Hydroxyl radical chlorinated hydrocarbons

The reaction of volatile chlorinated hydrocarbons with hydroxyl radicals is temperature dependent and thus varies with the seasons, although such variation in the atmospheric concentration of trichloroethylene may be minimal because of its brief residence time (EPA 1985c). The degradation products of this reaction include phosgene, dichloroacetyl chloride, and formyl chloride (Atkinson 1985 Gay et al. 1976 Kirchner et al. 1990). Reaction of trichloroethylene with ozone in the atmosphere is too slow to be an effective agent in trichloroethylene removal (Atkinson and Carter 1984). 

A similar statement could probably be made concerning ketones. These compounds are commonly used as solvents, and they are known to form fi radicals when photolyzed. Many chlorinated hydrocarbons, which are also widely used as solvents, can be attacked by hydroxyl radicals and thus contribute to peroxy radical formation. 

Masten S J, Hoignd J (1992) Comparison of Ozone and Hydroxyl Radical-Induced Oxidation of Chlorinated Hydrocarbons in Water, Ozone Science Engineering 14 197-214. 

Masten, S.J. and Hoigne, J., Comparison of ozone and hydroxyl radical-induced oxidation of chlorinated hydrocarbons in water, Ozone Sci. Eng., 14, 197-213, 1992. 

S Halogenated Compounds Many halogenated compounds, either because of their extensive use or their persistence are of environmental interest. The smaller, halogenated aliphatics have been used extensively and are common ground water contaminants," while the persistence of the PCBs and chlorinated dibenzo dioxins is well documented. Hydroxyl radicals are the more important oxidants in the vapor phase and the hydrocarbon structure determines the nature of the reaction. Second-order rate constants for some of these compounds are summarized in Table 6.27. 

Halons Halons are hydrocarbons in which atoms from halogens replace one or more hydrogen atoms. Halons include fluorine, chlorine, bromine, or iodine. There are a variety of halon systems. Extinguishing involves interruption of the hydroxyl radical of combustion. Some systems are discontinued because of toxicity and corrosion effects. Today two halons are in general use Halon 1211 (bromochlorodi-fluoromethane) and Halon 1301 (bromotrifluoromethane). Effectiveness is a function of many factors. Halon 1301 is best suited for total flooding applications, whereas Halon 1211 is weU suited for local application systems. Halon... 

The latter two properties create the problem Because of the insolubility in water, they are not removed by rainfall, and they are inert towards the hydroxyl radical. The reaction with this radical to form water is the process, that initiates the oxidation of hydrocarbons. Thus the CFCs are not removed by the common cleansing mechanisms that operate in the lower atmosphere, instead they rise into the stratosphere, where they are destroyed by solar short-wave UV-radiation releasing the ozone-depleting chlorine atoms. Because transport into the stratosphere is very slow, the residence time for CFC s in the environment is extremely long, up to the order of one century, so they accumulate in the atmosphere. 

This means that in sulphuric acid the two hydroxyl groups are exactly the same, for whichever one is replaced by chlorine and then by a hydrocarbon radical the resulting compound is the same. Therefore sulphuric acid must be. ... 

The kinetics of the reactions of many xenobiotics with hydroxyl and nitrate radicals have been examined under simulated atmospheric conditions and include (1) aliphatic and aromatic hydrocarbons (Tuazon et al. 1986) and substituted monocyclic aromatic compounds (Atkinson et al. 1987c) (2) terpenes (Atkinson et al. 1985a) (3) amines (Atkinson et al. 1987a) (4) heterocyclic compounds (Atkinson et al. 1985b) and (5) chlorinated aromatic hydrocarbons (Kwok et al. 1995). For PCBs (Anderson and Hites 1996), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 days for 3-chlorobiphenyl to 34 days for 2,2, 3,5, 6-pentachlorbiphenyl. It was estimated that loss by hydroxylation in the atmosphere was a primary process for removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a... 

The collapsing bubble interface results in the formation of hydroxyl and hydrogen radicals. These radicals destroy chlorinated orgamcs and petroleum hydrocarbons very effectively. 

What has been described as biomimetic control of chemical selectivity is already possible. When the steroid 3a-cholestanol is esterified with 4-iodophenylacetic acid and treated with chlorine in the dark the iododichloride can generate free radicals which attack the C-17 of the steroid. The shorter ester derived from benzoic acid attacks C-9 (Figure 6.33). In another context metal porphyrin complexes have been devised which can hydroxylate hydrocarbons (Figure 6.34). Many more ideas of this kind are likely to follow a better understanding of the mechanisms of enzyme catalysis. 

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