In organic chemistry, ozonolysis is a class of chemical reactions in which ozone is employed for the oxidative or reductive cleavage of organic compounds. However, it cleaves the double bond and triple bond of alkenes and alkynes and results in the formation of carbonyl compounds.
Note that, some alkynes can also undergo ozonolysis to give acid anhydride and diketones. In addition, this reaction is used for the identification of the structure of alkenes.
History of ozonolysis
Ozone (O3), is a triatomic form of oxygen. It can be represented by the two most stable lewis structures:
Christian Friedrick Schonbein discovered ozone in 1840. He also performed certain reactions in which bond cleavage using ozone is done, back in 1845. He observed that after the reaction of ethylene with ozone, the smell of ozone and ethylene was not perceivable. Although the ozonolysis of alkene is also referred to as “Harries ozonolysis”. This is because there are some attributes of this reaction to Carl Dietrich Harries.
There are two types of ozonolysis as given below:
1. Reductive ozonolysis
“The addition of ozone followed by hydrolysis in the presence of reducing agent is known as reductive ozonolysis.”
This mainly happens in the case of alkenes.
2. Oxidative ozonolysis
“The addition of ozone followed by the hydrolysis in the absence of reducing g agents is known as oxidative ozonolysis.”
However, this mainly happens in the case of alkynes.
Examples of ozonolysis
Ozonolysis of alkenes
Alkenes undergo ozonolysis to form aldehydes, alcohols, ketones, and carboxylic acids. The formation of the product depends on the structure of the alkene.
In the first step, the reaction begins by passing the ozone-containing air through a solution of an alkene. This is done in the presence of an inert solvent such as chloroform. Thus, the temperature is maintained at about -80ºC. The initially formed product molozonide rearranges into ozonide.
In the second step, the highly unstable ozonide is decomposed into cleaved products by hydrolysis. In this way, the double-bonded carbon in the alkene changed into the double bond to oxygen.
However, the byproduct hydrogen peroxide may oxidize aldehyde or ketone into a carboxylic acid. Therefore, the decomposition of ozonide is done under reduction conditions. For instance, zinc dust in a dilute acid can be used.
Ozonolysis of alkynes
When alkynes react with ozone carboxylic acids are produced. Some examples are given below:
This reaction is used in the structure determination of the alkynes. However, by identifying the structure of carboxylic acid, we can determine the structure of alkyne.
Mechanism of ozonolysis of 2-Butyne
In the first step, ozone is added to the alkynes. This results in the formation of ozonide. Which on hydrolysis gives 1,2 dicarbonyl compounds.
In the second step, the byproduct hydrogen peroxide affects the oxidative cleavage of carbonyl compounds forming a mixture of carboxylic acid.
Ozonolysis of benzene
The benzene reacts with ozone and produces three molecules of glyoxal through the formation of triozonide. Hence, this reaction provides the existence of three double bonds in benzene.
However, alkyl benzene on treating with ozone in the presence of hydrogen peroxide yields aliphatic carboxylic acid.
Ozonolysis of propylene
Propylene decomposes into carboxylic acids similar to alkenes as shown below:
The cleavage of the bond using ozone is used to locate the position of the double bond. However, it can also determine the position of the triple bond. It is also used in determining the structure of natural products. For example terpenes.
How many types of ozonolysis are there?
There are two different types of reductive and oxidative cleavage of bonds using ozone gas.
What does O3 do in reaction?
The ozone is used for the cleavage of the double bond of alkenes. However, it is also used for the cleavage of the triple bond of alkynes. In this way, carbonyl compounds such as aldehyde, ketone, and carboxylic acid are produced.
What is the application of ozonolysis?
It is mainly used for the identification of the position of double bonds and triple bonds.
Can Ozonolysis of an internal alkyne ever produce aldehydes?
The bond cleavage using ozone of terminal alkynes produces dicarboxylic acid. Which undergo oxidative cleavage and produce a mixture of carboxylic acid.
- Organic Reactions Mechanism with Problems by Dr. Rajpal Tyagi (M.M.H Post Graduate College Ghaziabad)
- A textbook of Organic Chemistry by M. Younas (University of Punjab)
- Chapter 6: Alkene (University of Calgary)