Reaction with atomic chlorine table

The replacements and alternates for the CFCs (Table 13.1) are characterized by having abstractable hydrogen atoms, and hence they are removed to varying extents by reaction with OH in the troposphere before reaching the stratosphere. The HFCs do not contain chlorine at all, so that their ODPs are very small, essentially zero (Table 13.3). In this section we discuss briefly the tropospheric chemistry of HCFCs and HFCs. 

Due to the highly exothermic nature of the process, the replacement of primary, secondary and tertiary hydrogens upon reaction with electrophilic fluorine atoms is not as selective as for other radicals. For example, early work by Tedder [30,34], showed that the order of selectivity follows the usual pattern, i. e. tert > sec > prim, but the relative selectivity of fluorine atoms is less than chlorine atoms (Table 2). 

The halocarbons, which are not destroyed in the troposphere by reactions with hydroxyl, pass into the stratosphere where they are photo-dissociated to liberate chlorine atoms which attack ozone. Only one of them is of natural origin, methyl chloride CH3CI, but there are also several industrial products, especially carbon tetrachloride, CC14, trichlorofluo-romethane, CFC13, and dichlorodifluoromethane. Methyl chloride (Table III) has a natural marine origin (for details, see ref. 12), but it is certainly present also in the smoke produced when polyvinyl and other products containing chlorine are burnt. In addition, it is produced naturally not only in forest fires, but also in tropical agriculture based on the cultivation... 

The deuterium isotope effect in the photo-induced chlorine atom exchange reaction with HCl has been investigated by Klein et over the range 30-150 °C by using the competition for chlorine atoms between D2 and DCl (or HCl). The exchange reactions were labeled with radioactive C1. Known mixtures of Cl2, D2, and DCl (or HCl) were irradiated followed by measurement of the residual D2 as well as the activity of the DCl. From this data the rate of the isotope exchange reaction could be determined with respect to the rate of chlorination of deuterium. The results are listed in Table 12 for the reactions... 

A variety of methods has been devised for the confirmation of heptachlor residues (Table II). The presence in the heptachlor molecule (Figure 1) of a reactive allylic chlorine atom has been the basis of three confirmatory tests based on its ease of replacement. Reaction with a silver acetate-glacial acetic acid mixture produced 1-acetoxychlordene which, with the GLC conditions used, had a retention time close to heptachlor epoxide 44). Of the common organochlorine pesticides, only heptachlor reacted quantitatively. Endrin reacts to a small extent with the glacial acetic acid to give a secondary endrin ketone peak. When the reaction of heptachlor with silver salts was extended to silver carbonate in aqueous alcohol, 1-hydroxychlordene was obtained which can easily be converted to the more volatile and GG-responsive silyl ether. Unfortunately, this silyl ether has a Rt identical to aldrin. With silver carbonate, hepta-... 

Bromine is potentially able to interact with stratospheric ozone in the same manner as chlorine (Wofsy et al., 1975). The catalytic cycle for bromine is expected to be quite efficient, because its reaction with methane is slower than that of Cl atoms in addition, the reaction of OH with HBr is faster than that of OH with HC1. The major bromine compound in the troposphere is methyl bromide, which has a natural origin and occurs with a mixing ratio of about 10 pptv (see Table 6-14). This seems small enough to neglect bromine to a first approximation. 

Acyl radicals can fragment by loss of carbon monoxide. Decarbonylation is slower than decarboxylation, but the rate also depends on the stability of the radical that is formed. For example, rates for decarbonylations giving tertiary benzylic radicals are on the order of 10 s whereas the benzoyl radical decarbonylates to phenyl radical with a rate on the order of 1 s (see also Table 11.3, Entries 45 to 48). When reaction of isobutyraldehyde with carbon tetrachloride is initiated by f-butyl peroxide, both isopropyl chloride and isobutyroyl chloride are formed, indicating that decarbonylation is competitive with the chlorine atom transfer. 

The determination of absolute rate coefficients of transfer reactions of bromine atoms is much more favourable than for the corresponding reactions of fluorine or chlorine atoms. This arises because the dissociation constant of molecular bromine is high at normal experimental temperatures and the chain lengths in bromination are relatively short. The rate constant of the reaction of bromine atoms with molecular hydrogen was the first quantitative kinetic study of a radical reaction [96]. Fettis and Knox [52] evaluated the data for the Br—Hj reaction and their results are given in Table 7. Trotman-Dickenson [1] has pointed out that the subsequent data of Timmons and Weston [80] for the reaction with Hj, HD and HT are not fully compatible with the conclusions of Pettis and Knox [52]. 

The data for the chlorine atom reactions with the chloroethylenes are collected in Table 13. If the log k values are correct, and in fact the reaction has a very small temperature coefficient, then it is interesting to see how little difference in the reactivity there is between the various chloroethylenes. Thus, trichloroethylene and tetrachloroethylene may be roughly one-half and one-tenth as reactive as ethylene, respectively, but that is a pretty restricted reactivity range, compared with that found for many other radicals. 

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