Chlorine photolysis rate

The photolysis rate of several chlorinated dioxins was determined in methanol (20) (Figure 1). Solutions of 2,7-dichlorodibenzo-p-dioxin (5 mg/liter), TCDD (5 mg/liter), and octachlorodibenzo-p-dioxin (2.2 mg/ liter) were irradiated with light having an intensity of about 100 /mW/cm ... 

Figure 1. Photolysis rates of chlorinated dibenzo-p-dioxins in methanol under ultraviolet light 2J-dichlorodibenzo-p-dioxin (III) (5 mg/liter), 2,3,7,8-tetrachlorodiben-zo-p-dioxin (I) (5 mg/liter), 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin (IV) (2.2 mg/ liter) (20), 1971 by AAAS...

It has been argued466 that the solution photolysis rates of polychlorinated dibenzo-/ -dioxins may be explained by the preferential photodissociation of chlorine atoms from a lateral vs a non-lateral position to yield the corresponding aryl radical and/or aryl cation-aryl carbene intermediate. 

Bromine nitrate possesses a similar spectrum (Figure 4.51) to that of chlorine nitrate. It is, however, shifted toward slightly longer wavelengths such that the photolysis rate of BrON02 is faster than that of CIONO2. Like chlorine nitrate, the absorption cross sections are temperature dependent. They have been measured in the laboratory by Spencer and Rowland (1978) and more recently by Burkholder et al. (1995). Nickolaisen and Sander (1996) have shown that the quantum yields should be approximately 0.71 for the... 

The formation rate of the CIO dimer and the photolysis rate of C1202 are critical pieces of information to calculate the ozone loss in the polar regions (Avallone and Toohey, 2001). A second cycle, which introduces an interaction between the chlorine and bromine families also contributes to the destruction of ozone in polar regions, as well as at middle latitudes ... 

Chlorine atom rate constants can be derived analogously from the above equation. Chlorine atoms were generated directly from the photolysis of molecular chlorine. Gas chromatography (FID) was employed to carry out the quantitative analysis of the concentrations of substrate and reference compounds. 

Chlorine monoxide CIO, bromine monoxide BrO, and iodine monoxide lO are the main chain carrier of stratospheric and tropospheric halogen chain reactions. CIO is partially photolyzed only in the stratosphere, and the photolytic rates of BrO and lO are large also in the troposphere. Their photolysis rates have to be considered in the evaluation of ozone depleting chain reactions. 

The growth of long chains ( > 10 ) in the perfectly mixed 1 1 crystals of ethylene with chlorine and bromine at 20-70 K was studied in detail by Wight et al. [1993]. Active radicals were generated by pulse photolysis of CI2 or Br2. The rate constant was found to be /Cc = 8-12s below Tc = 45 K. The chain grows according to the well known radical mechanism including the reactions... 

Chlorinated dibenzo ip-dioxins are contaminants of phenol-based pesticides and may enter the environment where they are subject to the action of sunlight. Rate measurements showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is more rapidly photolyzed in methanol than octachlorodi-benzo-p-dioxin. Initially TCDD yields 2,3,7-trichlorodiben-zo-p-dioxin, and subsequent reductive dechlorination is accompanied by ring fission. Pure dibenzo-p-dioxin gave polymeric material and some 2,2 -dihydroxybiphenyl on irradiation. Riboflavin-sensitized photolysis of the potential precursors of dioxins, 2,4-dichlorophenol and 2,4,5-trichloro-phenol, in water gave no detectable dioxins. The products identified were chlorinated phenoxyphenols and dihydroxy-biphenyls. In contrast, aqueous alkaline solutions of purified pentachlorophenol gave traces of octachlorodibenzo-p-dioxin on irradiation. 

Octachlorodibenzo-p-dioxin was photolyzed much more slowly than TCDD (Figure 1). The rate of dioxin photolysis increased as the number of substituent chlorine atoms decreased. Octachlorodibenzo-p-dioxin gave what seemed to be a series of chlorinated dioxins of decreasing chlorine content (20). 

Atrazine is successively transformed to 2,4,6-trihydroxy-l,3,5-triazine (Pelizzetti et al. 1990) by dealkylation of the alkylamine side chains and hydrolytic displacement of the ring chlorine and amino groups (Figure 1.3). A comparison has been made between direct photolysis and nitrate-mediated hydroxyl radical reactions (Torrents et al. 1997) the rates of the latter were much greater under the conditions of this experiment, and the major difference in the products was the absence of ring hydroxylation with loss of chloride. 

