Ozonolysis

What is ozonolysis:

Ozone O₃ is a reactive allotrope of oxygen. Using ozone alkenes and alkynes can be oxidized and broken into carbonyl compounds. With alkenes, it reacts vigorously to form molozonide, a transient intermediate that spontaneously rearranges to form an ozonide (1,2,4-trioxolane).

Ozonides are unstable compounds and are reduced to form carbonyl compounds(aldehydes or ketones) by treatment with H₂O and zinc.

H₂O₂ + Zn →  ZnO +  H₂O

Uses:

This reaction is often used to identify the position of the double bond in an alkene by breaking them down into easily identifiable pieces.

Ozonolysis of ethene:

Ozone reacts with ethene and oxidizes it into formaldehyde. During this reaction, a transient intermediate molozonide is formed that spontaneously rearranges to form an ozonide.

Ozonide is quickly reduced to form methanal (formaldehyde) when treated with H₂O and zinc.

H₂O₂ + Zn →  ZnO +  H₂O

Ozonolysis of Benzene:

Ozone reacts with Benzene and oxidizes it into glyoxal. During this reaction, an unstable intermediate benzene triozonide is formed. On treatment with water and zinc benzene triozonide decomposes into ethandials (glyoxals).

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:

Ozone reacts with ethene to form an unstable molozonide. Molozonide quickly decomposes into an ozonide through the formation of formaldehyde and formaldehyde oxide.

Step-2:

In this step, a water molecule reacts with ethylene oxide (ozonide) and yields formaldehyde.

Zn is employed to reduce H₂O₂ into H₂O.

H₂O₂ + Zn →  ZnO +  H₂O

In the absence of Zn, H₂O₂ oxidizes formaldehyde to formic acid.