Draw the structure of an eight-carbon alkene that would yield the following compound (and no others) after treatment with ozone followed by dimethyl sulfide.
The Correct Answer and Explanation is:
To answer this question, we need to reverse-engineer the product of ozonolysis, a reaction that cleaves carbon–carbon double bonds and forms carbonyl compounds (usually aldehydes or ketones). You’re asked to draw an eight-carbon alkene that yields only one ozonolysis product, so let’s go through the logic.
Step 1: Understand Ozonolysis with Reductive Workup
When an alkene is treated with ozone (O₃) followed by dimethyl sulfide (DMS) as the reducing agent, the double bond is cleaved, and each carbon of the double bond becomes the carbon of a carbonyl group:
- C=C → 2 × C=O (either aldehydes or ketones)
For example:
- CH₂=CH₂ (ethene) → 2 × HCHO (formaldehyde)
- CH₃CH=CHCH₃ → 2 × CH₃CHO (acetaldehyde)
Step 2: Only One Product Is Formed
This is key. You’re told that only one compound is produced upon ozonolysis. That means the alkene must be symmetrical, and both sides of the double bond must be identical so that cleaving it gives two identical carbonyl molecules.
Step 3: Determine the Product
Let’s suppose ozonolysis gives only 2 molecules of butanal (CH₃CH₂CH₂CHO).
This is a 4-carbon aldehyde. If we reverse that, the alkene must have had two identical 4-carbon fragments joined by a double bond between carbon 4 and carbon 5.
So the starting alkene must be:
CH₃CH₂CH=CHCH₂CH₂CH₃
But this has 7 carbons. To get 8, we must build an 8-carbon symmetrical alkene.
✅ Correct Answer: 4-octene (cis or trans)
Structure:
CH₃CH₂CH₂CH=CHCH₂CH₂CH₃This is 4-octene, which has a double bond between C-4 and C-5.
On ozonolysis:
- The double bond cleaves between C-4 and C-5.
- Each carbon becomes the carbon of a butanal (CH₃CH₂CH₂CHO).
- Since the alkene is symmetrical, only butanal is formed.
Final Notes
- This reaction demonstrates retrosynthetic analysis—thinking backward from the product.
- Symmetry in the alkene is crucial to yield only one ozonolysis product.
- 4-octene is the only eight-carbon straight-chain alkene that meets this requirement.