Reaction rate constants are postulated as shown in Table II for degradation in water (biolysis and photolysis), in bottom sediments (probably biolysis), and for permanent burial of sediment. The values were selected from a perusal of the literature and must be regarded as speculative. A factor of 20 reduction in reaction rate constant is assumed for addition of each chlorine. 

The chain length, i.e. number of RH —> RC1 conversions per Cl produced by photolysis, is wlO6 for CH4, and the reaction can be explosive in sunlight. Chlorination can also be initiated thermolytically, but considerably elevated temperatures are required to effect Cl2 — 2C1, and the rate of chlorination of C2H6 in the dark at 120° is virtually indetectable. It becomes extremely rapid on the introduction of traces of PbEt4, however, as this decomposes to yield ethyl radicals, Et, at this temperature, and these can act as initiators Et- + Cl—Cl —> Et—Cl + Cl. Chlorination of simple alkanes such as these is seldom useful for the preparation of mono-chloro derivatives, as this first product readily undergoes further attack by the highly reactive chlorine, and complex product mixtures are often obtained. 

The chlorine atom adds in the gas phase to propadiene (la) with a rate constant that is close to the gas-kinetic limit. According to the data from laser flash photolysis experiments, this step furnishes exclusively the 2-chloroallyl radical (2a) [16, 36], A computational analysis of this reaction indicates that the chlorine atom encounters no detectable energy barrier as it adds either to Ca or to Cp in diene la to furnish chlorinated radical 2a or 3a. A comparison between experimental and computed heats of formation points to a significant thermochemical preference for 2-chloroal-lyl radical formation in this reaction (Scheme 11.2). Due to the exothermicity of both addition steps, intermediates 2a and 3a are formed with considerable excess energy, thus allowing isomerizations of the primary adducts to follow. 

Irradiation of gaseous formaldehyde containing an excess of nitrogen dioxide over chlorine yielded ozone, carbon monoxide, nitrogen pentoxide, nitryl chloride, nitric and hydrochloric acids. Peroxynitric acid was the major photolysis product when chlorine concentration exceeded the nitrogen dioxide concentration (Hanst and Gay, 1977). Formaldehyde also reacts with NO3 in the atmosphere at a rate of 3.2 x 10 cmVmolecule-sec (Atkinson and Lloyd, 1984). 

The hydrolysis reaction (46) of N2Os under many conditions in the atmosphere becomes limited by the rate of N2Os formation, which only occurs at a significant rate at night (because of the rapid photolysis of the N03 precursor during the day). Hence under these conditions, reactions (39), (41), and (42) followed by photolysis of the chlorine-containing products become primarily responsible for the removal of gas-phase NOy and increase in CIO (Keim et al., 1996). 

Indeed, these reactions play an important role in the Antarctic ozone hole and they have important implications for control strategies, particularly of the bromi-nated compounds. For example, Danilin et al. (1996) examined the effects of ClO -BrO coupling on the cumulative loss of O-, in the Antarctic ozone hole from August 1 until the time of maximum ozone depletion. Increased bromine increased the rate of ozone loss under the denitrified conditions assumed in the calculations by converting CIO to Cl, primarily via reactions (31b) and (31c) (followed by photolysis of BrCl). Danilin et al. (1996) estimate that the efficiency of ozone destruction per bromine atom (a) is 33-55 times that per chlorine atom (the bromine enhancement factor ) under these conditions in the center of the Antarctic polar vortex, a 60 calculated as a global average over all latitudes, seasons, and altitudes (WMO, 1999). 

Such a process would have an intensity exponent of unity as observed. A further fact in support of the disproportionation mechanism is that the yield of CF2CI2 is largely independent of the ketone pressure at room temperature an abstraction mechanism would require a first-order dependence upon ketone pressure. While the dimer of CF2, tetrafluoro-ethylene, has never been observed in the reaction mixture, a white solid collected in the cell which was probably a polymer of CF2. While the experimental conditions are not strictly comparable, it is significant that the absorption spectrum of CF2 has been observed in the flash photolysis of 1,3-dichlorotetrafluoroacetone.39 When the temperature is raised, however, the yield of CF2C12 in normal photolysis, increases rapidly suggesting an energy of activation and this process can only be chlorine abstraction. The rate function ... 

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