Reverse Ozonolysis: Finding the Alkene from its Products
Ozonolysis is famous for acting like a "chemical scissor," slicing carbon-carbon double bonds and attaching oxygen atoms to form aldehydes or ketones. But in JEE and NEET, the examiners often run the reaction backward!
You will be given the product(s) and asked to find the parent alkene. This is known as Reverse Ozonolysis. Let's learn the visual "stitching" method to solve these instantly.
Video Tutorial: Solving an Intramolecular Ozonolysis
Watch Abhishek Sengar sir from CHEMCA solve a tricky variation where a single molecule contains two carbonyl groups, meaning the original alkene must have been a ring (cycloalkene)!
The "Stitch-It-Back" Trick
Solved Example: 2-methyl-6-oxoheptanal
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Draw the given product:
Heptanal means a 7-carbon chain. An aldehyde (CHO) is always at Carbon 1. A methyl group is at C2, and an oxo (ketone) group is at C6.
O=CH(1) - CH(CH3)(2) - CH2(3) - CH2(4) - CH2(5) - C(=O)(6) - CH3(7) -
Identify the Carbonyl Carbons:
The double bonds to oxygen exist at C1 and C6. Because they are on the same molecule, stitching them together will form a closed ring. -
Form the Ring:
Count the atoms in the new ring: C1, C2, C3, C4, C5, and C6. This forms a 6-membered ring (cyclohexene).
Attach the leftover groups: C2 has a methyl group. C6 also has a methyl group (which was previously C7). -
IUPAC Naming of the New Alkene:
We have a cyclohexene ring with two methyl groups on the double bond carbons. According to IUPAC rules, we must number the double bond carbons as 1 and 2, keeping substituent numbers as low as possible.
Let the original C6 become the new C1 (it has a methyl). Then the original C1 becomes the new C2. The original C2 (which has the other methyl) becomes the new C3.
The final name is 1,3-dimethylcyclohexene.
Fig: Removing the Oxygen atoms allows C1 and C6 to bond together, forming the ring.
Practice Questions for JEE & NEET
Apply the "Stitch-It-Back" trick to solve these two classic textbook problems.
Question 1: An unknown alkene undergoes reductive ozonolysis to yield exactly two moles of Propan-2-one (Acetone). Identify the alkene.
Answer: 2,3-dimethylbut-2-ene
Reasoning:
- Draw two molecules of Acetone facing each other: (CH3)2C=O and O=C(CH3)2.
- Remove the oxygen atoms.
- Connect the central carbons with a double bond: (CH3)2C=C(CH3)2.
- The longest chain is 4 carbons long, with a double bond at C2, and methyl groups at C2 and C3. Hence, 2,3-dimethylbut-2-ene.
Question 2: Ozonolysis of a certain alkene produces a mixture of Methanal (Formaldehyde) and Propan-2-one (Acetone). What is the name of the original alkene?
Answer: 2-methylpropene (Isobutylene)
Reasoning:
- Draw Methanal: CH2=O
- Draw Acetone: O=C(CH3)2
- Face the oxygens towards each other, erase them, and form a double bond.
- Resulting structure: CH2=C(CH3)2.
- The longest chain is 3 carbons long, with a methyl group at C2. Hence, 2-methylpropene.
Your tip about numbering the carbon chain before doing anything else saved my life. I always used to lose a carbon when turning a dialdehyde back into a ring
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