Ch. 10 - Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 10 - Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds

Problem 92

Chapter 11, Problem 92

A vicinal diol has OH groups on adjacent carbons. The dehydration of a vicinal diol is accompanied by a rearrangement called the pinacol rearrangement. Propose a mechanism for this reaction.

Verified step by step guidance

Step 1: Protonation of one of the hydroxyl groups occurs in the presence of sulfuric acid (H₂SO₄), which acts as a strong acid. This converts the hydroxyl group into a better leaving group, forming water.

Step 2: The water molecule leaves, generating a carbocation intermediate. This carbocation is formed on the carbon that originally had the protonated hydroxyl group.

Step 3: A rearrangement occurs to stabilize the carbocation. In this case, a methyl group migrates from the adjacent carbon to the carbocation-bearing carbon, resulting in a more stable tertiary carbocation.

Step 4: The remaining hydroxyl group undergoes deprotonation, forming a carbonyl group (C=O). This step completes the rearrangement and dehydration process.

Step 5: The final product is a ketone, specifically pinacolone, along with the release of a water molecule as a byproduct.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Vicinal Diols

Vicinal diols, also known as glycols, are organic compounds that contain two hydroxyl (-OH) groups attached to adjacent carbon atoms. Their structure is crucial in organic chemistry as they can undergo various reactions, including dehydration, which leads to the formation of alkenes or other functional groups. Understanding their properties and reactivity is essential for predicting the outcomes of reactions involving these compounds.

Dehydration Reaction

A dehydration reaction involves the removal of a water molecule from a compound, often resulting in the formation of a double bond or a new functional group. In the case of vicinal diols, dehydration typically leads to the formation of alkenes. This reaction is significant in organic synthesis as it can alter the structure and reactivity of the original molecule, making it a key step in many organic transformations.

Pinacol Rearrangement

The pinacol rearrangement is a specific type of rearrangement reaction that occurs when a vicinal diol is dehydrated, leading to the formation of a ketone or aldehyde. This reaction involves the migration of a carbon skeleton and the formation of a carbocation intermediate, which can rearrange to yield more stable products. Understanding this mechanism is vital for predicting the products of reactions involving vicinal diols and for grasping the principles of carbocation stability and rearrangement in organic chemistry.