Isoprene, reaction

Isoprene is highly reactive both as a diene and through its allyhc hydrogens, and its reactions are similar to those of butadiene (qv) (8). Apart from polymerisation, the most widely investigated isoprene reactions are the formation of six-membered rings by the Diels-Alder reaction ... 

From Atkinson and Aschmann (1993) and Atkinson (1997a) Donahue et al. (1998) have reported OH yields of 0.4-0.8 at low pressures (4-6 Torr) in N2 from the ethene, rra/w-2-butene, 2,3-di-methyl-2-butene, and isoprene reactions see Fig. 6.22 for biogen-ics structures. 

For example, in the isoprene reaction with O, H202 has been observed as a product, with its yield increasing as the relative humidity increased from 1 to 9% (Sauer et al., 1999). The yields of methacrolein and methyl vinyl ketone also increased simultaneously, as expected if they were also formed in part from the decomposition of an intermediate hydroxyhydroperox-ide. 

A small portion of the reaction proceeds by what appears to be abstraction of the allylic hydrogen atom from the weaker C-H bond (e.g., Lee and Rowland, 1977). In the case of the isoprene reaction, for example, about 15% of the overall reaction at 1 atm pressure and room temperature proceeds by abstraction (Ragains and Finlayson-Pitts, 1997). It should be noted,... 

FIGURE 10.5 Mechanistic representation of the ring-closure reactions for the -hydroxy-peroxy radicals arising from the OH-isoprene reactions. 

Analogous studies on the dimerization of cyclopentadiene in water revealed a stabilization of the transition structure relative to the initial structure as a result of a difference in solvation of 1.7 kcalmol. Unfortunately, at least to our knowledge, reliable experimental data for this process are not available. Recently, in a similar approach, the Gibbs enthalpies of hydration of the Diels-Alder reaction of cyclopentadiene with isoprene and methyl vinyl ketone were determined. Surprisingly, it was observed that water stabilized the transition structure of the cyclopentadiene - - isoprene reaction more than that of the... 

Table 4. Relative rates of isoprene reactions with OH, NO3, O3 and 804 in the gas phase (rx,g) and in the aqueous phase (rx,aq) at 25 C.

Karl M., Th. Brauers, H.-P. Dom, F. Holland, M. Komenda, D. Poppe, F. Rohrer, L. Rupp, A. Schaub and A. Wahner, Kinetic study of the OH-isoprene and Os-isoprene reaction in the atmosphere simulation chamber, SAPHIR, Geophys. Res. Lett. 31 (2004), L05117, doi 10.1029/2003GL019189. 

These reactions proceed by initial ozone attack on the C = C bond of the olefin. An intermediate ozonide is formed, which rapidly decomposes to a carbonyl and a biradical. The biradical can be stabilized, or it can decompose. Paulson et al. (1991b) found the products methacrolein, methyl vinyl ketone, and propene, in yields of 68%, 25%, and 7%, respectively. Based on the presence of epoxides in the ozone/isoprene system, Paulson et al. concluded that 0(3P) was being formed. Calculations indicated that 0.45 0(3P) radicals were formed for every ozone/isoprene reaction. However, Atkinson et al. (1993) recently showed that the epoxides were formed directly from the reaction with ozone rather than the reaction with 0(3P). The epoxides formed were l,2-epoxy-2-methy 1-3-butene and l,2-epoxy-3-methyl-3-butene, in yields of 0.028 and 0.011, respectively. There was also definite evidence for the formation of OH radicals in the ozone system, thus causing difficulties in product analyses. Each ozone/isoprene reaction yielded 0.68 OH radicals (Paulson et al., 1991b). 

Limited product data are available for the nitrate radical reaction with isoprene and a- and /3-pinene. For the isoprene reaction, Barnes et al. (1990) determined molar yields of total nitrates of 80%, CO of 4%, and formaldehyde of 11%. Methacrolein was also detected, but an accurate product yield could not be determined. Additional work with a- and /3-pinene was not conclusive. Although nitrate features were observed as initial reaction products, these compounds quickly transferred into aerosols, thus preventing identification via the methods employed in the study (FTIR spectroscopy). 

Paulson, S. E Flagan, R. C., and Seinfeld, J. H., Atmospheric photooxidation of isoprene. Part 2. The ozone-isoprene reaction. Int. J. Chem. Kinet. 24, 103 (1991b). 

The OH-isoprene reaction proceeds almost entirely by addition of the OH radical to the C=C double bonds. Formaldehyde, methacrolein (CH2=C(CH3)CHO), and methyl vinyl ketone (CH2=CHC(0)CH3) have been identified in the laboratory as major products of the OH-isoprene reaction. Figure 6.18 shows the initial steps of OH attack on... 

The 03-isoprene reaction proceeds by initial addition of 03 to the C=C double bonds to form two primary ozonides, each of which decomposes to two sets of carbonyl plus biradical products (a minor channel is apparently formation of a 1,2-epoxymethyl butene). This mechanism is consistent with the formation of formaldehyde, methacrolein, and methyl vinyl ketone. As in other 03-alkene reactions, OH radicals are observed in significant yield, about 0.27 molecule of OH per 03-isoprene reaction. 

The OH-isoprene reaction proceeds almost entirely by addition of the OH radical to the C=C double bonds. Formaldehyde, methacrolein (CH2=C(CH3)CHO), and methyl vinyl ketone (CH2=CHC(0)CH3) have been identified in the laboratory as major products of the OH-isoprene reaction. Figure 5.12a shows the initial steps of OH attack on isoprene. OH can add to four different positions in the isoprene molecule. The third and fourth radicals from the top of the figure are allylic radicals that have two radical centers, each of which yields a different peroxy radical when reacting with O2. The result is six different peroxy radicals, shown at the right of Figure 5.12a. Each of these peroxy radicals may react with NO, HO2, and RO2. We show, in Figure 5.12b, only the NO reaction pathways. The major products, methacrolein. 

The major products of the OH-, NO,-, and 03-isoprene reactions, such as methacrolein, methyl vinyl ketone, and nitrato-substituted unsaturated Cj-carbonyls, are themselves reactive toward OH and O3 (Atkinson, 1994), and methacrolein and methyl vinyl ketone may also photolyze. 

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