Step 1
The electrophilic addition of ozone to the carbon-carbon bond forms the molozonide intermediate which is quite unstable. Due to this unstable nature, the molozonide continues reacting – breaking apart to form a carbonyl molecule and a carbonyl oxide molecule as shown below:
Step 2
The carbonyl molecule and the carbonyl oxide molecule formed in step 1 rearrange themselves, reforming to create a more stable ozonide intermediate. This ozonide intermediate can be subjected to either an oxidative workup or a reductive workup. The oxidative workup will give carboxylic acid as the product whereas the reductive workup will yield aldehydes or ketones.
The formation of the ozonide intermediate is illustrated below.
Oxidative Workup
When oxidant hydrogen peroxide is used instead of zinc or dimethyl sulfide to treat the ozonide, the aldehydes formed are oxidized to carboxylic acids. Potassium Permanganate in the presence of hot acid can also be used in the oxidative workup.
Reductive Workup
Here, the ozonide is treated with mild reducing agents such as dimethyl sulfide and zinc metal with water. The ozonide is reduced as shown below.
Ozonolysis of Alkynes
Alkynes undergo ozonolysis to give an acid anhydride or a diketone as the final product. The fragmentation is not complete in this reaction (alkenes undergo complete fragmentation). No reducing agents are required as a simple aqueous workup is followed. If the reaction happens in the presence of water, the acid anhydride undergoes hydrolyzation to give two carboxylic acids. Ozonolysis can also be used to determine the position of the triple bond in an unknown alkyne.
Ozonolysis of Alkynes Mechanism
The alkyne reacts with the ozone, causing the breakage of the alkyne. This gives rise to the ozonide intermediate. A simple aqueous workup follows with the help of zinc metal, finally yielding a dicarbonyl compound.
Ozonolysis of Elastomers – Ozone Cracking
The attack of ozone (present in trace amounts in the atmosphere) on the elastomer creates cracks. The double bond in the rubber chain is attacked by the ozone here. If the rubber product is under tension, ozone cracks begin to form. The cracks can appear around the circumference of a bent rubber tube since they are oriented at right angles to the axis of strain. These cracks are extremely dangerous when they occur in fuel pipes, growing inward from the exposed exterior surface. This can even lead to fuel leakage and fires.
Thus, ozone can be employed to cleave unsaturated bonds in alkenes, alkynes, and azo compounds. This cleaving is used in the organic chemical reaction called ozonolysis.
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