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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12:23 minutes

Problem 80

Textbook Question

When the following seven-membered ring alcohol is dehydrated, three alkenes are formed. Propose a mechanism for their formation.

Verified step by step guidance

Step 1: Recognize that the reaction involves acid-catalyzed dehydration of the alcohol. Sulfuric acid (H₂SO₄) acts as a catalyst, and heat (Δ) promotes the elimination of water to form alkenes.

Step 2: Protonation of the hydroxyl group occurs first. The alcohol group (-OH) 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 a carbocation intermediate. After the departure of water, a carbocation is formed at the carbon where the hydroxyl group was originally attached. The stability of this carbocation is influenced by hyperconjugation and alkyl substituents.

Step 4: Rearrangement of the carbocation. The seven-membered ring allows for possible hydride or alkyl shifts to form more stable carbocations. This rearrangement can lead to different carbocation intermediates, which will ultimately result in the formation of multiple alkenes.

Step 5: Elimination of a proton (E1 mechanism). A base (often the conjugate base of H₂SO₄) abstracts a proton from a β-carbon adjacent to the carbocation, leading to the formation of double bonds (alkenes). Depending on the position of the β-hydrogens, three different alkenes are formed as shown in the image.

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Key Concepts

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

Dehydration Reaction

Dehydration reactions involve the removal of a water molecule from a compound, often resulting in the formation of a double bond. In the context of alcohols, this process typically occurs under acidic conditions, where the hydroxyl group is protonated, making it a better leaving group. The elimination of water leads to the formation of alkenes, which can vary based on the structure of the starting material.

Mechanism of Alkene Formation

The mechanism for alkene formation from alcohols generally follows an E1 or E2 pathway. In the E1 mechanism, the first step involves the formation of a carbocation after the leaving of water, followed by the elimination of a proton to form a double bond. In the E2 mechanism, the elimination occurs in a single concerted step, where a base abstracts a proton while the leaving group departs, leading to the formation of the alkene.

Regioselectivity and Stereochemistry

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others. In the dehydration of the seven-membered ring alcohol, multiple alkenes can form due to different positions of double bond formation. Stereochemistry also plays a role, as the spatial arrangement of substituents around the double bond can lead to different geometric isomers, influencing the overall product distribution.

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Textbook Question

Starting from bromobenzene and any other reagents and solvents you need, show how you would synthesize the following compounds. Any of these products may be used as starting materials in subsequent parts of this problem.

c. 1-phenylpropan-2-ol

Textbook Question

Explain why the acid-catalyzed dehydration of an alcohol is a reversible reaction, whereas the base-promoted dehydrohalogenation of an alkyl halide is an irreversible reaction.

Textbook Question

What stereoisomers are formed in the following reactions? Which stereoisomer is the major product?

a. the acid-catalyzed dehydration of 1-pentanol to 2-pentene

Textbook Question

Explain why the following alcohols, when heated with acid, form the same alkene.

Textbook Question

When heated with H₂SO4, both 3,3-dimethyl-2-butanol and 2,3-dimethyl-2-butanol are dehydrated to form 2,3-dimethyl-2-butene. Which alcohol dehydrates more rapidly?

Textbook Question

Write the appropriate reagent over each arrow.

Textbook Question

What is the major product(s) of each of the following reactions?

Textbook Question

What is the major product obtained when each of the following alcohols is heated in the presence of H₂SO₄?

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