Dichlorobenzenes, photolysis

Based on its tendency to sublime, volatilization rather than transformation is the most likely fate process for 1,4-dichlorobenzene from surface soil. Transformation of 1,4-dichlorobenzene by biodegradation, photolysis, chemical hydrolysis, and oxidation appear to be relatively minor processes. Leaching of... 

Wilkinson s catalyst. Irradiation at 366 nm of 0.001 M RhCPPh NO and 1 M cyclohexene in o-dichlorobenzene was carried out under 1 atm H2 at room temperature. The hydrogen uptake was monitored using a mercury manometer attached to the reaction flask. Hydrogen was added periodically in order to maintain 1 atm pressure in the system. The solvent and olefin were distilled twice and degassed by three freeze-pump-thaw cycles before use. A 1000 watt Hg lamp filtered with a glass filter to isolate the 366 nm Hg line was used for all photolysis experiments. The light intensity, measured by ferrioxalate actinometry, was 1.0 x 10 6 einsteins/min. 

Figure 1 UV-visible absorption spectra of matrix-isolated p-dichlorobenzene and of its radical cation (a) spectrum of p-dichlorobenzene before photolysis (b) spectrum after 15 min photolysis at = 220-1000nm (c) spectrum after 15 min Ar resonance photolysis. The much increased yield of product upon vacuum UV photolysis can clearly be seen. (Reprinted with permission from Friedman et al., J. Phys. Chem., 1984, 88, 1944. 1984 American Chemical Society)...

An interesting result has been obtained in a study of the dechlorination of polychlorobenzenes by direct irradiation in acetonitrile in addition to dechlorination, products resulting from rearrangement of the chlorine atoms to positions mefa to their original point of attachment are observed. Thus orfho-dichlorobenzene was found to yield mainly chlorobenzene along with small amounts of para-dichlorobenzene the authors speculate that the rearrangement products are formed by recombination of an aryl-chlorine radical pair. In another novel result photolysis of pentachlorophenol in acetonitrile is found to give small amounts of the benzoxazole (160). ... 

PROBABLE FATE photolysis, expected to oecur slowly oxidation no data available on aqueous oxidation, oxidized by hydroxyl radicals in atmosphere hydrolysis not important process first-order hydrolytic half-life >879 yrs volatilization volatilizes at a relatively rapid rate, half-life is about 10 hr volatilization from soil surfaces is expected to be a signifieant transport mechanism sorption sorbed by organic materials adsorption to sediment expected to be a major environmental fate process based on research in the Great Lakes area biological processes bioaccumulates more than chlorobenzene, biodegradation is not as significant as volatilization slightly persistent in water, half-life 2-20 days approximately 98.5% of 1,3-dichlorobenzene ends up in air 1% ends up in water the rest is divided equally between terrestrial soils and aquatic sediments. 

We recently developed efficient synthetic routes to the required 1,3- and -1,4 - dichlorobenzene Cr( CO) 3 (2) complexes for the preparation of pol3rmers such as 1 [13]. Complex 2 can be further modified by photolysis in the presence of tributylphosphine to produce the new monomer 3 (Scheme I). A relative rate study between 2 and 3 showed the latter complex to be less reactive in the palladium-catalyzed cross-coupling reaction with 1-(trimethylstannyl)-2-phenylethyne. The palladiiun-catalyzed polycondensation of monomers 3 and 4 resulted in high yields of polymer 5 (>90%). A reaction temperature of 65 °C and the use Of THF as solvent proved to be the optimum polymerization conditions. Lower temperatures or a solvent change to toluene afforded little if any polymer after several hours of reaction. 

The quadricyclanone derivative (471), available from the thermal cycloaddition of diphenylcyclopropenone to (472), suffered rearrangement when heated in boiling o-dichlorobenzene to give the tricyclo[4,l,0,0 ]heptane derivative (473 64%) photolysis of(471) in benzene solution gave the keten (474 95 %) which, on heating in boiling xylene, yielded (473). The chemistry of the intermediate products from (472)... 

Photolysis of 1,2-dichlorobenzene in trimethylphosphite at bO C for 5 days gave 1,2-bis(dimethoxyphosphoryl)benzene (17) in 50% yield along with the monosubstitution product in 20% yield [184] (see Scheme 4.35). The same reaction, when applied to the case of l-iodo-2-bromobenzene, led to disubstitution affording product 17 in 87% yield [185]. 

Homolytic cleavage of the aryl carbon-halogen bond affords aryl free radicals, which can either arylate a suitable aromatic reaction partner or abstract hydrogen from a hydrogen atom donor (reductive dehalogenation), as first reported over 35 years ago. A recent example is the report by Meunier et al. that photolysis of 1,4-dichlorobenzene in the absence of oxygen yielded 4,4 -dichlorobiphenyl and 2,4, 5-trichlorobiphenyl as reaction products. The general reaction mechanism proceeds as follows ... 

Photolysis of ferric ions (and also nitrate ions) has also been used" as a hydroxyl radical source for the degradation of 1,4-dichlorobenzene. The major product, 2,5-dichlorophenol, was formed by hydrox-ylation at one of the unsubstituted ring positions. 

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