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青州's Question
Chemistry
Posted 3 months ago

Question 6 Mechanism I (5 marks): In the space provided below please give the detailed mechanism for the following transformation. Please be certain to use good curved arrow notation for your mechanism.
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Answer from Sia
Posted 3 months ago
Solution
1
Protonation of the hydroxyl group: The first step in the mechanism involves the protonation of the hydroxyl group (-OH) on the six-membered ring. This is facilitated by the acidic environment, which provides H+H^+ ions. The lone pair on the oxygen atom of the hydroxyl group attacks a proton, leading to the formation of a water molecule (H2OH_2O) which is a good leaving group
2
Formation of a carbocation: After the hydroxyl group is protonated and leaves as water, a carbocation is formed at the position where the hydroxyl group was attached. This is a key intermediate in the reaction mechanism
3
Ring expansion: The five-membered ring adjacent to the carbocation will undergo a ring expansion to stabilize the carbocation. This involves the migration of a bond from the five-membered ring to the six-membered ring, resulting in a new six-membered ring and a seven-membered ring
4
Deprotonation to form the final product: The final step involves the deprotonation of the carbocation intermediate to form the final product, which is a bicyclic compound with a six-membered ring fused to a seven-membered ring containing an oxygen atom. The carboxylic acid group remains unchanged throughout the reaction
Answer
The detailed mechanism involves protonation of the hydroxyl group, formation of a carbocation, ring expansion, and deprotonation to form the final product.
Key Concept
Acid-catalyzed ring expansion in bicyclic compounds
Explanation
The reaction proceeds through a series of steps starting with protonation, followed by the formation of a more stable carbocation through ring expansion, and ending with deprotonation to yield the final product.

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