Ozone-Alkene Chemistry

Another potential dark source of in the atmosphere, more particularly in the boundary layer, is from the reactions between ozone and alkenes. The ozonolysis of alkenes can lead to the direct production of the OH radical at varying yields (between 7 and 100%) depending on the structure of the alkene, normally accompanied by the co-production of an (organic) peroxy radical. As compared to both the reactions of OH and NO3 with alkenes the initial rate of the reaction of ozone with an alkene is relatively slow, this can be olfset under regimes where there are high concentrations of alkenes and/or ozone. For example, under typical rural conditions the atmospheric lifetimes for the reaction of ethene with OH, O3 and NO3 are 20 h, 9.7 days and 5.2 months, respectively in contrast, for the same reactants with 2-methyl-2-butene the atmospheric lifetimes are 2.0 h, 0.9 h and 0.09 h. 

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Grosjean, D., Grosjean, E., Williams, II, E.L. (1994) Atmospheric chemistry of olefins A product study of the ozone — alkene reaction with cyclohexane added to scavenge OH. Environ. Sci. Technol. 26, 186-196. 

It should be noted that while the mechanism outlined in this section describes the overall features of 0,-alkene chemistry, there are also other minor paths as well. For example, small yields of epoxides that appear to be formed in the primary reaction have been observed as products of the reactions of some dienes and cycloalkenes (e.g., see Paulson et al., 1992b and Atkinson et al., 1994a, 1994b). The reader should consult the rather extensive ozone literature for further details on both the condensed- and gas-phase reactions. 

Grosjean, D E. Grosjean, and E. L. Williams, Atmospheric Chemistry of Olefins A Product Study of the Ozone-Alkene Reaction with Cyclohexane Added to Scavenge OH, Emiron. Sci. TechnoL, 28, 186-196 (1994c). 

All carbonyl oxides proved to be highly photolabile, and on photolysis yield dioxiranes 3 or split off oxygen atoms to produce ketones. Oxygen atoms are also formed thermally from vibrationally excited 1. Thus, if the large exothermicity of the ozonolysis reaction is taken into account, 1 might be a source of O atoms and OH radicals in the troposphere. The role of dioxiranes has not yet been discussed in context with atmospheric chemistry, although the formation of these species in contrast to the isomeric carbonyl oxides - in ozone/alkene reactions has been unequivocally demonstrated [13]. 

Addition compounds called ozonides are produced when alkenes react with ozone and reductive cleavage of these compounds is used extensively in preparative and diagnostic organic chemistry. 

Further chemistry of alkenes and alkynes is described in this chapter, with emphasis on addition reactions that lead to reduction and oxidation of carbon-carbon multiple bonds. First we explain what is meant by the terms reduction and oxidation as applied to carbon compounds. Then we emphasize hydrogenation, which is reduction through addition of hydrogen, and oxidative addition reactions with reagents such as ozone, peroxides, permanganate, and osmium tetroxide. We conclude with a section on the special nature of 1-alkynes— their acidic behavior and how the conjugate bases of alkynes can be used in synthesis to form carbon-carbon bonds. 

In addition to OH radicals, unsaturated bonds are reactive towards O3 and NO3 radicals and reaction with these species is an important atmospheric degradation mechanism for unsaturated compounds. Table 4 lists rate constants for the reactions of 03 and NO3 radicals with selected alkenes and acetylene. To place such rate constants into perspective we need to consider the typical ambient atmospheric concentrations of O3 and NO3 radicals. Typical ozone concentrations in pristine environments are 20-40 ppb while concentrations in the range 100-200 ppb are experienced in polluted air. The ambient concentration of NO3 is limited by the availability of NO sources. In remote marine environments the NO levels are extremely low (a few ppt) and NO3 radicals do not play an important role in atmospheric chemistry. In continental and urban areas the NO levels are much higher (up to several hundred ppb in polluted urban areas) and NO3 radicals can build up to 5-100 ppt at night (N03 radicals are photolyzed rapidly and are not present in appreciable amounts during the day). For the purposes of the present discussion we have calculated the atmospheric lifetimes of selected unsaturated compounds in Table 4 in the presence of 100 ppb (2.5 x 1012 cm 3) of O3 and 10 ppt (2.5 x 108 cnr3) of NO3. Lifetimes in other environments can be evaluated by appropriate scaling of the data in Table 4. As seen from Table 4, the more reactive unsaturated compounds have lifetimes with respect to reaction with O3 and NO3 radicals of only a few minutes ... 

The cleavage of alkenes by ozone, usually to give carbonyl compounds as products, is a reaction which has been widely exploited in synthesis and on which a great deal of mechanistic work has been carried out. Reviews of this chemistry are available.The ozonolysis usually leads to the formation of one of two distinct types of peroxidic product (Scheme 34) the cyclic peroxides (53), which are formed in nonnucleophilic solvents, and acyclic hydroperoxides, such as (54X which are formed in the... 

Ozone reacts with alkenes (olefins) through addition to the double bond. The initially formed ozonides decompose further. Schbnbein appears to have performed the first such identified ozonolysis (on ethylene). Turpentine is a complex mixture of olefinic terpenes (see P.S. Bailey, Ozonation in Organic Chemistry, Academic Press, New York, 1978, pp. 1—4, for a brief historical perspective). 

