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:
Correct Answer:
The product after ozonolysis is:
2 molecules of butanal (CH₃CH₂CH₂CHO)
To yield only butanal (an aldehyde with four carbon atoms), the original alkene must cleave symmetrically into two identical four-carbon fragments.
Structure of the Alkene:
The only eight-carbon alkene that will give two molecules of butanal upon ozonolysis is:
3-octene (cis or trans)
Structure of 3-octene:
CH₃CH₂CH=CHCH₂CH₂CH₃
Explanation
Ozonolysis is a powerful oxidative cleavage reaction used to break double bonds in alkenes. It involves the reaction of ozone (O₃) with the carbon-carbon double bond (C=C), resulting in the cleavage of the double bond and formation of two carbonyl-containing fragments — aldehydes or ketones depending on the substitution of the double bond.
After ozonolysis, if dimethyl sulfide (DMS, (CH₃)₂S) is used as the reducing agent, any ozonide intermediate is reduced to aldehydes or ketones, without further oxidation (i.e., no carboxylic acids form).
In this question, the only ozonolysis product is butanal (CH₃CH₂CH₂CHO) — an aldehyde with four carbon atoms. Since ozonolysis cleaves a double bond into two carbonyl fragments, the original alkene must have been split at the double bond to form two identical butanal molecules.
To get two butanal molecules, the original alkene must be symmetrical, with the double bond located such that each resulting fragment has four carbons.
Therefore, the original alkene is:
3-octene, with the structure:
CH₃CH₂CH=CHCH₂CH₂CH₃
Upon ozonolysis:
- The double bond between carbon 3 and carbon 4 is cleaved.
- Each side becomes a butanal molecule (CH₃CH₂CH₂CHO).
No other carbonyl compounds are formed, making 3-octene the only correct structure for this scenario. Both cis- and trans-3-octene would yield the same products under these conditions.
