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CH3CHO + ch3ch2ch2-

CH3- + CH3CH2CH2CHO

CH3CH(OH)CH2CH2CH2-

CH3C(0)CH2CH2CH3 + HO2

Rate constants for alkoxy radical isomerizations can be combined with rate constants for alkoxy radical decomposition and reaction with 02 to predict the relative importance of the three pathways (Atkinson 1994). Alkoxy radicals can also react with NO and N02, but under ambient tropospheric conditions these reactions are generally of negligible importance.

Atmospheric Photooxidation of Propane To illustrate alkane chemistry, consider the atmospheric photooxidation of propane:

Internal H-atom abstraction predominates in the initial OH attack. As we have been doing, we can eliminate the fast reactions from the mechanism by combining them with the foregoing rate-determining step. Thus the four reactions above may be expressed more concisely as

If we further take the liberty of assuming that the CH3CH(02-)CH3 radical is produced and consumed only in these two reactions, a good assumption in this case, these two reactions can be written as a single overall reaction:

where the reaction converts one molecule of NO to one molecule of N02- If we assume further that the sole fate of the h02 radical is reaction with NO, we can

CH3CH2CH3 + OH- —*CH3CHCH3 4- H20 CH3CHCH3 TÛ2 —» CH3CH(02-)CH3

fast

CH3CH(02-)CH3 + N0 —>N02 + CH3CH(0-)CH3 CH3CH(0-)CH3 +O2 —» CH3C(0)CH3 + HO2

fas( (Acetone)

CH3CH2CH3 + OH- CH3CH(02-)CH3 CH3CH(02-)CH3 + NO N02 + CH3C(0)CH3 + HO2-

eliminate H02 from the right-hand side:

In writing the reaction this way, we can clearly see that the net effect of the hydroxy! radical attack on propane is conversion of two molecules of NO to N02, the production of one molecule of acetone, CH3C(0)CH3 and the regeneration of the hydroxy] radical. Thus the photooxidation of propane can be viewed as a chain reaction mechanism in which the active species, the hydroxy 1 radical, is regenerated. For larger alkanes, such as n-butane, the atmospheric photooxidation mechanisms become more complex, although they continue to exhibit the same essential features of the propane degradation path. Two important issues arise in the reaction mechanisms of the higher alkanes. The first is that some fraction of the peroxyalkyl-NO reactions lead to alkyl nitrates rather than N02 and an alkoxy radical. The second is that the larger alkoxy radicals may isomerize as well as react with 02. Figure 6.13 shows the mechanism of the n-butane-OH reaction (Jungkamp et al. 1997),

, CHjCHJCHJCHJ |

CH,CHJCH2CH;0,' NO

CH^CHXHiCHjONO,

CH3CH,02- + |CHjCHO| |CH,CH,CH,CH0| CH2(0, ■) CH2CH2CH2OH

1"

ICH^CHOl +

CH2(0 *) CHJCHjCHJOH isom.

CH2(OH)CH

,CH,CHOH

FIGURE 6.13 Atmospheric photooxidation mechanism for n-butane. The only significant reaction of n-butane is with the hydroxyl radical. Approximately 85% of that reaction involves H-atom abstraction from an internal carbon atom and 15% from a terminal carbon atom. In the terminal H-atom abstraction path, the CH3CH2CH2CH20- alkoxy radical is estimated to react with 02 25% of the time and isomerize 75% of the time. The second isomerization is estimated to be a factor of 5 faster than the first isomerization of the CH3CH2CH2 CH20- radical, so that competition with 02 reaction ts not considered at this step. The predominant fate of «-hydroxy radicals is reaction with 02. For example, ■CHjOH + Oj —HCHO + HO,-, and CH3CHOH + 02 —> CH3CHO + HOr. In the n-butane mechanism, the a-hydroxy radical, CH2(0H)CH2CH2CH0H reacts rapidly with 02 to form 4-hydroxy- 1-butanal, CH2(0H)CH2CH2CH0. In the internal H-atom abstraction path, the alkoxy radical CH3CH2CH(0)CH3 reacts with 02 to yield methyl ethyl ketone (MEK), CH3CH2C{0)CH3, and decomposes to form CH3CHO and CH3CH2-, which, after reaction with 02 and NO and 02 again, yields another molecule of CH3CHO and H02.

