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Reductive and Oxidative Ozonolysis of Alkynes- Ozonolysis, Mechanisms, Types, Practice Problems and FAQs

Reductive and Oxidative Ozonolysis of Alkynes- Ozonolysis, Mechanisms, Types, Practice Problems and FAQs

In villages and towns people consume directly either the municipal water or well water. But city people are very health conscious and look for drink waters marked as ‘mineral water, UV sterilized water. Ozone concentrated water’ etc. Why and how do these help health?

Talking about ozone rich water, Ozone is one form of our vital oxygen having some special characteristics to normal oxygen. This oxygen-enriched water apart from giving additional oxygen than what is normally present in water(8ppm) is a very reactive species capable of attacking a wide range of organic compounds such as alkene or alkyne, as well as all microorganisms through a reaction called ozonolysis. Water treated with ozone is effective against infection by microorganisms as well as the degradation of organic pollutants.

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 Let us now look at what reductive and oxidative ozonolysis are all about.

Table of Contents

  • Ozonolysis Reaction
  • Ozonolysis of alkynes
  • Mechanism of ozonolysis reaction of alkyne
  • Reductive ozonolysis
  • Oxidative Ozonolysis
  • Practice Problems
  • Frequently Asked Questions

Ozonolysis Reaction

Ozone is an isotope of oxygen and is highly reactive. Unsaturated compounds, in the presence of ozone molecules, undergo an oxidative cleavage. The unsaturated bonds present in the compound are replaced with C-O bonds, thereby giving rise to an intermediate complex called the ozonide. The intermediate ozonide molecule is subjected to zinc dust to remove oxygen from it. The removal of oxygen from the ozonide is a reductive reaction resulting in the formation of carbonyl compounds.

Ozonolysis also takes place in compounds such as elastomers and azo compounds. The oxidative cleavage of elastomers is known as ozone cracking. The oxidation of azo compounds results in the formation of nitrosamines.

Ozonolysis of alkynes

Alkynes contain 2 pi bonds. The triple bond of alkynes, in the presence of an ozone molecule, undergoes oxidative cleavage. Alkynes undergo oxidation resulting in the formation of end products such as diketones and acid anhydrides. In the presence of water, the acid anhydride gives rise to two carboxylic acids with the help of hydrolysis.

Let us understand the reaction mechanism of ozonolysis taking place in an alkyne.

Mechanism of ozonolysis reaction of alkyne

The reaction mechanism of ozonolysis involves 3 steps.

Step 1: Attack of the ozone molecule

The ozone molecule consists of three oxygen atoms. One of the oxygen atoms acquires a partial negative charge and the other one acquires a partial positive charge. The negatively charged oxygen ion attacks the triple-bonded carbon atom. The ozone molecule gets attached to the reactant. Rearrangement occurs within the structure resulting in the formation of an intermediate structure called the molozonide.

Step 2: Formation of ozonide intermediate

The molozonide intermediate further rearranges itself resulting in the formation of a stable ozonide intermediate. In ozonide, we can see that the triple bond is reduced to a single bond. Later on, the single bond between two carbon atoms also breaks, yielding different carbonyl compounds.


                                            Formation of Molozonide and Ozonide 

Step 3: Formation of carbonyl compounds

It can be performed by two types of mechanisms, which are given as follows:

1. Reductive ozonolysis
2. Oxidative ozonolysis

Reductive ozonolysis

This type of ozonolysis of alkyne involves the addition of an ozone molecule to an alkyne to form an ozonide. Ozonide is unstable and decomposes easily by reduction into , -diketones. The reagent that is used in reductive ozonolysis is given as follows:

(i) Zn and H2O

(ii) (CH3)2S and H2O

(iii) CH3COOH

The formation of , -diketones can be given as:


Oxidative Ozonolysis

This type of ozonolysis of alkynes involves the addition of an ozone molecule to an alkyne to form an ozonide, followed by the oxidation of the ozonide to smaller oxidized molecules like acid. The reagent which is used in oxidative ozonolysis is given as follows:

(i) H2O

(ii) H2O2

The formation of carboxylic acids can be given as:


In oxidative ozonolysis, if formic acid is formed, it will further decompose into carbon dioxide and water

HCOOHCO2+H2O

Practice Problems

Q1. Predict the product of reaction when Acetylene is treated with ozone molecule in the presence of hydrogen peroxide?

a. Acetic Acid
b. Formic Acid
c. Carbon dioxide
d. Buta-1,3-diene

Answer: This type of alkyne ozonolysis involves the addition of an ozone molecule to an alkyne to form an ozonide, which is then oxidized to smaller oxidized molecules such as acid. Hence, when acetylene is treated with O3 and H2O2, the product should be formic acid.

