Ozonolysis of Alkenes

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.

Understanding the Mechanism

Step 1: [3+2] Dipolar Cycloaddition

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.

Step 2: Ring Breakage

The molozonide ring is unstable and quickly breaks down to form a carbonyl compound (either an aldehyde or a ketone) and another dipole species.

Step 3: Formation of Final Ozonide

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.

Workup Techniques

After forming the ozonide, the next step involves a workup to yield the final product. There are two primary workup techniques:

1. Reductive Workup with Dimethylsulfide (DMS): DMS is effective but stinky.
2. Reductive Workup with Fresh Zinc in Acid: This method is cleaner but less effective and sensitive to zinc quality.

Alternative: Oxidative Workup

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.

Practical Example: 4-Methylhept-3-ene

Let’s consider 4-methylhept-3-ene undergoing reductive workup with DMS:

1. First, the alkene reacts with ozone to form molozonide.
2. The molozonide ring breaks to form a carbonyl and a dipole species.
3. These react to form the final ozonide.
4. Reductive workup with DMS yields the final products.

Simplifying Predictions: A Handy Trick

1. Stretch the Double Bond: Draw your alkene with an exaggerated double bond.
2. Erase the Middle: Erase the middle part of the exaggerated double bond.
3. Add Oxygens: Stick an oxygen atom at the end of each of the remaining bonds.

By following these steps, you can easily predict your ozonolysis products.

Multiple Double Bonds and Cyclic Molecules

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.