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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4:44 minutes

Problem 54

Textbook Question

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

Verified step by step guidance

Step 1: Protonation of the hydroxyl group - Under acidic conditions, the hydroxyl group (-OH) of tetrahydrofurfuryl alcohol is protonated by H⁺, forming a positively charged oxonium ion. This step increases the electrophilicity of the oxygen atom and prepares the molecule for subsequent reactions.

Step 2: Formation of a carbocation - The protonated hydroxyl group undergoes elimination of water (H₂O), leading to the formation of a carbocation at the carbon adjacent to the oxygen atom. This carbocation is stabilized by resonance with the oxygen atom in the tetrahydrofuran ring.

Step 3: Intramolecular nucleophilic attack - The oxygen atom in the tetrahydrofuran ring acts as a nucleophile and attacks the carbocation, forming a new bond between the oxygen and the carbocation carbon. This step results in the formation of a six-membered ring intermediate.

Step 4: Deprotonation to form the double bond - A proton is removed from the intermediate, leading to the formation of a double bond in the six-membered ring. This step completes the synthesis of dihydropyran.

Step 5: Final product - The reaction yields dihydropyran, a six-membered ring containing an oxygen atom and a double bond, as the final product.

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Key Concepts

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

Acid Catalysis

Acid catalysis involves the acceleration of a chemical reaction by the presence of an acid, which donates protons (H+) to the reactants. This process can enhance the electrophilicity of certain functional groups, making them more reactive. In the context of organic reactions, acids can facilitate the formation of carbocations, which are key intermediates in many mechanisms, including the conversion of tetrahydrofurfuryl alcohol to dihydropyran.

Mechanism of Dihydropyran Formation

The formation of dihydropyran from tetrahydrofurfuryl alcohol typically involves a series of steps including protonation, nucleophilic attack, and dehydration. Initially, the alcohol is protonated to form a more reactive carbocation. This carbocation can then undergo intramolecular nucleophilic attack by an alcohol group, leading to the formation of a cyclic ether, followed by loss of water to yield dihydropyran.

Carbocation Stability

Carbocation stability is a crucial concept in organic chemistry, as the stability of these positively charged intermediates influences reaction pathways. Tertiary carbocations are more stable than secondary or primary ones due to hyperconjugation and inductive effects. In the proposed mechanism, the formation of a stable carbocation intermediate from tetrahydrofurfuryl alcohol is essential for the efficient synthesis of dihydropyran under acid catalysis.

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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

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

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)

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)

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