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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4:56 minutes

Problem 29c

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)

Verified step by step guidance

Step 1: Analyze the structures of the two compounds. The first compound is benzyl alcohol, which contains a hydroxyl group (-OH) attached to a benzene ring. The second compound is cyclohexanol, which contains a hydroxyl group attached to a cyclohexane ring.

Step 2: Understand the dehydration reaction. Dehydration involves the removal of a water molecule, typically under acidic conditions, to form an alkene. In this case, benzyl alcohol will form a conjugated alkene (styrene), while cyclohexanol will form cyclohexene.

Step 3: Consider the stability of the carbocation intermediate. During dehydration, the hydroxyl group is protonated and leaves as water, forming a carbocation intermediate. The benzyl carbocation formed from benzyl alcohol is stabilized by resonance with the benzene ring, making it much more stable than the cyclohexyl carbocation formed from cyclohexanol, which lacks resonance stabilization.

Step 4: Evaluate the role of conjugation. The product of benzyl alcohol dehydration (styrene) is conjugated, meaning the double bond is in resonance with the benzene ring. This conjugation further stabilizes the product, making the reaction more favorable compared to the non-conjugated cyclohexene product formed from cyclohexanol.

Step 5: Conclude the enhanced reactivity. Benzyl alcohol dehydrates more easily than cyclohexanol due to the resonance stabilization of the benzyl carbocation intermediate and the conjugation in the final product, which lowers the activation energy and increases the equilibrium constant for the reaction.

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Key Concepts

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

Conjugation

Conjugation refers to the overlap of p-orbitals across adjacent double bonds or lone pairs, allowing for delocalization of electrons. This delocalization stabilizes the molecule, making it more reactive in certain reactions, such as dehydration. In the context of the compounds shown, the more conjugated system can facilitate the loss of water more readily due to this stabilization.

Dehydration Reaction

A dehydration reaction involves the removal of a water molecule from a compound, often resulting in the formation of a double bond. This reaction is crucial in organic chemistry for synthesizing alkenes from alcohols. The ease of dehydration can be influenced by the structure of the alcohol, where steric hindrance and electronic effects play significant roles in determining the reaction rate.

Stability of Carbocation Intermediates

During dehydration, a carbocation intermediate may form, and its stability is critical for the reaction's rate. More stable carbocations, such as tertiary or resonance-stabilized carbocations, will form more readily and lead to faster reactions. In the compounds presented, the one that dehydrates more easily likely forms a more stable carbocation, enhancing its reactivity compared to the less conjugated system.

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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)

Key Concepts

Conjugation

Dehydration Reaction

Stability of Carbocation Intermediates

Watch next

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)