In an aqueous solution, ozone may act on various compounds in one of two ways:
- by direct reaction with the molecular ozone, and
- by indirect reaction with the radical species (hydroxyl radical) that are formed when ozone decomposes in water.
These two properties of ozone enable it to react in three ways:
- Cyclo addition (Criegee mechanism)
- Electorphilic reaction
- Nucleophilic reaction
Ozone molecules may lead to the formation of primary ozonide. The primary ozonide decomposes into a carbonyl compound (aldehyde or ketone) and a zwitterion that quickly leads to a hydroxy-hydroperoxide. The hydroxy-hydroperoxide, in turn, decomposes into a carbonyl compound and hydrogen peroxide.
The electrophilic reaction is restricted to molecular sites with a strong electron density, in particular certain aromatic compounds. Aromatics substituted with electron donor groups (OH, NH2 and similar compounds) are highly reactive with ozone (at the ortho and para positions). Aromatics substituted with electron withdrawing groups (-COOH, -NO2) are weakly reactive with ozone. The initial attack on the substituted positions leads to the formation of ortho- and para-hydroxylated by-products, although these compounds are highly susceptible to further ozonation. The compounds lead to the formation of quinoid and, due to the opening of the aromatic cycle, to the formation of aliphatic products with carbonyl and carboxyl functions.
The nucleophilic reaction is found locally on molecular sites showing an electronic deficit and, more frequently, on carbons carrying electron withdrawing groups. The molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds as well as the specific functional groups.