Anchimeric assistance to bond homolysis by radical complexation is not a new concept, having been documented twenty years ago in thermal free radical chemistry by product analysis and activation parameters (27).…”

Section: Adtiitirjes Inclucling Energy-transfer Agentsmentioning

confidence: 99%

Photocyclization of aryl halides. Part I. 5-(2-Halogenophenyl)-1,3-diphenylpyrazoles; homolytic fission assisted by radical complexation

Grimshaw¹,

Silva²

1980

Can. J. Chem.

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Can. J. Chem. 58, 1880(1980). The photocyclization of 5-(2-halogenopheny1)-1,3-diphenyl pyrazoles proceeds efficiently with the exclusion ofsolvent participation hydrogen abstraction reactions. In agiven solvent, the quantum yields of cyclization (4) of CI, Br, and I compounds show no correlation with carbon-halogen bond dissociation energies. In a series of solvents, 4 varies smoothly with solvent viscosity and is independent of solvent polarity. The 4 values are independent of triplet quenchers, air, intensity and wavelength of radiation, and reactant concentration. Photocyclization can be triplet sensitized at up to 80% of the direct reaction rate and the reactant triplet energies are about 65 kcal mol-I while DC-Br is 81 and DcPcl is 95 kcal mol-'. A mechanism of homolytic fission assisted by radical complexation is proposed to accommodate the results. Photocyclization of the H-and MeO-substituted analogues proceeds inefficiently and with 4 values dependent on solvent polarity. An electrocyclic mechanism is followed in these cases. The assisted homolysis model explains several anomalies in the field of aryl halide photochemistry. [Traduit par lejournal]

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“…Investigations in relation to IPC persistence in the environment indicated a half-life of 254 days with unidentified metabolites in water under simulated summer sunshine [16]. However, in presence of TiO 2 /H 2 O 2 and UV, the half-life of IPC was shortened to few hours [20,21], giving a variety of degradation products including ortho-and para-hydroxy analogues, benzoquenone, isopropyl aminobenzoate and aminophenol derivatives [22][23][24][25][26][27]. In view of these conflicting reports and inadequate data on propham photo-product, herein we report the IPC photodegradation in various media with its respective photoproducts; however, such data are not fully available and the demand for it is important both to chemists and environmentalists.…”

Section: Introductionmentioning

confidence: 99%

Analysis and Characterization of Isopropyl Carbanilate Herbicide and Its Photoproducts

ZA'ATER¹

2011

AJAC

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The phototransformation of the herbicide Isopropyl carbanilate (IPC) has been investigated under UV light. Irradiation of the herbicide at room temperature in aqueous and organic solvents such as hexane and methanol afforded new photo-products formed as a consequence of various processes including photo-Fries rearrangement, ring solvolysis, hydrolysis of the amide/carbamoyl and ester bonds, ring coupling and polymerization. The percentage remaining of the herbicide as a function of time was followed periodically starting from zero time up to three hours. Analyses were performed by GC-FID equipped with a semipolar glass column operated at 170?C. The rate of photo disappearance of IPC under controlled lab condition followed 1st order kinetics and found to be solvent dependent in the manner of non polar > polar solvents. The photo-products were successfully separated by GC and preparative TLC (Silica gel F-254) and were identified using either GC-MS and/or MS. Identifications were assigned on the bases of molecular ions, mass fragmentation pattern and whenever possible by comparison with the mass spectra of literature analogues

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The photochemistry of chloroaromatic compounds. Is ".pi.-chlorobenzene" an intermediate?

Cited by 33 publications

References 0 publications

2,5-Dimethyl-2,4-hexadiene induced photodechlorination of 9,10-dichloroanthracene

2,5-Dimethyl-2,4-hexadiene induced photodechlorination of 9,10-dichloroanthracene

Photocyclization of aryl halides. Part I. 5-(2-Halogenophenyl)-1,3-diphenylpyrazoles; homolytic fission assisted by radical complexation

Analysis and Characterization of Isopropyl Carbanilate Herbicide and Its Photoproducts

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