Textbook Question
Alcohols combine with ketones and aldehydes to form interesting derivatives, which we will discuss in Chapter 18. The following reactions show the hydrolysis of two such derivatives. Propose mechanisms for these reactions.
(b)
21. Aldehydes and Ketones: Nucleophilic Addition
Ketone and Aldehyde Synthesis Reactions
7:13 minutesProblem 100a(ii)
Textbook Question
Draw the mechanism for the acid-catalyzed pinacol rearrangement of the following diols.
(a) 
Verified step by step guidance1
Identify the diol structure given in the problem. Pinacol rearrangement typically involves a 1,2-diol, so ensure you have the correct starting material.
Protonate one of the hydroxyl groups using the acid catalyst. This step increases the leaving group ability of the hydroxyl group by converting it into a better leaving group, such as water.
Once the hydroxyl group is protonated, it can leave as a water molecule, forming a carbocation at the carbon where the hydroxyl group was attached.
Rearrange the carbocation to form a more stable carbocation. This often involves a 1,2-shift of an alkyl or aryl group adjacent to the carbocation, resulting in a new carbocation at a different position.
Deprotonate the oxonium ion formed after the rearrangement to yield the final ketone product. This step involves the removal of a proton to restore neutrality to the molecule.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Pinacol Rearrangement
Pinacol rearrangement is a chemical reaction where a 1,2-diol undergoes acid-catalyzed dehydration to form a ketone or aldehyde. The reaction involves the migration of an alkyl group and is typically initiated by protonation of one of the hydroxyl groups, leading to the formation of a carbocation intermediate.
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Definition of Claisen Rearrangement
Carbocation Stability
Carbocation stability is crucial in organic reactions, including rearrangements. In pinacol rearrangement, the formation of a stable carbocation intermediate is essential for the migration step. Stability is influenced by factors such as hyperconjugation and resonance, with tertiary carbocations generally being more stable than secondary or primary ones.
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Acid Catalysis
Acid catalysis involves the use of an acid to increase the rate of a chemical reaction. In the pinacol rearrangement, the acid protonates the hydroxyl group, facilitating the formation of a carbocation. This protonation step is critical as it initiates the rearrangement process, allowing the subsequent migration and dehydration steps to occur.
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