Figure 1 Ozone: Allotrope of Oxygen

Ozone, O3, is an allotrope of oxygen. It is not possible to draw a single structure for ozone in which each oxygen atom has a filled valence shell and is, at the same time, uncharged. Rather, resonance theory describes the structure of this compound as a hybrid of the three resonance contributors shown in Figure 1.

Allotrope of Oxygen

In a process called ozonolysis, an alkene is treated with ozone to produce intermediates called ozonides, which are reduced directly, generally with zinc metal in acetic acid, to yield aldehydes or ketones, depending on the substituents attached to the double bond of the initial alkene. Equations 6 -8 provide three specific examples.

Note that an aromatic ring is resistant to ozone.

The value of ozonolysis lies in the structural insight it affords a chemist who is trying to determine the identity of an unknown compound. Figure 2 illustrates this idea.

Structural Elucidation with Ozonolysis

Figure 2 : Structural Elucidation with Ozonolysis

The unknown is degraded into smaller, simpler molecules that are more readily identified. Once identified, these fragments are then mentally reconnected by joining the carbonyl carbons together to create an alkene.