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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7:44 minutes

Problem 27

Textbook Question

When the following substituted cycloheptanol undergoes dehydration, one of the minor products has undergone a ring contraction. Propose a mechanism to show how this ring contraction occurs.

Verified step by step guidance

Step 1: Protonation of the hydroxyl group - In the presence of H₂SO₄ (a strong acid), the hydroxyl group (-OH) on the cycloheptanol is protonated to form water, which is a good leaving group. This step increases the likelihood of dehydration.

Step 2: Formation of a carbocation - The water molecule leaves, resulting in the formation of a carbocation at the carbon where the hydroxyl group was originally attached. This carbocation is stabilized by hyperconjugation and inductive effects from the adjacent alkyl groups.

Step 3: Ring contraction via a hydride shift - A hydride shift occurs from one of the adjacent carbons to the carbocation center. This shift leads to the formation of a new carbocation, which is now located on a six-membered ring due to the contraction of the original seven-membered ring.

Step 4: Formation of the double bond - A proton is eliminated from the carbocation intermediate, resulting in the formation of a double bond. This elimination step produces the minor product, which is a substituted cyclohexene.

Step 5: Explanation of minor product formation - The ring contraction occurs because the six-membered ring is more stable than the seven-membered ring due to reduced ring strain. This stability drives the formation of the minor product during the dehydration process.

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Key Concepts

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

Cycloheptanol Structure

Cycloheptanol is a seven-membered cyclic alcohol with a hydroxyl (-OH) group attached to one of the carbon atoms. Understanding its structure is crucial for analyzing reactions involving this compound, particularly how the hydroxyl group influences reactivity during dehydration and subsequent transformations.

Dehydration Mechanism

Dehydration is a chemical reaction that involves the removal of water (H2O) from a molecule. In the case of cycloheptanol, dehydration typically leads to the formation of an alkene. Recognizing the steps of this mechanism, including protonation of the alcohol and elimination of water, is essential for predicting the products and any potential ring contractions.

Ring Contraction

Ring contraction refers to the process where a cyclic compound undergoes a reduction in the number of atoms in the ring, often resulting in a smaller ring structure. This phenomenon can occur during dehydration reactions, particularly when the stability of the resulting carbocation is enhanced by forming a more stable, smaller ring, which is a key aspect to consider in the proposed mechanism.

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Related Practice

Textbook Question

Indicate which alcohol in each pair undergoes an elimination reaction more rapidly when heated with H₂SO₄.

Textbook Question

Propose mechanisms for the following reactions.

(b)

HINT: Alcohol dehydrations usually go through E1 elimination of the protonated alcohol, with a carbocation intermediate. Rearrangements are common.

Textbook Question

Propose mechanisms for the following reactions.

HINT: Alcohol dehydrations usually go through E1 elimination of the protonated alcohol, with a carbocation intermediate. Rearrangements are common.

Textbook Question

Under acid catalysis, tetrahydrofurfuryl alcohol reacts to give surprisingly good yields of dihydropyran. Propose a mechanism to explain this useful synthesis.

Textbook Question

Propose a mechanism for each reaction.

(c)

Textbook Question

Propose a mechanism for each reaction.

(a)

Textbook Question

Propose mechanisms for the following reactions. In most cases, more products are formed than are shown here. You only need to explain the formation of the products shown, however.

(b)

Textbook Question

Write a balanced equation for each reaction, showing the major product you expect.

(a)

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