The Most Predominant Fate In Air For Most Simple Radicals Is Reaction With

(Here and elsewhere in this book we write Lewis structures that assume that d otbitals in atoms such as sulfur and phosphorus allow these elements to form double bonds.) Hydroxyl radical does not add to carbon dioxide, O—C=0, since the molecule contains only very strong C=0 bonds. However, OH' addition does occur to the carbon atom in carbon monoxide, CO, since the triple bond is thereby converted to the very stable double bond and a new single bond is also formed:

This process is exothermic because the third C—O bond in carbon monoxide is weak relative to the other two.

Peroxy Radicals Reaction Produce
Gas is inert in troposphere; will rise to stratosphere

FIGURE 5-1 (a) Decision tree illustrating the fate of gases emitted into the air. (b) Decision tree illustrating the fate of airborne free radicals.

Peroxy Radicals Reaction Produce
02 adds to radical site to produce peroxy radical

* Under conditions with significant nitric oxide present and before radical + radieat reactions become important.

Generally, OH* does not add to multiple bonds in any fully oxidized species such as CO2, SO3, and N205, since such processes are endothermic and therefore are very slow to occur at atmospheric temperatures. Similarly, N2 does not react with OH* because the component of the nitrogen-to-nitrogen bond that would be destroyed is stronger than the N—O bond that would be formed. It doesn't react with 02 because a high activation energy is required for this reaction to occur.

For molecules that do not have a reactive multiple bond but do contain hydrogen, OH" reacts with them by the abstraction of a hydrogen atom to form a water molecule and a new reactive free radical. For CH4, NH3, H2S, and CH3CI, for instance, the reactions are

CH+ + OH' —> CH; + H2O NH-j + OH' —»NH; + H2O

CH3CI + OH'-> CH2C1' + H2O

Because the H—OH bond formed in these reactions is very strong, the processes are all exothermic; thus only small activation energy barriers exist to impede these reactions (see Box 1-2).


Why aren't gases such as CF2C12 (a CFC) readily oxidized in the troposphere? Would the same be true for CH2C12?


The abstraction of the hydrogen atom in HF by OH * is endothermic. Comment briefly on the expected rate of this reaction: Would it be (at least potentially) fast, or necessarily very slow in the troposphere?


The hydroxyl radical does not react with gaseous nitrous oxide, N20, even though the molecule contains multiple bonds. What can you deduce about the probable energetics (endothermic or exothermic character) of this reaction from the observed lack of reactivity?

A few gases emitted into air can absorb some of either the UV-A or the visible component of sunlight, and this input of energy is sufficient to break one of the bonds in the molecule, thereby producing two free radicals. For example, most molecules of atmospheric formaldehyde gas, H2CO, react by photochemical decomposition after absorption of UV-A from sunlight:

H2CO UV'A(A<338"m)> H" + HCO"

In all the cases discussed, the initial reaction of a gas emitted into air produces free radicals, almost all of which are extremely reactive. The predominant fate in tropospheric air for most simple radicals is reaction with diatomic oxygen, often by an addition process: One of the oxygen atoms attaches, or "adds on," to the other reactant, usually at the site of the unpaired electron. For instance, 02 reacts by addition with the methyl radical, CHj:

Notice that CH-.OO' itself is a free radical; the terminal oxygen forms only one bond and carries the unpaired electron;

Species such as HOO" and CH3OO" are called peroxy radicals since they contain a peroxide-like O—O bond; recall that HOO* is the hydroperoxy radical.

As radicals go, peroxy radicals are less reactive than most. They do not readily abstract hydrogen since the resulting peroxides would not be very stable energetically. Since the transfer of H to the peroxy radical would be endothermic and thus would possess a large activation energy, abstraction reactions for peroxy radicals are usually so slow that they are of negligible importance (in contrast to those for OH'). Peroxy radicals in the troposphere do not react with atomic oxygen because of the extremely low concentrations of the free atom in this region of the atmosphere. The most common fate of peroxy radicals in tropospheric air, except for the cleanest type of air, such as that over oceans, is reaction with nitric oxide, NO', by the transfer of the "loose" oxygen atom (see the later section on stratospheric chemistry), thereby forming nitrogen dioxide, NOj, and a radical that has one fewer oxygen atoms:

CHjOO" + NO'->CH30" + N02

It is by this type of reaction that most atmospheric NO" is oxidized to NO'2, at least in polluted air. Recall that this reaction also is typical of the types encountered in stratospheric chemistry (see Chapters 1 and 2) and that NO" oxidation by ozone in sunlit conditions yields a null reaction.

For free radicals that contain nonperoxy oxygen atoms, the reaction with molecular oxygen frequently involves the abstraction of an H atom by 02. This process occurs provided that, as a result, another new bond within the system is formed: A single bond involving oxygen is converted to a double one, or a double bond involving oxygen is converted to a triple one. As examples, consider the three reactions below in which a C—O single (or double) bond is converted to a double (or triple) one as a consequence of the loss of a hydrogen atom:

Such processes do not occur unless a new bond is created in the product free radical, since the strength of the newly created H—OO bond alone is not sufficient to compensate for the breaking of the original bond to hydrogen.

If there is no suitable hydrogen atom for 02 to abstract, then when it collides with a radical, it instead adds to it at the site of the unpaired electron, as was previously discussed for simple radicals. For example, radicals of the type R—C=0, where R is a chain of carbon atoms, add 02 to form a peroxy radical:

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