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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5:52 minutes

Problem 34a

Textbook Question

Propose mechanisms for the following reactions.
(a)

Verified step by step guidance

Step 1: Recognize that this reaction involves the dehydration of cyclopentanol to form cyclopentene. Dehydration reactions typically occur in the presence of a strong acid like H2SO4 and heat, and they follow an elimination (E1) mechanism.

Step 2: Protonation of the alcohol group: The hydroxyl (-OH) group of cyclopentanol is protonated by H2SO4, converting it into a better leaving group (water, H2O). This step increases the electrophilicity of the carbon attached to the hydroxyl group.

Step 3: Formation of the carbocation intermediate: The protonated hydroxyl group leaves as water, generating a carbocation intermediate on the cyclopentane ring. This step is the rate-determining step of the E1 mechanism.

Step 4: Rearrangement (if necessary): Check if the carbocation can undergo rearrangement to form a more stable carbocation. In this case, no rearrangement is needed because the cyclopentyl carbocation is already stable.

Step 5: Elimination of a proton: A base (often HSO4⁻ from the acid) abstracts a proton from a β-carbon adjacent to the carbocation, leading to the formation of a double bond (cyclopentene) and completing the elimination process.

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Key Concepts

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

Elimination Reactions

Elimination reactions involve the removal of atoms or groups from a molecule, resulting in the formation of a double bond. In the context of alcohols, dehydration is a common elimination reaction where a hydroxyl group (OH) is removed along with a hydrogen atom from an adjacent carbon, leading to the formation of an alkene. This process is often facilitated by heat and an acid catalyst.

Acid-Catalyzed Dehydration

Acid-catalyzed dehydration is a specific type of elimination reaction where an alcohol is converted into an alkene in the presence of an acid, such as sulfuric acid (H2SO4). The acid protonates the hydroxyl group, making it a better leaving group, and the subsequent loss of water leads to the formation of a double bond. This mechanism is crucial for understanding how cyclopentanol transforms into cyclopentene.

Carbocation Stability

Carbocation stability is a key factor in determining the pathway of elimination reactions. When the hydroxyl group is removed, a carbocation intermediate may form. The stability of this carbocation influences the likelihood of elimination occurring. More stable carbocations (such as tertiary over secondary or primary) are favored, which can affect the regioselectivity and product distribution in the reaction.

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Related Practice

Textbook Question

Show the product(s) you expect from dehydration of the following alcohols when they are heated in sulfuric or phosphoric acid. In each case, use a mechanism to show how the products are formed.

(a)

Textbook Question

Propose mechanisms for the following reactions.

(d)

Textbook Question

Propose mechanisms for the following reactions.

(c)

Textbook Question

Propose mechanisms for the following reactions.

(b)

Textbook Question

Show the product(s) you expect from dehydration of the following alcohols when they are heated in sulfuric or phosphoric acid. In each case, use a mechanism to show how the products are formed.

(b)

Textbook Question

The acid-catalyzed hydration we learned here in Chapter 8 is reversible.

(a) Propose a mechanism for the formation of an alkene from an alcohol.

Textbook Question

The acid-catalyzed hydration we learned here in Chapter 8 is reversible:

(d) How might you shift the equilibrium to the right?

Textbook Question

The acid-catalyzed hydration we learned here in Chapter 8 is reversible:

(e) How might you shift the equilibrium to the left?

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