Ozonolysis of alkenes
What is ozonolysis:
Ozonolysis is a chemical reaction where ozone (O3), a highly reactive allotrope of oxygen, oxidizes and cleaves alkenes and alkynes into carbonyl compounds. When reacting with alkenes, the process involves the formation of an unstable intermediate called molozonide, which spontaneously rearranges into an ozonide (1,2,4-trioxolane).
Ozonides are unstable and are typically reduced to form carbonyl compounds (aldehydes or ketones) through the treatment with water (H2O) and zinc (Zn). The general reduction reaction is as follows:
H2O2 + Zn → ZnO + H2O
Uses:
Ozonolysis is a valuable reaction for identifying the position of double bonds in alkenes by breaking them down into easily identifiable carbonyl compounds.
Ozonolysis of ethene:
When ozone reacts with ethene, it oxidizes the ethene into formaldehyde. During this reaction, a transient intermediate called molozonide is formed, which rearranges into ethylene ozonide.
The ozonide is then quickly reduced to methanal (formaldehyde) in the presence of water and zinc.
H2O2 + Zn → ZnO + H2O
Ozonolysis of Benzene:
In the ozonolysis of benzene, ozone reacts to form glyoxal. An unstable intermediate, benzene triozonide, is produced during this reaction. This intermediate decomposes into ethandial (glyoxal) when treated with water and zinc.
Mechanism of Ozonolysis:
Ozonolysis of Ethene involves two steps. In the first step, ozone will oxidize ethene into an ozonide, and in the second step, water.
Step 1: Formation of Molozonide and Ozonide
Ozone reacts with ethene to form an unstable molozonide intermediate. This intermediate quickly decomposes to form an ozonide through the intermediate formation of formaldehyde and formaldehyde oxide.
Step 2: Reduction of Ozonide
The ozonide formed in the first step reacts with a water molecule, yielding formaldehyde.
Zinc is used to reduce hydrogen peroxide (H2O2) to water (H2O).
H2O2 + Zn → ZnO + H2O
In the absence of Zn, H2O2, can further oxidize formaldehyde to form formic acid.
