Hydroxyl radicals atom reaction with

Experiments performed at room temperature and at SF6 pressures of 250-1000 mbar show that the kinetics of decay of the C2F5 radicals does not depend on the total pressure. The reaction of the oxygen atoms produced in the hydroxyl radical self-reaction with the ethyl and hydroxyl radicals is not important in this pressure range and has been omitted in the reaction scheme. The value of k,+2 = (1.1 0.2) x 10"10 cm3molecule"1s"1 estimated by Fagerstrom et al.203 can be considered as an upper limit of the high-pressure limiting rate constant kl oo for reaction (1). 

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. 

Since the oxygen atom then just reacts with O2 to replenish O, this path consisting of reactions 5.21b, 5.22, and 5.2 is Just another null cycle. Occasionally, however, 0( D) collides with H2O and produces two hydroxyl radicals (recall reaction 4.17),... 

An alternative route involves end-capping to produce a terminal hydroxyl followed by reaction with trichloroacetyl isocyanate (Equation 5.23). This new, reactive end group can be used to initiate the growth of a second block via the photoreduction method proposed by Bamford in which magnesium or ihenium carbonyls are excited by UV or visible radiation and extract a chloride atom from the terminal unit, thereby creating a radical site. As only one radical is formed, this is a much cleaner reaction compared with (1) however, block lengths are more difficult to control in both these radical reactions, and the exact structure of the product formed can depend on the mechanism of the termination reaction. 

The chemistry of organolead compounds in the environment has been reviewed elsewhere [93-96], but a few salient aspects are summarized here. First, tetra-alkyllead compounds are volatile Henry s Law constant of 4.7 x 10" and 6.9 X 10" (Pam )/mol for tetramethyl and tetraethyllead, respectively, according to Wang et al. [97]. However, only a small fraction of the Pb leaving an automobile as exhaust is in this form, e.g., typically 0.1-10% [95]. In addition to the relatively low emission factor from leaded gasoline combustion, tetra-alkyllead compounds are rapidly decomposed by homogeneous gas phase reactions such as photolysis, reaction with ozone, triplet atomic oxygen, or hydroxyl radical [98,99] with half-lives of less than 10 h in summer and 40 h in... 

Taking the order of magnitude, these results are, of course, not surprising considering that a sulfur-hydrogen bond in thiols is generally weaker by up to about 20 kcal mol" (ca. 85 kJ moE ) than a carbon-hydrogen bond. An even faster H-atom abstraction is achieved by hydroxyl radicals which occurs with rate constants of several 10 M s [13], reaction (2) ... 

Effect of Hydroxyl Radicals on Ozone Depletion. Hydroxyl radicals, formed by reaction of ( D) oxygen atoms with water or CH, can destroy ozone catalyticahy (11,32) as shown in the following reactions. 

A chlorohydrin has been defined (1) as a compound containing both chloio and hydroxyl radicals, and chlorohydrins have been described as compounds having the chloro and the hydroxyl groups on adjacent carbon atoms (2). Common usage of the term appHes to aUphatic compounds and does not include aromatic compounds. Chlorohydrins are most easily prepared by the reaction of an alkene with chlorine and water, though other methods of preparation ate possible. The principal use of chlorohydrins has been as intermediates in the production of various oxitane compounds through dehydrochlorination. 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

For polychlorinated biphenyls (PCBs), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 d for 3-chlorobiphenyl to 34 d for 236-25 pentachlorobiphenyl (Anderson and Hites 1996). It was estimated that loss by hydroxy-lation in the atmosphere was a primary process for the removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a hydroxyl radical at the 1-position followed by transannular dioxygenation at the 2- and 5-positions followed by ring fission (Brubaker and Hites 1998). Reactions of hydroxyl radicals with polychlorinated dibenzo[l,4]dioxins and dibenzofurans also play an important role for their removal from the atmosphere (Brubaker and Hites 1997). The gas phase and the particulate phase are in equilibrium, and the results show that gas-phase reactions with hydroxyl radicals are important for the... 

The stability of perchlorofluoroalkanes is due to the absence of hydrogen atoms that may be abstracted by reaction with hydroxyl radicals. Attention has therefore been directed to chlorofluo-roalkanes containing at least one hydrogen atom (Hayman and Derwent 1997). Considerable effort has also been directed to the reactions of chloroalkanes and chloroalkenes, and this deserves a rather more detailed examination in the light of interest in the products that are formed. 

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