Question

Which of the following alkenes on oxidation by $\text{KMnO}_4$ in dil. $\text{H}_2\text{SO}_4$ forms adipic acid?

• A. Hex-3-ene
• B. Hex-1-ene
• C. Hex-2-ene
• D. Cyclohexene

Hint:

To synthesize a dicarboxylic acid using oxidative cleavage, you must always choose a cyclic alkene. The total number of carbons inside the starting ring will correspond exactly to the total number of carbons in the final linear dicarboxylic product.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The problem asks to determine which starting alkene yields a six-carbon dicarboxylic acid (adipic acid) upon vigorous oxidative cleavage with hot, acidified potassium permanganate ($\text{KMnO}_4$).

Step 2: Key Formula or Approach:
Vigorous oxidation of an alkene using acidified $\text{KMnO}_4$ completely cleaves the carbon-carbon double bond (C=C). Open-chain alkenes break apart into separate fragment molecules (ketones or carboxylic acids). Cyclic alkenes undergo ring-opening oxidative cleavage at the double bond site to yield a single straight-chain dicarboxylic acid molecule containing the same total number of carbon atoms.

Step 3: Detailed Explanation:
Let's consider the structure of

cyclohexene. It is a six-carbon cyclic alkene. When cyclohexene is treated with $\text{KMnO}_4$ in dilute $\text{H}_2\text{SO}_4$, the double bond between carbon-1 and carbon-2 is entirely broken. Both alkene carbons are completely oxidized into carboxylic acid functional groups ($-\text{COOH}$): $$\text{Cyclohexene} \xrightarrow{\text{KMnO}_4 / \text{dil. }\text{H}_2\text{SO}_4} \text{HOOC-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-COOH}$$ The resulting molecule has six carbons arranged linearly with a carboxylic acid group at each terminal end. This compound is systematically named hexane-1,6-dioic acid, commonly known as

adipic acid. Open-chain isomers like hex-1-ene, hex-2-ene, or hex-3-ene would yield smaller fragmented short-chain acids (e.g., pentanoic acid, butanoic acid, or propanoic acid) instead of a single continuous dicarboxylic chain.

Step 4: Final Answer:
The alkene that oxidizes to form adipic acid is cyclohexene, matching option (D).