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 :
First, the question says:
- We have an 8-carbon alkene.
- After ozonolysis (ozone followed by dimethyl sulfide, which is a reductive workup), it yields a single compound (and no others).
When an alkene undergoes ozonolysis + reductive workup, each carbon-carbon double bond is cleaved, and each carbon of the double bond becomes a carbonyl group (C=O).
Depending on the position of the double bond, ozonolysis produces:
- Aldehydes (if the double-bonded carbon is bonded to at least one H)
- Ketones (if the double-bonded carbon is bonded to two carbons)
Now, since only one product forms after ozonolysis, both halves after cleavage must be identical — symmetrical.
Thus, the alkene must be symmetrical around the double bond.
Let’s work through it:
We need an 8-carbon alkene, symmetrical, so after ozonolysis, the two pieces are the same.
Suppose we choose 4 carbons on each side of the double bond.
Thus, the structure is:
CH₃–CH₂–CH₂–CH=CH–CH₂–CH₂–CH₃
This is 4-octene, specifically the trans-4-octene or cis-4-octene form.
After ozonolysis:
- The C=C bond between C4 and C5 is cleaved.
- Each side becomes a carbonyl group.
- Thus, we get two identical fragments: butanal (CH₃–CH₂–CH₂–CHO)
Butanal is an aldehyde (4 carbons).
Final Answer:
The structure of the starting alkene is 4-octene:
Structure:
CH₃–CH₂–CH₂–CH=CH–CH₂–CH₂–CH₃Full Explanation (300 words)
When an alkene undergoes ozonolysis followed by reductive workup (dimethyl sulfide), the carbon-carbon double bond is cleaved and each carbon formerly involved in the double bond becomes a carbonyl group. If an alkene yields only a single product after this process, it indicates that the molecule must be symmetrical, meaning the two sides of the double bond are identical. Since the alkene must have eight carbons and produce only one product, it must be symmetrically arranged around the double bond.
4-octene fits these criteria perfectly. Its structure, CH₃–CH₂–CH₂–CH=CH–CH₂–CH₂–CH₃, places the double bond between the fourth and fifth carbons, ensuring symmetry. Upon ozonolysis, the double bond is cleaved, and each carbon atom at the cleavage point is converted into an aldehyde group. The resulting product is butanal (CH₃–CH₂–CH₂–CHO), a four-carbon aldehyde. Since the molecule was symmetric, the same product is generated from both sides of the original double bond, leading to only one distinct compound after ozonolysis.
If the starting material had been asymmetric (e.g., with different groups on either side of the double bond), two different products would have resulted, which contradicts the problem’s statement. Therefore, 4-octene is the correct and only answer that matches all conditions provided.