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A vicinal diol with a cyclopentane ring on each adjacent carbon is heated with dilute acid (H+/delta). Which of the following best describes the product?

1. Two cyclopentane rings connected by a C=C double bond
2. Two cyclopentane rings connected through an oxygen bridge forming a cyclic ether
3. A ketone with a cyclopentane ring on each side of the carbonyl carbon
4. A ketone with one cyclohexane ring and one cyclopentane ring on each side of the carbonyl

Correct answer: A ketone with a cyclopentane ring on each side of the carbonyl carbon

Solution

The reaction is a pinacol rearrangement. The starting material is a 1,2-diol (vicinal diol) with cyclopentyl groups on each carbon bearing -OH. Under H+/heat: Step 1: protonation of one -OH gives a good leaving group (water). Step 2: loss of water generates a tertiary carbocation. Step 3: 1,2-alkyl shift (the cyclopentyl group migrates to the adjacent carbocation). Step 4: deprotonation of the remaining -OH+ gives the ketone. The product is dicyclopentyl ketone (a ketone with a cyclopentane ring on each side of the C=O).

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