But formic being unstable further decomposed to form carbon dioxide and water. Hence, the correct answer is an option (C).


Q2. Hexane-1,2,4,5-tetraone is formed from a reagent X treated with ozone and acetic acid. What is the possible IUPAC name of X?

a. 1,3-Hexadiyne
b. 1,4-Hexadiyne
c. 2,4-Hexadiyne
d. 2,3-Hexadiyne

Answer: This type of alkyne ozonolysis involves adding an ozone molecule to an alkyne to form an ozonide. Ozonide is unstable and quickly decomposes with acetic acid by reduction into -diketones. Here, when reagent X is treated with O3 and CH3COOH, the product formed is Hexane-1,2,4,5-tetraone. The reagent should be 1,4-Hexadiyne. Hence, the correct answer is option (B).


Q3. In a laboratory, while doing ozonolysis of a compound, compounds A, B and C are formed. The compound A with molecular formula C2H4O2, when treated by sodium bicarbonate gives a colourless gas, D with effervescence. The compounds B and D are the same. Predict the IUPAC name of starting reagent and the nature of ozonolysis?

a. But-1-yne, oxidative ozonolysis
b. Propa-1,2-diene, reductive ozonolysis
c. Propyne, oxidative ozonolysis
d. Propyne, reductive ozonolysis

Answer: The compound A with molecular and formula C2H4O2 is treated by sodium bicarbonate test to give a colourless gas D with effervescence. This is a test for the presence of the carboxylic acid group. So, compound A should be Acetic acid (CH3COOH). The gas released during this test is carbon dioxide. Hence the compounds B and D are CO2. If the product is a carboxylic acid, then the nature of ozonolysis must be oxidative. The compound B is CO2, then it must be formed from formic acids. The suggested mechanism is


So, the correct answer is an option (C).

Q4. What should be the expected product when cyclohexyne is treated with ozone in the presence of hydrogen peroxide and (dimethyl sulphide and water) separately

a. Cyclohexane-1,2-dione, Cyclohexane-1,6-dioic acid
b. Cyclohexane-1,6-dioic acid, Cyclohexane-1,2-dione
c. Cyclohexane-1,2-dione, Cyclohexane-1,2-dione
d. Cyclohexane-1,6-dioic acid, Cyclohexane-1,6-dioic acid

Answer: When cyclohexyne is treated with ozone in the presence of Hydrogen peroxide (O3 and H2O2), this is oxidative ozonolysis which involves the addition of an ozone molecule to an alkyne to form an ozonide, followed by the oxidation to form a carboxylic acid. So, the product should be Cyclohexane-1,6-dioic acid.

When cyclohexyne is treated with ozone in the presence of dimethyl sulphide and water ((CH3)2S and H2O), this is reductive ozonolysis which involves the addition of an ozone molecule to alkyne to form an ozonide, decomposes easily by reduction into , -diketones. So, the product should be Cyclohexane-1,2-dione.
So, the correct answer should be (B).

Frequently Asked Questions

Q 1. What are the chemistry applications of ozonolysis?
Answer: Ozonolysis has the following applications: Ozonolysis is used in organic chemistry to determine the location of double and triple bonds in alkenes and alkynes. Ozonolysis is a technique used to determine the structure of long alkenes and alkynes.

Q 2. What are the industrial applications of Ozonolysis?
Answer: Bleaching employs ozonolysis. Ozonolysis is used in wastewater treatment. Ozonolysis is used to produce alcohols, carboxylic acids, aldehydes, and ketones.

Q 3. What role does zinc play in ozonolysis?
Answer: Zinc dust is used in ozonolysis to prevent further oxidation of the chemical. Zn halts the reaction by preventing the molecule from forming additional oxygen bonds.

Q 4. Is ozone an electrophile or a nucleophile?
Answer: Molecular ozone has the properties of a dipole, an electrophilic agent, and a nucleophilic agent. The ozone molecule's dipolar structure may result in 1-3 dipolar cycloadditions on unsaturated bonds, resulting in the formation of primary ozonide.

Related topic

Physical and Chemical Properties of Alkynes

Distinguishing Test of Terminal and Non-Terminal Alkynes

Reaction of Alkynes

Halogenation of Alkynes

Oxidation of Alkynes

Catalytic hydrogenation of alkenes and alkynes

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