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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5:24 minutes

Problem 39d

Textbook Question

Propose mechanisms for the following reactions.
d.
HINT: Alcohol dehydrations usually go through E1 elimination of the protonated alcohol, with a carbocation intermediate. Rearrangements are common.

Verified step by step guidance

Step 1: Protonation of the alcohol group - The reaction begins with the alcohol group being protonated by sulfuric acid (H₂SO₄). This converts the hydroxyl group (-OH) into a better leaving group, forming a protonated alcohol (H₂O⁺).

Step 2: Formation of the carbocation intermediate - The protonated alcohol undergoes elimination of water (H₂O), leaving behind a carbocation intermediate. This carbocation is initially formed at the carbon where the alcohol group was attached.

Step 3: Carbocation rearrangement - Carbocations are prone to rearrangements to form more stable intermediates. In this case, a hydride shift or ring expansion may occur to stabilize the carbocation. Analyze the structure to determine the most stable carbocation configuration.

Step 4: Elimination to form alkenes - The carbocation intermediate undergoes elimination of a proton (H⁺) to form double bonds (alkenes). Depending on the position of the carbocation and the rearrangements, multiple alkenes can be formed as products.

Step 5: Product analysis - The reaction yields a mixture of alkenes, including the ones shown in the image. These products are formed due to the different possible elimination pathways and carbocation rearrangements during the reaction.

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Key Concepts

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

E1 Mechanism

The E1 mechanism is a type of elimination reaction where the rate-determining step involves the formation of a carbocation intermediate after the loss of a leaving group, such as water from a protonated alcohol. This mechanism typically occurs in two steps: first, the alcohol is protonated to form a better leaving group, and then the leaving group departs, forming a carbocation that can lose a proton to form an alkene.

Carbocation Stability

Carbocation stability is crucial in determining the pathway of E1 reactions. Carbocations are positively charged species that can be classified as primary, secondary, or tertiary based on the number of alkyl groups attached to the positively charged carbon. Tertiary carbocations are the most stable due to hyperconjugation and inductive effects, making them more favorable intermediates in elimination reactions.

Rearrangements in Organic Reactions

Rearrangements refer to the structural changes that can occur during the formation of carbocations, often leading to more stable carbocation intermediates. In the context of dehydration reactions, a carbocation may undergo a hydride or alkyl shift to form a more stable carbocation before elimination occurs, resulting in different alkene products. This is a common phenomenon in E1 mechanisms and can lead to a variety of products.

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Textbook Question

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

Textbook Question

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

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

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

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.

Textbook Question

Propose a mechanism for each reaction.

(c)

Textbook Question

Propose a mechanism for each reaction.

(a)

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