Ozonolysis Mechanism

Ozonolysis is an organic redox reaction in which unsaturated carbon-carbon bonds (either double or triple bonds) of alkenes, alkynes or azo compounds are cleaved with ozone. Azo compounds are those compounds which have functional group diazenyl (R-N=N-R’) This reaction has various uses such as it is used for production of alcohols, aldehydes, ketones, carboxylic acids etc.

Ozone

To understand the ozonolysis reaction and its mechanism, you need to first understand the structure of the ozone molecule.

Ozone is a reactive allotrope of oxygen. It is also called trioxygen. As it has three oxygen atoms in its one molecule. It is an inorganic molecule. Its chemical formula is O3. Ozone is much less stable than oxygen.

Ozone has a bent structure and it is a polar molecule. As the distribution of electrons across the atoms in ozone is uneven, so the central atom has less electron density which results in polarity of molecules. We can describe the charge distribution by stating that the central atom has a formal charge of +1 and other two oxygen atoms in ozone have -1/2 charge on each atom.

Alkenes and Alkynes 

Before understanding ozonolysis of alkenes and alkynes, you need to understand in brief about what alkenes and alkynes are. Alkenes are also known as olefins. Alkenes are unsaturated hydrocarbons which consist of at least one carbon-carbon double bond in their molecule. Their general formula is CnH2n. Alkynes are also unsaturated hydrocarbons, but they have at least one carbon – carbon triple bond in their molecule. Their general formula is CnH2n-2. 

Ozonolysis

Ozonolysis is a type of reaction that is used in organic chemistry to determine the position of a carbon-carbon double bond in unsaturated compounds. Ozonolysis involves the reaction of the compound with ozone leading to the formation of an ozonide, and the ozonide yields on hydrogenation or treatment with acid a mixture containing aldehydes, ketones, or carboxylic acids. Ozonolysis mechanism occurs under an oxidative cleavage reaction. The ozone is not only responsible for breaking the carbon pi bond but also the carbon-carbon sigma bond. This mechanism involves the attack of ozone on the given reactant to form an ozonide. To eliminate the oxygen content in this intermediate stage, zinc dust is employed though it forms zinc oxide with the oxygen. The final product may be somewhat different based on the type of reactant and the workup.

Ozonolysis of Alkenes

Alkenes consists of one pi bond so the double bond of alkenes, when in the presence of the ozone molecule, undergoes oxidative cleavage. Alkenes undergo the process of oxidation resulting in the formation of end products such as ketones, aldehydes, alcohols, or carboxylic acids. The alkene is generally subjected to ozonolysis in the presence of ozone molecules. Out of the three oxygen atoms that are present in the ozone molecule, one has a positive charge whereas the other has a negative charge. The oxygen atom that is negatively charged will attack one end of the carbon-carbon double bond. The positively charged oxygen atom will attack the other end of the carbon-carbon double bond. A cyclic structure is obtained and is known as molozonide, it is highly unstable. 

In this structure, carbon atoms are still attached with a single bond. It will then rearrange itself to form a relatively stable intermediate called ozonide. In the ozonide intermediate the bond between two carbon atoms is lost. The carbon atoms in the ozonide intermediate are directly linked via oxygen atoms. Ozonide will then undergo a reduction in the presence of dimethyl sulfide and zinc dust. Two products are created as the result of the reduction reaction and in both the products, a double bond exists between carbon and oxygen atoms and this refers to the ozonolysis reaction. Ozonolysis is the lysis of the double bond in the presence of an ozone molecule. The products formed at the end after the ozonolysis of alkenes would be either ketones, alcohols, aldehydes, or carboxylic acids.

Ozonolysis of Alkynes

Alkynes consists of two pi bonds. The triple bond of alkynes undergoes oxidative cleavage in the presence of an ozone molecule. Alkynes undergo the process of oxidation resulting in the formation of end products such as diketones and acid anhydrides. Basically in the presence of water, the acid anhydride gives rise to two carboxylic acids with the help of hydrolysis. The alkyne is generally subjected to ozonolysis in the presence of ozone molecules. The oxygen atom that is negatively charged will attack one end of the carbon-carbon triple bond. The positively charged oxygen atom will attack the other end of the carbon-carbon triple bond. An intermediate consisting of a carbon-carbon double bond is created and this intermediate is unstable and undergoes rearrangement. Double bond between two carbon atoms is broken and that results in the formation of a stable ozonide. In ozonide the triple bond is reduced to a single bond. And then, the single bond between two carbon atoms also breaks, yielding a diketone compound. The diketone compound will then undergo hydrolysis resulting in the formation of two carboxylic acids.

Applications of Ozonolysis 

• It is used in organic chemistry to know the location of double or triple bonds in alkenes and alkynes respectively. 

• It is used to know the structure of long alkenes and alkynes. 

• It is used in bleaching. 

• It is used in wastewater disinfection. 

• It is used in the synthesis of alcohols, carboxylic acids, aldehydes and ketones. 

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