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co h2o no2

Ethyl benzene

m - Ethyl toluene p - Ethyl toluene

Aromatic compounds of interest in tropospheric chemistry.

C2H5

o - Ethyl toluene

FIGURE 6.16

m - Ethyl toluene p - Ethyl toluene

Aromatic compounds of interest in tropospheric chemistry.

The aromatic-OH radical reaction proceeds via two pathways: (a) a minor one (of order 10%) involving H-atom abstraction from C—H bonds of, for benzene, the aromatic ring, or for alkyl-substituted aromatic hydrocarbons, the alkyl-substituent groups; and (b) a major reaction pathway (of order 90%) involving OH radical addition to the aromatic ring. For example, for toluene these reaction pathways are:

For the first addition product the structure above denotes the radicals:

The H-atom abstraction pathway leads mainly to the formation of aromatic aldehydes

Benzaldehyde

Benzaldehyde

As noted above, this H-atom abstraction pathway is minor, accounting for <10% of the overall OH radical reaction for benzene and the alkyl-substituted aromatic hydrocarbons.

The radicals resulting from OH addition to the aromatic ring are named as follows:

For toluene, and other aromatics, there are several possible sites of attack for the OH radical. Some sites are less sterically hindered than others or are favored because of stabilizations resulting from group interactions. Andino et al. (1996) have performed ab initio calculations to determine the most energetically favored structures resulting from OH addition to aromatic compounds. For toluene the most favored structure is that resulting from OH addition at the ortho position:9

[In general, the preferred place of OH addition to an aromatic is a position ortho to a substituent methyl group (Andino et al. 1996).]

Following formation of the OH adduct, the adduct can react with 02 or N02. The 02 reaction path is

H OH

Hydroxycyclohexadienyl radical (from benzene)

Methyl hydroxycyclohexadienyl radicals (from toluene)

H OH

Hydroxycyclohexadienyl radical (from benzene)

Methyl hydroxycyclohexadienyl radicals (from toluene)

ho2- Abstraction o - Cresol ho2- Abstraction o - Cresol

9OH addition to the meta and para positions of toluene yield structures that are only 1 to 2 kcal mor1 less favorable than addition at the ortho site and thus cannot be ruled out categorically. For our purposes, we will consider OH addition at the ortho site only.

Addition

9OH addition to the meta and para positions of toluene yield structures that are only 1 to 2 kcal mor1 less favorable than addition at the ortho site and thus cannot be ruled out categorically. For our purposes, we will consider OH addition at the ortho site only.

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The location of 02 addition in the product shown above is that most favored energetically (Andino et al. 1996). The H-atom abstraction reaction to yield phenolic compounds, such as o-cresol, has been shown to be relatively minor, accounting for ~ 16% of the overall OH radical mechanism for toluene (Calvert et al. 2002). The N02 reaction of the OH adduct leads to nitroaromatics:

m-Nitrotoluene m-Nitrotoluene

Rate constants for the methyl hydroxycyclohexadienyl radical with 02 and N02 are ~5 x 10-16cm3 molecule-1 s-1 and ~3xl0-11cm3 molecule-1 s-1, respectively (Knispel et al. 1990; Zetzsch et al. 1990; Goumri et al. 1992; Atkinson 1994). Based on these rate constants, the N02 reaction with the toluene-OH adduct will be of significance for N02 concentrations exceeding about 9 x 1012 molecules cm-3 (300 ppb).

Alkyl peroxy radicals generally react with NO to form alkoxy radicals (assuming sufficient NO is present). Aromatic peroxy radicals, in contrast, are believed to cyclicize, forming bicyclic radicals. For the product of reaction b above, the energetically favored bicyclic radical is

After bicyclic radical formation, 02 rapidly adds to the radical, forming a bicyclic peroxy radical, for example

This radical is then expected to react with NO to form a bicyclic oxy radical and N02, such as

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The only path for this bicyclic oxy radical is fragmentation via favorable p-scission reactions. For the above radical, such scission would produce

Decomposition B

Decomposition A

Observed ring fragmentation products of the toluene-OH reaction include the following:

OO n ii hcch

Glyoxal

Il II

ch3c ch

Methylglyoxal o ch30

II I II

hcch=c ch

Methyl butenedial o o

Il II

HCCH=CHCH

1,4-Butenedial

6.10.4 Aldehydes

Aldehydes are important constituents of atmospheric chemistry. We have already seen the role played by formaldehyde in the chemistry of the background troposphere. Aldehydes are formed in the atmosphere from the photochemical degradation of other organic compounds. Aldehydes undergo photolysis, reaction with OH radicals, and reaction with N03 radicals. Reaction with NO3 radicals is of relatively minor importance as a consumption process for aldehydes, thus the major loss processes involve photolysis and reaction with OH radicals.

