Alkenes
An alkene X undergoes catalytic hydrogenation (H₂/Ni) to give n-butane. The same alkene X decolorises bromine water and also decolorises cold, dilute KMnO₄ solution. When X is treated with ozone followed by oxidative workup (H₂O₂), it gives only propanoic acid as the organic product. What is the IUPAC name of X?
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About This Question
- Subject
- chemistry
- Chapter
- hydrocarbons
- Topic
- alkenes
- Difficulty
- Hard
- Year
- 2025
Solution
Correct Answer:
But-2-ene
Step 1 — Hydrogenation gives n-butane (C₄H₁₀), so X must be a butene (C₄H₈). Step 2 — It decolorises Br₂/H₂O (confirms C=C present) and cold dilute KMnO₄ (confirms alkene, not aromatic). Step 3 — Ozonolysis with oxidative workup converts each carbon of the C=C to a carboxylic acid (if it bears at least one H) or a ketone (if it bears no H). Only propanoic acid (CH₃CH₂COOH) is produced, meaning both fragments are identical 3-carbon units: CH₃CH₂– on each carbon of the double bond. This is consistent with but-2-ene (CH₃–CH=CH–CH₃), where each C of the double bond carries one CH₃ and one H → each gives CH₃CH₂COOH upon oxidative ozonolysis. But-1-ene would give methanal + propanal (different fragments). Therefore X is but-2-ene.
This hard difficulty chemistry question is from the chapter hydrocarbons, covering the topic of alkenes. It appeared in the 2025 exam.
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