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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4:20 minutes

Problem 74

Textbook Question

The following reaction is called the pinacol rearrangement. The reaction begins with an acid-promoted ionization to give a carbocation. This carbocation undergoes a methyl shift to give a more stable, resonance-stabilized cation. Loss of a proton gives the observed product. Propose a mechanism for the pinacol rearrangement.

Verified step by step guidance

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

Step 2: The water molecule leaves, generating a carbocation at the carbon where the hydroxyl group was initially attached. This carbocation is relatively unstable.

Step 3: A methyl group undergoes a 1,2-shift (methyl migration) to stabilize the carbocation. This results in the formation of a resonance-stabilized tertiary carbocation.

Step 4: A deprotonation step occurs, where a proton is removed from the carbocation intermediate. This leads to the formation of a ketone group (C=O) at the migrated carbon.

Step 5: The final product, pinacolone (H3CC(=O)C(CH3)3), is formed as a result of the rearrangement and stabilization of the molecule.

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Key Concepts

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

Carbocation Stability

Carbocations are positively charged species that are highly reactive. Their stability is influenced by the degree of substitution; tertiary carbocations are more stable than secondary or primary ones due to hyperconjugation and inductive effects. In the pinacol rearrangement, the formation of a more stable carbocation through a methyl shift is crucial for the reaction's progression.

Methyl Shift

A methyl shift is a type of rearrangement where a methyl group moves from one carbon to an adjacent carbon, resulting in the formation of a more stable carbocation. This process is essential in the pinacol rearrangement, as it allows the reaction to proceed through a more stable intermediate, ultimately leading to the formation of the product, pinacolone.

Acid-Catalyzed Rearrangement

The pinacol rearrangement is an acid-catalyzed reaction, meaning that the presence of an acid (like H2SO4) facilitates the ionization of the starting material, pinacol. This ionization generates a carbocation, which is a key intermediate in the reaction mechanism. The acid not only promotes the formation of the carbocation but also helps in the subsequent steps leading to the final product.

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Related Practice

Textbook Question

Propose a mechanism for each of the following reactions:

Textbook Question

Propose a mechanism for each of the following reactions:

Textbook Question

Propose a mechanism for each reaction.

(a)

(b)

Textbook Question

Suggest an alkene that, in two steps, could be converted into each of the following ketones. Each sequence should involve a pinacol rearrangement.

(c)

Textbook Question

Treatment of the following alcohol was expected to give alkene A. Instead, B was produced as the major product. Suggest a mechanism by which B formed.

Textbook Question

Although 2-methyl-1,2-propanediol is an unsymmetrical vicinal diol, only one product is obtained when it is dehydrated in an acidic solution.

a. What is this product?

Textbook Question

A student was studying terpene synthesis, and she wanted to make the compound shown here. First she converted 3-bromo-6-methylcyclohexene to alcohol A. She heated alcohol A with sulfuric acid and purified one of the components (compound B) from the resulting mixture. Compound B has the correct molecular formula for the desired product.

Textbook Question

What is the product of the following reaction?

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