Formaldehyde photolyzes and reacts with OH, as seen earlier. Acetaldehyde photolyzes by

Data on absorption cross sections and quantum yields for higher aldehydes are summarized by Atkinson (1994).

Hydroxyl radical reaction with aldehydes involves H-atom abstraction to produce the corresponding acyl (RCO) radical

which rapidly adds 02 to yield an acyl peroxy radical:

fast

These acyl peroxy radicals then react with NO or N02, the latter leading to peroxyacyl nitrates, RC(0)00N02:

RC(0)00- +NO —y RC(0)0- + N02 RC(0)0-—>R- + C02 RC(0)00- + N02 + M ^ RC(0)00N02 + M

6.10.5 Ketones

This class of organic compounds is exemplified by acetone and its higher homologues. As for the aldehydes, photolysis and reaction with the OH radical are the major atmospheric loss processes (Atkinson 1989; Mellouki et al. 2003). The limited experimental data available indicate that, with the exception of acetone (see Figure 6.17), photolysis is probably of minor importance. Reaction with the OH radical is then the major

Ethanol Atmospheric Reaction

ho2-

figure 6.17 Atmospheric photooxidation mechanism for acetone.

tropospheric loss process. For example, for methyl ethyl ketone the OH radical can attack any of the three carbon atoms that contain hydrogen atoms

OH +CH3CH2C(0)CH3 H20 + CH3CHC(0)CH3

H20 + CH3CH2C(0)CH200-

H20 + 00CH2CH2C(0)CH3

with a being the major reaction pathway. Subsequent reaction of this particular radical with NO leads to

The major reaction products from the atmospheric reactions of the ketones are aldehydes and PAN precursors.

6.10.6 a, P-Unsaturated Carbonyls

These compounds, exemplified by acrolein (CH2=CHCHO), crotonaldehyde (CH3CH= CHCHO), and methyl vinyl ketone (CH2=CHC(0)CH3), are known to react with ozone and with OH radicals. Photolysis and N03 radical reaction are of minor importance. Under atmospheric conditions the 03 reactions are also of minor significance, leaving the OH radical reaction as the major loss process. For the aldehydes, OH radical reaction can proceed via two reaction pathways: OH radical addition to the double bond and H-atom abstraction from the—CHO group (Atkinson, 1989). These a, P-unsaturated aldehydes are expected to ultimately give rise to a-dicarbonyls such as glyoxal and methylglyoxal. For the a, P-unsaturated ketones such as methyl vinyl ketone the major atmospheric reaction with the OH radical occurs only by OH radical addition to the double bond. Again, oe-dicarbonyls, together with aldehydes and hydroxyaldehydes, are formed as products.

6.10.7 Ethers

The aliphatic ethers, such as dimethyl ether and diethyl ether, react under atmospheric conditions essentially solely with the OH radical, via H-atom abstraction from C—H bonds (Wallington et al. 1988, 1989; Atkinson 1989; Japar et al. 1990, 1991; Wallington and Japar 1991; Mellouki et al. 2003). The reaction mechanism for dimethyl ether is ch3och3 + oh- ^ ch30ch202-ch3och2o2- + no —> n02 + ch30ch20-ch3och2o- + o2 —>hc(o)och3 + ho2-

where the carbon-containing product in the last reaction is methyl formate.

6.10.8 Alcohols

The reaction sequences for the simpler aliphatic alcohols under atmospheric conditions are known (Atkinson 1989; Mellouki et al. 2003); these involve H-atom abstraction, mainly from the a C—H bonds. For example, the methanol-OH reaction is

with the first reaction pathway accounting for ~ 85% of the overall reaction at 298 K. Since, as shown earlier, both the -CH2OH and CH30- radicals react with 02 to yield formaldehyde and H02, the overall methanol-OH reaction can be written as

The ethanol-OH reaction proceeds as follows

OH- + CH3CH2OH —► H20 + CH2CH2OH 5%) —* HzO + CH3CHOH 90%) -^H20 + CH3CH20- (~5%)

where the branching ratios are those at 298 K. The second two channels result in identical products under atmospheric conditions, H02 + CH3CHO. The first channel forms the intermediate CH2CH2OH, which, under atmospheric conditions, leads to the same products as the OH + ethene reaction. Using the ethene-OH mechanism given earlier, the overall ethanol-OH reaction mechanism can be written as

C2H5OH + OH- + 0.05 NO —► 0.05 N02 + 0.014 HOCH2CHO + 0.072 HCHO

where the principal products are acetaldehyde and the H02 radical.

Free tropospheric concentrations of methanol range from about 700 ppt at northern midlatitudes to about 400 ppt at southern latitudes (Singh et al. 1995). In general, ethanol abundance in the free troposphere is an order of magnitude lower than that of methanol. Average lifetimes of CH3OH and C2H5OH in the atmosphere are on the order of 16 days and 4 days, respectively.

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  • stephanie
    What is most energetically favored to add to toluene?
    8 years ago

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