CHEMISTRY OF NONMETHANE ORGANIC COMPOUNDS IN THE TROPOSPHERE 247 6.10.2 Alkenes

We now proceed to the atmospheric chemistry of alkenes (or olefins). Alkenes are constituents of gasoline fuels and motor vehicle exhaust emissions. This class of organic compounds accounts for about 10% of the nonmethane organic compound concentration in the Los Angeles air basin (Lurmann and Main 1992) and other U.S. cities (Chameides et al. 1992). Because of their high reactivity with respect to ozone formation, alkenes are important contributors to overall ozone formation in urban areas. By now we fully expect that alkenes will react with the hydroxyl radical, and that is indeed the case. Because of the double bonded carbon atoms in alkene molecules, they will also react with ozone, the N03 radical, and atomic oxygen. The reaction with ozone can be an important alkene oxidation path, whereas that with oxygen atoms is generally not competitive with the other paths because of the extremely low concentration of O atoms. Let us begin with the hydroxyl radical reaction mechanism and focus on the simplest alkene, ethene (C2H4).

a. OH Reaction We just saw that the initial step in OH attack on an alkane molecule is abstraction of a hydrogen atom to form a water molecule and an alkyl radical. In the case of alkenes, OH adds to the double bond rather than abstracting a hydrogen atom.7 The ethene-OH reaction mechanism is

C2H4 + OH- —> HOCH2CH2-HOCH2CH2- + 02—> H0CH2CH202-

fast

H0CH2CH202- + NO —► N02 + H0CH2CH20-The H0CH2CH20- radical then decomposes and reacts with 02:

H0CH2CH20- ^ HCHO + CH2OH

H0CH2CH20 + 02 ^ HOCH2CHO + HOr

The numbers over the arrows indicate the fraction of the reactions that lead to the indicated products at 298 K. Finally, the CH2OH radical reacts with 02 to give formaldehyde and a hydroperoxyl radical:

fast

7Hydrogen atom abstraction from —CH3 groups accounts generally for <5% of the overall OH reaction of ethene and the methyl-substituted ethenes (propene, 2-methyl propene, the 2-butenes, 2-methyl-2-butene, and 2,3-dimethyl-2-butene). For alkenes with alkyl side chains, perhaps up to 10% of the OH reaction proceeds by H-atom abstraction, but we will neglect that path here.

Following our procedure of condensing the mechanism by eliminating the fast reactions, we have

Finally, assuming that H0CH2CH202 is produced and consumed only in these two reactions, we can write an overall reaction as

C2H4 + OH- + NO —> N02 + 1.44 HCHO + 0.28 HOCH2CHO + H02 • As we did with propane, we can assume that H02 reacts solely with NO, to give C2H4 + OH + 2 NO —>2 NOz + 1.44 HCHO + 0.28 HOCH2CHO + OH-

We see that the overall result of hydroxyl radical attack on ethene is conversion of two molecules of NO to N02, formation of 1.44 molecules of formaldehyde and 0.28 molecules of glycol aldehyde (HOCH2CHO), and regeneration of a hydroxyl radical. By comparing this mechanism to that of propane, we see the similarities in the NO to N02 conversion and the formation of oxygenated products.

