A Selection of the Chemistry of Volatile Organic Compounds

A Selection of the Chemistry of Volatile Organic CompoundsRyan Hartsook1 May 2014

1Volatile Organic Compounds

methanem-xylene

isoprene

acetonehexanal

formaldehyde

benzene

Volatile organic compounds come in all shapes and sizes. There are over one thousand compounds that fall into this category, including methane, isoprene, hexanal, formaldehyde, acetone, xylene, and benzene. Methane is the most abundant by mixing ratio but is not the most reactive. It is emitted by livestock, rice cultivation, and coal burning, to name a few. In general, VOCs are emitted by plants and animals, escape from soil and sediment, and are produced in industrial and combustion processes. Various hexane derivatives generate the odor of green tea, and others are responsible for the smell of flowers, attracting pollinating insects, or protecting plants. We will mostly consider methane as we look at some VOC chemistry, but the reactions can easily be extended to larger molecules.2Methane Oxidation

Methane is reasonably stable, so we need a reactive species to start things off. Hydroxyl radicals will do the trick. One source is ozone photolysis, which we have seen before. Hydroxyl radicals can attack methane, starting a reaction sequence that can produce a couple of things. If NOx is not present, the methylperoxide radical can react with a hydrogen peroxide radical, producing methyl peroxide and molecular oxygen. Methyl peroxide can rain out, so it can represent a termination. If NOx is present, the methylperoxide radical will attack nitrogen monoxide, ultimately producing formaldehyde and two nitrogen dioxides. Notice a hydroxyl radical is also produced, which makes this sequence catalytic.3Methane Oxidation

Net:Catalytic OH, NOx

The two nitrogen dioxides produced by methane oxidation can photolyze to nitrogen monoxide and atomic oxygen. Atomic oxygen then combines with molecule oxygen, and we get ozone. This is a source of tropospheric ozone, which is not necessarily a good thing. The nitrogen monoxide can go back and react with VOCs again to produce more nitrogen dioxide. The net result of all this is one methane can produce two ozone in the presence of nitrogen oxides.4At Night

X

Now, remember that all this started with a VOC reacting with a hydroxyl radical. The major source of hydroxyl radicals is ozone, which requires sunlight and is therefore limited to daytime. At night, there is another pathway that VOCs can follow. The nitrate radical is easily photolyzed, but at night it is around to react. Simple alkanes can be turned in to radicals that can then participate in the reactions discussed previously. Notice nitric acid is produced this way, which can dissolve in rain.5At Night

+

If alkenes are around, an alkyl radical can be formed, which then can, among other things, react with oxygen to form a peroxide radical. This can do any number of things, including react with NOx or O2 to form these species. The one on the right, alpha nitro-oxy-acetone, may be a mutagen.6Works CitedHolloway, Ann M. and Richard P. Wayne. Atmospheric Chemistry. Cambridge: RSC Publishing, 2010. Print.