ChemTalk

Ozonolysis

ozonolysis

Core Concepts

This article serves as an in-depth guide to what the ozonolysis reaction is in organic chemistry. After reading this article, you will be able to understand and describe its reaction mechanism, its applications in the real world, and its role in advancing various fields in chemistry.

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What is Ozonolysis?

The ozonolysis reaction plays a fundamental role in organic chemistry and has significantly contributed to our understanding of complex organic compounds. This reaction involves using ozone (O3) to cleave carbon-carbon double bonds (C=C) in organic molecules such as alkenes and alkynes. This can produce various carbonyl compounds such as aldehydes, carboxylic acids, and ketones. Furthermore, this reaction serves as a versatile and essential tool in organic chemistry. For instance, it has found applications in elucidating chemical structures, synthesizing valuable chemical intermediates, and developing environmentally friendly processes.

ozonolysis reaction
A visual of the ozonolysis reaction which shows an alkene reacting with ozone to form two carbonyl compounds.

Ozonolysis Mechanism

mechanism of ozonolysis
A diagram showing the mechanism of ozonolysis.

Reaction Steps

The process of ozonolysis begins with the initial step of ozone addition and ends with ozonide cleavage. This reaction exclusively cleaves the carbon-carbon double bond while leaving all other functional groups unaffected. In the initial step, ozone reacts with a carbon-carbon double bond in the presence of a reducing agent which yields molozonide. This highly unstable intermediate is an unstable five-membered cyclic structure containing three adjacent oxygen atoms. Chemists may refer to molozonide as 1,2,3-trioxolane by IUPAC naming conventions.

Molozonide then rearranges, leading to the formation of an ozonide commonly referred to as 1,2,4-trioxolane by IUPAC naming conventions. Although more stable than molozonide, it is still an unstable intermediate. Additionally, there is a zwitterion intermediate that appears between molozanide and the ozonide known as the Criegee intermediate. The final step of the reaction is the cleavage of the ozonide and it rearranges into more stable carbonyl compounds as products. The cleavage of ozonide may occur either through a reductive workup or oxidative workup pathway.

molozonide structure
The chemical structure of molozonide (1,2,3-trioxalane).
trioxolane structure
The chemical structure of 1,2,4-trioxolane which is the ozonide formed as an intermediate during ozonolysis.
criegee intermediate in ozonolysis
The structure of the Criegee intermediate.

Reductive Workup and Oxidative Workup

  • Reductive workup: In the presence of reducing agents such as dimethyl sulfide (DMS), acetic acid, or zinc, the ozonide formed undergoes reduction. This will yield aldehydes (CHO) and ketones (CH3COCH3) as products.
  • Oxidative workup: In the absence of a reducing agent, ozonolysis can yield aldehydes, ketones, and carboxylic acids (COOH). Chemists use hydrogen peroxide (H2O2) as the reagent in the oxidative workup pathway to form carboxylic acids as products.

Reaction Products

Two factors dictate what the final reaction products will be. First, the R groups of the cleaved alkene. The second factor is whether the reaction underwent reductive workup or oxidative workup. The final product will be a ketone if the ends of an alkene have R groups on both sides. When there is one hydrogen atom on the end of an alkene, the reaction will yield an aldehyde. Formaldehyde, the simplest aldehyde, forms when the end of an alkene has hydrogen atoms on both sides. When performing an oxidative workup, hydrogen peroxide will oxidize aldehydes formed as products during cleavage into carboxylic acids.

ozonolysis reductive oxidative workup graphic
A more detailed diagram of the mechanism of ozonolysis showing what reaction products will be depending on whether a reductive workup or oxidative workup was done.

Applications of Ozonolysis

  1. Elucidation of chemical structure: Ozonolysis has a crucial role in the structural determination of complex organic compounds. Chemists can deduce atomical connectivity in a molecule by subjecting it to ozonolysis, which cleaves the molecule at every insaturation and adds a carbonyl at each cleavage location.
  2. Chemical synthesis: Chemists use ozonolysis to synthesize important intermediates for pharmaceuticals, fragrances, and other fine chemicals. As such, chemists can produce precise building blocks for more complex compounds by selectively cleaving specific carbon-carbon double bonds.
  3. Environmental remediation: Ozonolysis has environmental applications such as water treatment. In this case, the powerful oxidizing properties of ozone allow it to break down harmful organic pollutants into harmless byproducts. This makes water purification processes more sustainable and efficient.
  4. Polymer modification: Ozonolysis has seen use in modifying polymers. Introducing functional groups that contain oxygen onto polymer surfaces allows chemists to adjust properties such as wettability and adhesion as needed.

Further Reading