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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5:48 minutes

Problem 96b

Textbook Question

Propose a mechanism for each of the following reactions:
b.

Verified step by step guidance

Step 1: Protonation of the alcohol group - The reaction begins with the protonation of the hydroxyl (-OH) group by sulfuric acid (H₂SO₄). This converts the alcohol into a better leaving group, forming a positively charged oxonium ion.

Step 2: Formation of a carbocation - The protonated alcohol group leaves as water (H₂O), resulting in the formation of a carbocation at the tertiary carbon. This step is facilitated by the stability of the tertiary carbocation.

Step 3: Ring expansion - The carbocation undergoes a ring expansion to relieve ring strain in the cyclopentane ring. A hydride shift or alkyl shift occurs, leading to the formation of a more stable cyclohexane ring structure.

Step 4: Elimination to form the double bond - A proton is abstracted from a neighboring carbon by the conjugate base of sulfuric acid (HSO₄⁻), resulting in the formation of a double bond. This step completes the formation of the alkene product.

Step 5: Final product - The final product is a cyclohexene derivative with a double bond and a methyl group attached to the ring. This is the result of the elimination reaction and ring expansion.

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Key Concepts

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

Dehydration Reaction

A dehydration reaction involves the removal of a water molecule from a compound, typically resulting in the formation of a double bond. In organic chemistry, this process is often used to convert alcohols into alkenes. The reaction is facilitated by strong acids, such as sulfuric acid, which protonate the alcohol, making it a better leaving group.

Protonation and Carbocation Formation

In the presence of sulfuric acid, the hydroxyl group of the alcohol is protonated, leading to the formation of a carbocation intermediate. This step is crucial as it increases the electrophilicity of the molecule, allowing for the subsequent elimination of water. The stability of the carbocation significantly influences the reaction pathway and the final product.

Elimination Mechanism (E1 or E2)

The elimination mechanism can occur via either E1 or E2 pathways. In the E1 mechanism, the reaction proceeds through a two-step process involving carbocation formation followed by the loss of a proton to form the alkene. In contrast, the E2 mechanism is a concerted process where the base abstracts a proton while the leaving group departs, resulting in the formation of the double bond in a single step.

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Related Practice

Textbook Question

Predict the products of the sulfuric acid-catalyzed dehydration of the following alcohols. When more than one product is expected, label the major and minor products.

(d) 1-isopropylcyclohexanol

(e) 2-methylcyclohexanol

Textbook Question

Propose mechanisms for the following reactions. Additional products may be formed, but your mechanism only needs to explain the products shown.

(b)

Textbook Question

Four pairs of compounds are shown. In each pair, one of the compounds reacts more quickly, or with a more favorable equilibrium constant, than the less conjugated system. In each case, explain the enhanced reactivity.

(c)

Textbook Question

Predict the product of the following pinacol rearrangements.

(b)

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

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.

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Propose a mechanism for each of the following reactions: b.

Key Concepts

Dehydration Reaction

Protonation and Carbocation Formation

Elimination Mechanism (E1 or E2)

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Propose a mechanism for each of the following reactions:
b.