Ozonolysis Mechanism – Ozonolysis of Alkenes and Alkynes

What is Ozonolysis?

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. 

Ozonolysis Reaction-

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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.

 

Ozone shows two resonating structures which are shown below – 

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Skeletal formula of Ozone with partial charges is shown below- 

 

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Alkenes and Alkynes 

Before understanding ozonolysis of alkenes and alkynes, you need to understand in brief about what are alkenes and alkynes. 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

 

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                  Alkene                                                        Alkyne 

Ozonolysis of Alkenes 

In ozonolysis of alkenes double bonds between carbon – carbon of alkene molecules are cleaved, and oxygen atoms of ozone get attached to it. It is a type of oxidation-reduction reaction. Ozonolysis of alkene gives alcohols, aldehydes, ketones or carboxylic acids. Ozonolysis doesn’t lead to further oxidation of aldehydes and ketones. So, they can be recovered easily. Ozonolysis of alkenes take place naturally as well. 

Mechanism of Ozonolysis of Alkenes 

Mechanism of ozonolysis of Alkenes can be explained by following two steps – 

Step 1. Formation of molozonide and ozonide or oxidation of alkene

Ozone adds to the double bond of alkene and forms a five membered ring intermediate called molozonide. Now molozonide rearranges to form ozonide. (we are describing mechanism for ozonolysis of ethene)

Reaction is given below – 

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Step 2. Reduction of the ozonide 

Ozonide is reduced to aldehydes and ketones by using a mild reducing agent such as dimethyl sulphide or zinc. If dimethyl sulphide is used as a reducing agent then Sulphur also gets oxidized and forms dimethyl sulfoxide which is also called DMSO. 

Reaction is given below – 

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Ozonolysis of Alkynes 

In ozonolysis of alkynes triple bonds between carbon – carbon of alkyne molecules are cleaved, and oxygen atoms of ozone get attached to it. It is a type of oxidation-reduction reaction. Ozonolysis of alkyne gives acid anhydride or diketone product. 

Mechanism of Ozonolysis of Alkynes 

In the ozonolysis of alkynes reducing agent is not required. The exact mechanism is not completely known. Its mechanism can be described by following two steps –

Step 1. Formation of ozonide 

Alkyne reacts with ozone and the triple bond gets a break which results in a cyclic compound called ozonide. 

Reaction can be written as follows- 

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Step 2. Formation of diketones and Carboxylic acids 

Reduction of ozonide forms diketone compounds which on hydrolysis give carboxylic acids. 

Reaction can be written as follows – 

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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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