Isoprene hydroxyl radical reaction

The most important daytime loss process for the biogenics is reaction with OH radicals. Rate constants for the OH reaction with isoprene, a-and /3-pinene are 1.01 X 10-10 cm3 molecule-1 s-1, 5.37 X 10-11 cm3 molecule-1 s-1, and 7.89 X 10 11 cm3 molecule-1 s-1, respectively. Hydroxyl radical reactions with dialkenes and monoterpenes proceed primarily via OH addition across the double bond. Subsequent addition of oxygen to the radical produces a peroxy radical. The reactions of the resulting peroxy radical proceed in a manner similar to those of alkyl peroxy radicals. 

NMHC. A large number of hydrocarbons are present in petroleum deposits, and their release during refining or use of fuels and solvents, or during the combustion of fuels, results in the presence of more than a hundred different hydrocarbons in polluted air (43,44). These unnatural hydrocarbons join the natural terpenes such as isoprene and the pinenes in their reactions with tropospheric hydroxyl radical. In saturated hydrocarbons (containing all single carbon-carbon bonds) abstraction of a hydrogen (e,g, R4) is the sole tropospheric reaction, but in unsaturated hydrocarbons HO-addition to a carbon-carbon double bond is usually the dominant reaction pathway. 

Kleindienst, T.E., Harris, G.W., Pitts, Jr., J.N. (1982) Rates and temperature dependences of the reaction of hydroxyl radical with isoprene, its oxidation products, and selected terpenes. Environ. Sci. Technol. 16, 844-846. 

Paulson, S. E., R. C. Flagan, and J. H. Seinfeld, Atmospheric Photooxidation of Isoprene. 1. The Hydroxyl Radical and Ground State Atomic Oxygen Reactions, hit. J. Chem. Kinet., 24, 79-101 (1992a). 

Measurements of these relatively minor species will not only complete the budget of NO, but will also indicate if our understanding of the hydrocarbon oxidation schemes in the atmosphere is complete. The organic nitrates that completed the NO, budget in the example in Figure 9 arose primarily from the oxidation of the naturally emitted hydrocarbon, isoprene (2-methylbutadiene). To demonstrate the oxidation mechanisms believed to be involved in the production of multifunctional organic nitrates, a partial OH oxidation sequence for isoprene is discussed. The reaction pathways described are modeled closely to those described in reference 52 for propene. The first step in this oxidation is addition of the hydroxyl radical across a double bond. Subsequent addition of 02 results in the formation of a peroxy radical. With the two double bonds present in isoprene, there are four possible isomers, as shown in reactions 2-5 ... 

The further decomposition of acetyl nitrate in the atmosphere has not been studied. The oxidation of isoprene by the hydroxyl radical proceeds via repeated steps of OH addition across the double bond, followed by addition of 02 to form a peroxy radical. The peroxy radical then either oxidizes NO to N02 or adds NO to form an organic nitrate. The alkoxy radical produced in the former step underwent decomposition to form both stable and reactive products. A number of possible pathways exist for forming presumably stable organic nitrates (bold in reactions 7 through 16). 

In addition to being oxidized by the hydroxyl radical, alkenes may react with the N03 radical as has been described by several investigators (52, 56, 66). Listed in Table I are some of the organic nitrates that have been predicted to be produced via reaction of OH and N03 with isoprene and pro-pene. Analogous compounds would be expected from other simple alkenes and from terpenes such as a- and (3-pinene. Other possible organic nitrates may be produced via the oxidation of aromatic compounds (53, 54) and the oxidation of carbonaceous aerosols (67). Quantitative determination of these species has not been made in the ambient atmosphere. 

As Barr et al. (2003) pointed out, the importance of such emissions is determined mainly by their impact on the three processes taking place in the atmosphere. The first consists in that such NMHCs as isoprene form in the course of carboxylization in plants and contribute much thereby to the formation of biospheric carbon cycle. The second process is connected with NMHCs exhibiting high chemical activity with respect to such main oxidants as hydroxyl radicals (OH), ozone (03), and nitrate radicals (N03). Reactions with the participation of such components result in the formation of radicals of alkylperoxides (R02), which favor efficient transformation of nitrogen monoxide (NO) into nitrogen dioxide (N02), which favors an increase of ozone concentration in the ABL. Finally, NMHC oxidation leads to the formation of such carbonyl compounds as formaldehyde (HCHO), which stimulates the processes of 03 formation. Finally, the oxidation of monoterpenes and sesquiterpenes results in the intensive formation of fine carbon aerosol with a particle diameter of <0.4 pm... 

Pedersen and Sehested (2002) showed that the aqueous-phase reaction of isoprene with ozone was insignificant for the processing of isoprene in the atmosphere. They estimated the overall and individual lifetimes of isoprene due to reactions with ozone and the hydroxyl radical, at 25 "C and typical in-cloud conditions. The results (Table 3) indicate that clouds generally should not contribute much to the processing of isoprene in the atmosphere. Only in the aqueous phase, were the lifetimes of isoprene due to reactions with ozone and with OH radicals comparable. Similar conclusions were drawn for methyl vinyl ketone, while for methacrolein the clouds could reduce the overall atmospheric lifetime by 50 %. 

Scheme 3.30 Proposed route based on data from ESI-MS(/MS) monitoring of the formation of the oxygenated products 115-120 in the AIBN-initiated reaction of isoprene with hydroxyl radicals...

As exemplified by the stmctural formulas of a-pinene, P-pinene, A -carene, isoprene, and limonene, shown in Figure 16.1, terpenes contain alkenyl (olefinic) bonds, in some cases two or more per molecule. Because of these and other structural features, terpenes are among the most reactive compounds in the atmosphere. The reaction of terpenes with hydroxyl radical is very rapid, and terpenes also react with other oxidizing agents in the atmosphere, particularly ozone, O3. Turpentine, a mixture of terpenes, has been widely used in paint because it reacts with atmospheric oxygen to form a peroxide, then a hard resin. It is likely that compounds such as a-pinene and isoprene undergo similar reactions in the atmos-... 

Low concentrations of S02 and TBHP were used to initiate the polymerization of MMA and other vinyl monomers. DPPH and hydroquinone do not inhibit this MMA polymerization. End-group analysis indicates the incorporation of sulfonate and hydroxyl end groups in the polymers, and copolymerization results (MMA-isoprene and MMA-acrylic acid) with this S02-TBHP initiator system and AIBN are in good agreement. The over-all polymerization appears to be primarily radical in nature. Inert solvents (benzene, toluene, and xylene) enhance the rate of polymerization of MMA but not of other vinyl monomers (AN, Sty, V A, EM A, MA, etc.). An initiation mechanism involving monomer and solvent appears to be predominant in the case of MMA, while with other monomers an initiation reaction involving only the monomer is predominant. 

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