New Applications of Tetracyanoethylene (TCNE) in Organic Chemistry, A. J. Fatiadi (1986). This review with 501 references deals with reactions of tetracyanoethylene used in organic synthesis. Information on molecular complexes, ozonization of alkenes and acetylenes, dehydrogenation and tricyanovinylation, reactions of TCNE oxide, reactions with ketones and diketones, synthesis of heterocycles and cationic polymerizations are included in this survey. Some industrial and analytical applications are also discussed. 

Ozone also reacts with low concentrations of NO2 to form (Equation 4.24) an oxidant that is important in nighttime atmospheric chemistry, the nitrate radical, NO3. This species, which is unstable in the presence of light, reacts fairly rapidly with many compounds such as alkenes (including terpenes), phenols and other aromatic compounds,... 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

Ozone plays a major role in the degradation of unsaturated VOCs in the troposphere, especially during night-time. The rate constants of the ozonolysis of a variety of alkenes have been reported [1]. However, in most instances the fate of the primary products of the ozonolysis is unknown, although the secondary reaction products are of crucial importance for the overall understanding of the alkene/ozone chemistry. The classical Criegee mechanism of the ozonolysis reaction involves the primary ozonide (POZ, 1,2,3-trioxolane), which cleaves to the Criegee intermediate (carbonyl O oxide) and a carbonyl compound [2, 3]. The secondary ozonide (SOZ, 1,2,4-trioxolane) is formed from these components in a [l,3]-dipolar cycloaddition reaction. 

The cycloaddition can also be analyzed in terms of HOMO-LUMO interactions (see page 756), with the interaction of the LUMO of ozone and the HOMO of the alkene being dominant. For a discussion, see Kuczkowski, R. L. in Padwa, A., Ed. 1,3-Dipolar Cycloaddition Chemistry, Vol. 2 Wiley-Interscience New York, 1984 pp. 197-276. 

Mechanism of Ozonolysis (Criegee mechanism) The initial step of the reaction involves a 1,3-dipolar cycloaddition of ozone to the alkene leading to the formation of the primary ozonide (molozonide or 1,2,3-trioxolane), which decomposes to give a carbonyl oxide and a carbonyl compound. The carbonyl oxides are similar to ozone in being 1,3-dipolar compounds and undergo 1,3-dipolar cycloaddition to the carbonyl compound with the reverse regio-chemistry, leading to a relatively stable secondary ozonide (1,2,4-trioxolane) (Scheme 5.47). 

In previous sections we have mentioned that oxidation in organic chemistry includes the removal of hydrogen or the addition of oxygen. Alkenes can easily be oxidized in a reactions with ozone, O3. The reaction mechanism involves the formation of a reactive intermediate, the ozonide, which subsequently decomposes into two molecules that can be either aldehydes or ketones, depending on the structure of the starting alkene (see the next scheme). 

Atkinson, R. Gas-phase tropospheric chemistry of volatile organic compounds. 1. Alkanes and alkenes. J. Phys. Chem. Ref. Data 1997, 26(20), 215-290 Hisahiro Einaga, Shigeru Futamura. Oxidation behavior of cyclohexane on alumina-supported manganese oxides with ozone. Applied Catalysis B Environmental 2005, 6049-55. 

The formation of OH radicals by the reaction of alkenes and O3 in the polluted atmosphere is very important in atmospheric chemistry of ozone formation described in the next section. The yields of OH radicals under the atmospheric pressure are compiled by the lUPAC subcommittee (Atkinson et al. 2006) and summarized in Table 7.3. These values are those obtained by the batch type experiments with OH scavengers, while Kroll et al. (2001a) investigated pressure dependence (1 00 torr) of the initial OH yields in the reaction of O3 and alkene using a high-pressure flow system coupled with the direct detection of OH by LIF. From this experiment, strong pressure dependence was found for the OH formation yields from alkenes other than ethylene with the OH yields higher than unity at low... 

The photodecomposition of the various oxidation products of the alkanes, alkenes, and the aromatic hydrocarbons play important roles in the chemistry of the urban, mral, and remote atmospheres. These processes provide radical and other reachve products that help drive the chemistry that leads to ozone generation and other important chemistty in the troposphere. In this chapter, we have reviewed the evidence for the nature of the primary processes that occur in the aldehydes, ketones, alkyl nitrites, nittoalkanes, alkyl nitrates, peroxyacyl nitrates, alkyl peroxides, and some representative, ttopospheric, sunlight-absorbing aromatic compounds. Where sufficient data exist, estimates have been made of the rate of the photolytic processes that occur in these molecules by calculation of the photolysis frequencies ory-values. These rate coefficients allow estimation of the photochemical lifetimes of the various compounds in the atmosphere as well as the rates at which various reactive products are formed through photolysis. 

As we have seen in our discussions in this chapter, photochemistry provides an important part of the atmospheric chemistry that occurs within the troposphere. The direct photochemistry of the alkanes and alkenes is unimportant within the troposphere, since they do not absorb the available sunlight. Those aromatic hydrocarbons that absorb tropospheric sunlight (largely the polycyclic hydrocarbons) do not appear to photodecompose (Calvert et al., 2002). However, photochemistry is indirectly responsible for the decay of all of the hydrocarbons by the dominant pathway of OH attack. It is the photodecomposition of ozone that is the major source of the OH radicals that accounts for the majority of the observed destmction of the hydrocarbons (RH) and the subsequent generation of oxidation products including ozone build up in the troposphere ... 

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