For monoalkenes, dienes, or trienes with nonconjugated C = C bonds, the OH radical can add to either end of the C = C bond. For propene, for example, we obtain

For dienes with conjugated double bonds, such as 1,3-butadiene and isoprene (2-methyl-1,3-butadiene), OH radical addition to the C = C—C = C system is expected to occur at positions 1 and/or 4:

Alkene-OH reactions proceed via OH radical addition to the double bond to form a P-hydroxyalkyl radical (Atkinson, et al. 2000)

C2H4 + OH- H0CH2CH202-

H0CH2CH202- +N0^4 N02 + 0.72 (HCHO + HCHO + HOr) + 0.28(HOCH2CHO + HOr)

R2 R4

TABLE 6.4 Carbonyl Yields from 1-Alkene-OH Reactions

Yield

TABLE 6.4 Carbonyl Yields from 1-Alkene-OH Reactions

Yield

1-Alkene

HCHO

RCHO

Propene

0.86

0.98 (acetaldehyde)

1-Butene

0.94 (propanal)

1-Pentene

0.88

0.73 (butanal)

1 -Hexene

0.57

0.46 (pentanal)

I-Heptene

0.49

0.30 (hexanal)

1-Octene

0.39

0.21 (heptanal)

Source: Atkinson et al. (1995b).

Source: Atkinson et al. (1995b).

followed by rapid addition of 02 to yield the corresponding P-hydroxyalkyl peroxy radicals: OH „ OH OO ,

R2 R4

In the presence of NO, the P-hydroxyalkyl peroxy radical reacts with NO to form either the (3-hydroxylalkoxy radical plus N02 or the p-hydroxynitrate:8

Rate constants for the reactions of P-hydroxyalkyl peroxy radicals with NO are essentially identical to those for the reaction of NO with > C2 alkyl peroxy radicals formed from alkanes.

The p-hydroxyalkoxy radicals can then decompose, react with 02, or isomerize. Available data show that, apart from ethene, for which reaction of the H0CH2CH20- radical with 02 and decomposition are competitive, the P-hydroxyalkoxy radicals formed subsequent to OH addition to > C3 alkenes undergo decomposition and the reaction with 02 is negligible.

The decomposition reaction is

2 R4

O 11

R3CR4

Carbonyl yields from alkene-OH reactions are summarized in Table 6.4. The yields of HCHO and RCHO arising from cleavage of the —C=C— bond of 1-alkenes

8The P-hydroxynitrate formation pathway accounts for only ~ 1-1.5% of the overall NO reaction pathway at 298 K for propene (Shepson et al. 1985). The yields of P-hydroxynitrates from the propene-OH and 1-butene-OH reactions are about a factor of 2 lower than those of alkyl nitrates from the propane-OH and rc-butane-OH reactions. These observations suggest that the formation yields of P-hydroxynitrates from the OH reaction with higher 1 -alkenes could also be a factor of 2 lower than those from the reactions with the corresponding «-alkanes.

chjch

= ch2

o2

OH •

no ch3ch(02-) ch2oh I no ch3ch(oh) ch2o-

ch3ch(0-)ch20h ch3ch(oh) ch2o-

FIGURE 6.14 Propene-OH reaction mechanism.

RCH=CH2 decrease monotonically from > 0.90 for propene and 1-butene to 0.21 to 0.39 for 1-octene. H-atom abstraction from the CH2 groups in the 1-alkenes is expected to account for an increasing fraction of the overall OH radical reaction as the carbon number of the 1-alkenes increases, with about 15% of the 1-heptene reaction being estimated to proceed by H-atom abstraction from the secondary CH2 groups. The propene-OH reaction mechanism is shown in Figure 6.14.

b. N03 Reaction Because of its strong oxidizing capacity and its relatively high nighttime concentrations, the N03 radical can play an important role in the nighttime removal of atmospheric organic species. Although the reaction of N03 with trace gases is significantly slower than that of OH, N03 can be present in much higher concentrations than OH, so that the overall reaction with many species is comparable for OH and N03 radicals. Since the reaction of OH with organic species is 10 to 1000 times faster than that of N03, the oxidation potential of the two radicals is in the same ballpark.

Alkenes react with the nitrate radical (Atkinson et al. 2000). As in OH-alkene reactions, N03 adds to the double bond and H-atom abstraction is relatively insignificant:

ono2

This is followed by rapid 02 addition d 0N°2 R Rl\l / 3 c-c

to produce the (3-nitratoalkyl peroxy radical, subsequent reactions of which are 0n02

ono2

The latter reaction is expected to be minor.

