Ozonolysis of alkenes can appear daunting at first glance. However, in this tutorial, we will break it down into manageable steps. By understanding the underlying mechanism and applying some straightforward techniques, you can predict the products of alkene ozonolysis with confidence.
The reaction starts with an alkene and an ozone molecule. In the first step, a [3+2] dipolar cycloaddition occurs between these reactants. The name “[3+2] dipolar cycloaddition” might sound complicated, but it refers to the initial step where the alkene and ozone form a five-membered ring. This unstable ring is commonly known as a primary ozonide or molozonide.
The molozonide ring is unstable and quickly breaks down to form a carbonyl compound (either an aldehyde or a ketone) and another dipole species.
The carbonyl and the dipole species then react to form another five-membered ring, commonly known as an ozonide. This structure is a 1,2,4-trioxolane ring and is considerably more stable than molozonide. However, it is generally not isolated.
After forming the ozonide, the next step involves a workup to yield the final product. There are two primary workup techniques:
Using hydrogen peroxide as a workup agent results in an “oxidative workup,” converting aldehydes to carboxylic acids.
Typically, your instructor will not require you to know the mechanism for the workup steps, however, the mechanism for the alkene reaction with the ozone is a fair game.
Let’s consider 4-methylhept-3-ene undergoing reductive workup with DMS:
By following these steps, you can easily predict your ozonolysis products.
The same principles apply to alkenes with multiple double bonds and cyclic molecules. For cyclic molecules, imagine “cutting” through the double bond and then proceed as you would with an open-chain molecule.