Reaction rate constants, chlorinated

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. 

De, A.K., Chaudhuri, B., Bhattacharjee, S., Dutta, B.K., Estimation of OH radical reaction rate constants for phenol and chlorinated phenols using UV/H202 photo-oxidation, /. Haz. Mat., 64, 91-104, 1999. 

Table 5. First order reaction rate constants (L water/g Pd-min) for chlorinated ethylenes...

Air tropospheric lifetime was calculated based on the gas-phase reaction rate constant for a di-chlorinated PCDF with OH radicals to be 0.7-1.0 d (Atkinson 1991) ... 

In the second case, the phenoxide is monofunctional, but the chloro-monomer is difunctional. This reaction provided a test of the relative reactivities of the two chlorines on 4,4 -dichlorodiphenylsulfone. Figure 5 does show a slight curvature which indicates a difference in the reactivities. Using Equations 8 and 9, the reaction rate constants (fci and k2) were determined to be 0.10 and 0.053 liter/mole-min. Therefore, the assumption of equal chlorine reactivity (which was made in deriving these equations for the polymerization reaction) is not entirely correct. A complete theoretical analysis would be very involved at least two more reaction rate constants would be necessary, and the experimental data obtained would not be sufficient to determine all of these constants. 

Figure 11.11. Summary of reaction rate constants and half-time values of different compounds in the presence of chlorine dioxide versus pH. Assumptions for t /2 scale [CIO2] = 1 mM [P] [C102]consf (From Hoigne and Bader, 1994.)...

In the atmosphere, the vapor-phase reaction of PCBs with hydroxyl radicals (photochemicaUy formed by sunlight) is the dominant transformation process (Brubaker and Hites 1998). The calculated tropospheric lifetime values for this reaction increases as the number of chlorine substitutions increases. The tropospheric lifetime values (determined using the calculated OH radical reaction rate constant and assuming an annual diurnally averaged OH radical concentration of 5x10 molecule/cm ) are 5-11 days for monochlorobiphenyls, 8-17 days for dichlorobiphenyls, 14—30 days for trichlorobiphenyls,... 

Abiotic degradation of SCCPs in the air has been reported. [11,12] estimated that the second reaction rate constant of SCCPs with Cio-13 and 49-71 % chlorine with OH radical was 2.2-S.2 x 10 cm molecule s The author reported that the half-life in air was estimated to be 1.9-7.2 days, based on the assumption that the OH radical concentration in the air was 5 x 10 molecule cm. In this assessment, the half-life as a parameter for modeling is set to be 3.1 days, which is estimated by multiplying the median of the second reaction rate of... 

Self-Reaction Kinetics. Of all peroxy radical reactions, the self-reaction between two identical peroxy radicals is perhaps the most studied. The measurement of peroxy radical UV absorption cross sections, discussed above, often occurs under the assumption that all the chlorine or fluorine atoms produced by photolysis are converted quantitatively into peroxy radicals however, this assumption must be corrected for by the loss of peroxy radicals from self-reaction. Furthermore, studies of RO2 -b NO or RO2 -f HO2 reactions usually take place at sufficiently high RO2 concentrations to require knowledge of the self-reaction rate constant, in order to interpret the results of the kinetics measurements. Both concerns make laboratory studies of peroxy self-reaction kinetics an important issue. In contrast, the steady-state atmospheric concentrations of HCFC-based peroxy radicals are probably too small for their self-reactions to be relevant to atmospheric chemistry. In this context, the most important peroxy-peroxy radical reactions would be between the HCFC-based peroxy radicals and CH3O2, but such reactions have not been studied to date. 

Other secondary chlorine species (atomic Cl, CIO, ClOOCl etc.) have been made responsible for Arctic ozone depletion, whereas the sources of the chlorine atoms are poorly understood (Keil and Shepson 2006). The Cl atom reacts similarly to OH (e. g. in oxidation of volatile organic compounds Cai and Griffin 2006). However, the photolysis of HCl is too slow (even in the stratosphere) to provide atomic Cl. Thus, the only direct Cl source from HCl is due to its reaction with OH, but with a fairly low reaction rate constant (Rossi 2003). There are several chemical means of production of elemental Cl (and other halogens) from heterogeneous chemistry (see Chapter 5.8.2) in the troposphere the photolysis of chloroorganic is not very important, with a few exceptions (see Chapter 5.8.1). 

While OH radical reaction rate constants have been successfully generated to extend structure-activity predictive capability for thiocarbamates and chlorinated aromatics, further e q>erimental work will be inq>ortant to assess OH reaction rates for die more complex chemistries diat con rise die majority of high-use urban and agricultural pesticides. 

Example 17.1-3 Finding the reaction-rate constant from mass transfer data In studies with a wetted-wall absorption column, we find that the mass transfer coefficient for chlorine into water is 16 10 cm/sec. The chlorine presumably is irreversibly reacting with the water ... 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

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... 

Rate constants have been measured for the reactions of boron compounds with a series of bromomethanes and bromofluoromethanes. Previously it was shown that the reactivity of the chlorine in chlorofluoromethane is substantially reduced by increasing fluorine substitution. The corresponding decrease in the reactivity of bromolluoromethane u as not observed [ifS]. 

A single chlorine atom can bring about the decomposition of tens of thousands of ozone molecules. Bromine atoms can substitute for chlorine indeed the rate constant for the Br-catalyzed reaction is larger than that tor the reaction just cited. 

It has been proposed that aromatic solvents, carbon disulfide, and sulfur dioxide form a complex with atomic chlorine and that this substantially modifies both its overall reactivity and the specificity of its reactions.126 For example, in reactions of Cl with aliphatic hydrocarbons, there is a dramatic increase in Ihe specificity for abstraction of tertiary or secondary over primary hydrogens in benzene as opposed to aliphatic solvents. At the same time, the overall rate constant for abstraction is reduced by up to two orders of magnitude in the aromatic solvent.1"6 The exact nature of the complex responsible for this effect, whether a ji-coinplex (24) or a chlorocyclohexadienyl radical (25), is not yet resolved.126- 22... 

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