Ozonolysis

Introduction

Ozonolysis is a process that determines the position of a carbon-carbon double bond in unsaturated molecules. It involves the interaction of the chemical with ozone, which results in the creation of an ozonide, which then produces a mix of aldehydes, ketones or carboxylic acids after hydrogenation or acid treatment. It is feasible to detect the position of the double bond in the initial unsaturated chemical by understanding the shape of the aldehydes and ketones generated.

Ozonolysis reaction is a widely utilised technique to determine the structure of natural compounds, particularly terpenes. It has also been used to analyse aromatic compounds’ configuration and make uncommon aldehydes and ketones. Oleic acid produces n-nonanal (pelargonic aldehyde) and azelaic semialdehyde on ozonolysis.

Mechanism of ozonolysis

The process of ozonolysis is based on an oxidative cleavage reaction. The ozone destroys both the carbon-carbon pi and carbon-carbon sigma bonds. It entails the ozone attack on a specific reactant, resulting in the formation of an ozonide. Zinc dust removes oxygen from this intermediate step (forming zinc oxide with oxygen). The type of reactant and the workup impact the end product. 

There are three phases in the ozonolysis reaction mechanism.

Step 1: The ozone molecule is attacked

Three oxygen atoms make up the ozone molecule. One oxygen atom gains a partial negative charge, while the other gains a partial positive charge. The negatively charged oxygen ion attacks the double-bonded carbon atom. The reactant is linked to the ozone molecule. Rearrangement occurs within the structure, resulting in an intermediate structure known as molozonide.

Step 2: The ozonide intermediate is formed

The molozonide intermediate continues to rearrange, eventually forming a stable ozonide intermediate.

Step 3: Carbonyl compounds are formed

The resulting ozonide intermediate is exposed to zinc dust (Zn/water) and dimethyl sulphide. Two carbonyl compounds are produced in the presence of zinc dust.

Ozonolysis of alkenes

C-C=C-C-C + O3 + Zn → C-CHO + C-C-CHO ……. (Reductive ozonolysis)

C-C=C-C-C + O3 →   C-COOH + C-C-COOH ……… (Oxidative ozonolysis)

Oxidising alkenes with ozone can form alcohols, aldehydes, ketones and carboxylic acids. In the ozonolysis of alkenes process, ozone is bubbled through an alkene solution in methanol at 78°C until the solution turns a characteristic blue colour due to unreacted ozone. This signifies that the alkene has been completely consumed. Alternatively, by detecting the presence of ozone, several additional compounds can be employed as indicators of this endpoint. The gas that bubbles out can be routed via a potassium iodide solution if ozonolysis is done by bubbling a stream of ozone-enriched oxygen through the reaction mixture. When the solution stops absorbing ozone, the ozone in the bubbles oxidises the iodide to iodine, which has a visible violet tint.

An indicator, such as Sudan Red III can be added into the mixture for better reaction management. This indicator reacts with ozone more gradually than the targeted ozonolysis target. The indicator’s ozonolysis, which results in a significant colour shift, only occurs when the target has been ingested. If the substrate contains two alkenes that react with ozone at different rates, an indication whose oxidation rate is halfway between them can be used to stop the reaction when only the substrate’s most sensitive alkene has reacted. Otherwise, the existence of unreacted ozone in solution (as indicated by its blue hue) or in bubbles (as determined by iodide detection) shows that all alkenes have reacted.

Ozonolysis of alkynes

Ozonolysis of alkynes is a simple process. Two pi bonds occur in alkynes. In the presence of an ozone molecule, the triple bond of alkynes undergoes oxidative breakage. End products such as diketones and acid anhydrides form when alkynes are oxidised. With the support of hydrolysis, the acid anhydride breaks down into two carboxylic acids in the surrounding water.

Consider an example to help understand the ozonolysis reaction mechanism in an alkyne.

In the presence of ozone molecules, the alkyne undergoes ozonolysis. One end of the carbon-carbon triple bond is attacked by the negatively charged oxygen atom. The positively charged oxygen atom is attacked by the other end of the carbon-carbon triple bond. A carbon-carbon double bond is produced in the intermediate. This intermediate is unstable and experiences more changes. Because the double bonding of two carbon atoms is severed, a stable ozonide is formed.

We can observe that the triple bond is reduced to a single bond in ozonide. Later, the single link between two carbon atoms breaks, resulting in the formation of a diketone molecule. The hydrolysis of the diketone molecule results in the production of two carboxylic acids.

Natural rubber tubing cracking due to ozone

This approach identifies isoprene as the structural repeat unit in natural rubber. Ozone cracking, where traces of the gas in the environment induce deterioration in elastomers such as rubber products, polybutadiene, styrene-butadiene and nitrile rubber, is also a severe issue. If the polymer is exposed to light, ozonolysis forms surface ketone groups, triggering further slow breakdown via Norrish reactions.

Ozone cracking is a type of stress corrosion cracking in which active chemical species attack a molecule’s components. The rubber product must be under strain for crack development to occur.

The rubber product must be under tension for crack growth to occur. Ozone cracking was once commonly seen in tires’ sidewalls, where it could expand to cause a dangerous blowout but is now rare due to modern antiozonants. Other means of prevention include replacing susceptible rubbers with resistant elastomers such as polychloroprene, EPDM or Viton.

Conclusion

In this chapter, we learn about ozonolysis and the various methods of ozonolysis. It is a reaction to oxidise alkenes or alkynes. This reaction allows double or triple bonds to be exchanged with oxygen bonds. Ozonolysis may be done in both oxidative as well reductive manner.

Oxidative ozonolysis yields products like carboxylic acids if aldehydes are produced after hydrolysis due to production of hydrogen peroxide as a side product after hydrolysis which oxidises aldehydes produced into carboxylic acid further. But in reductive ozonolysis in presence of Zn dust aldehyde or ketones produced after hydrolysis don’t get oxidised further as hydrogen peroxide produced reacts with Zn dust.