Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

9. Alkenes and Alkynes

Dehydration Reaction

Organic Chemistry

9. Alkenes and Alkynes

Dehydration Reaction

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7:35 minutes

Problem 92

Textbook Question

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.

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