Further reactions of (3-nitratoalkoxy radicals include rkI

ono2

r)cr2

of which the last reaction is expeced to be minor. Figure 6.15 shows the mechanism of the propene-N03 reaction.

| (probably main path)

ch3ch(02-) ch2ono2 Jno ch3ch(0-)ch20n02

ch3ch

= ch2

o2

no3

o2

| (probably main path)

ch3ch(0n02)ch202-

i no ch3ch(0n02)ch20-

ch3c(q) ch2ono2

CH3CH0

+ hcho ch3ch(0n02)ch0

ch3cho

ho2-

FIGURE 6.15 Propene-n03 reaction mechanism.

c. Ozone Reaction The presence of the double bond renders alkenes susceptible to reaction with ozone. Reactions with ozone are, in fact, competitive with the daytime OH radical reactions and the nighttime N03 radical reaction as a tropospheric loss process for the alkenes. The ozone-alkene reaction proceeds via initial 03 addition to the olefinic double bond, followed by rapid decomposition of the resulting molozonide

2 R4

[R3R4COO

with the relative importance of the reaction pathways (a) and (b) being generally assumed to be approximately equal.

Carbonyl product yields for the reaction of alkenes with 03 are generally consistent with the initial alkene-03 reaction given above:

Alkene + 03 —> 1.0primary carbonyl + l.Obiradical

The kinetics and products of the gas-phase alkene-03 reaction have been studied extensively (Atkinson et al. 2000) and are reasonably well understood for a large number of the smaller alkenes. The major mechanistic issue concerns the fate, under atmospheric conditions, of the initially energy-rich Criegee biradical, which can be collisionally stabilized or can undergo unimolecular decomposition:

[R!CH2C(R2)o6] + —> R1CH2C(R2)00 (stabilization) —>RiCH2C(0)R2 + 0 —> [RiCH2C(0)0R2] + —► decomposition —> [RiCH = C(OOH)R2]+ —> R!CHC(0)R2 + OH-

At atmospheric pressure, O atoms are not formed in any appreciable amount, so the second path can generally be neglected.

Hydroxyl radicals have been observed to be formed from alkene-03 reactions, sometimes with close to a unit yield (1 molecule of OH per 1 molecule of alkene reacted). Atkinson et al. (1995a) reported OH radical yields from a series of alkene-03 reactions:

Alkene

OH Yield

1-Pentene

0.37

1-Hexene

0.32

1-Heptene

0.27

1-Octene

0.18

2,3-Dimethyl-1 -butene

0.50

Cyclopentene

0.61

1 -Methylcyclohexene

0.90

(Estimated uncertainties in these yields are a factor of ~ 1.5.)

The reaction pathways of Criegee biradicals are generally well established for the first two compounds in the series although the exact fractions that proceed via each individual path are still open to question (Atkinson et al. 2000):

[CH3CHOO ]+ch3choo-

The stabilized biradicals can react with a number of species:

rchoo + hzo —► rc(0)0h + h20 + no —+rcho + NO2

In addition, biradicals such as (CH3)2COO- may undergo unimolecular isomerization ch3 . ch3

Rate constants for reactions of CH200- radicals with the following species, relative to the reaction with S02, are

It appears that the reaction of stabilized biradicals with H20 will predominate under atmospheric conditions (Atkinson 1994).

6.10.3 Aromatics

Aromatic compounds are of great interest in the chemistry of the urban atmosphere because of their abundance in motor vehicle emissions and because of their reactivity with respect to ozone and organic aerosol formation. The major atmospheric sink for aromatics is reaction with the hydroxyl radical. Whereas rate constants for the OH reaction with aromatics have been well characterized (Calvert et al. 2002), mechanisms of aromatic oxidation following the initial OH attack have been highly uncertain. Aromatic compounds of concern in urban atmospheric chemistry are given in Figure 6.16.

HCHO

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