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

6. Thermodynamics and Kinetics

Carbocation Intermediate Rearrangements

Organic Chemistry

6. Thermodynamics and Kinetics

Carbocation Intermediate Rearrangements

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5:10 minutes

Problem 2

Textbook Question

In a strongly acidic solution, cyclohexa-1,4-diene tautomerizes to cyclohexa-1,3-diene. Propose a mechanism for this rearrangement, and explain why it is energetically favorable.

Verified step by step guidance

Step 1: Begin by understanding the concept of tautomerization. Tautomerization is a chemical reaction that results in the structural rearrangement of a compound, typically involving the migration of a hydrogen atom and a switch of a single bond and adjacent double bond. In this case, cyclohexa-1,4-diene is converting to cyclohexa-1,3-diene.

Step 2: Consider the role of the acidic solution. In a strongly acidic environment, protons (H⁺ ions) are abundant. These protons can facilitate the rearrangement by protonating the double bond, making it more susceptible to rearrangement.

Step 3: Propose the initial step of the mechanism. The protonation of cyclohexa-1,4-diene occurs at one of the double bonds, forming a carbocation intermediate. This is a key step as it sets the stage for the rearrangement.

Step 4: Describe the rearrangement process. The carbocation formed is stabilized by the migration of a hydrogen atom from the adjacent carbon, leading to the formation of a new double bond. This migration results in the shift from a 1,4-diene to a 1,3-diene structure.

Step 5: Explain the energetic favorability. The formation of cyclohexa-1,3-diene is energetically favorable due to increased conjugation. Conjugated dienes are more stable than isolated dienes because the delocalization of electrons across the conjugated system lowers the overall energy of the molecule.

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Key Concepts

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

Tautomerization

Tautomerization is a chemical process where a compound undergoes structural rearrangement, typically involving the migration of a hydrogen atom and a switch of a single bond and adjacent double bond. In the context of cyclohexa-1,4-diene, tautomerization involves the shift of double bonds to form cyclohexa-1,3-diene, which is facilitated by acidic conditions.

Acid Catalysis

Acid catalysis involves the use of an acid to increase the rate of a chemical reaction. In this mechanism, the acid donates a proton to the cyclohexa-1,4-diene, stabilizing the intermediate and facilitating the rearrangement of bonds. The presence of a strong acid helps in the protonation of the diene, making the tautomerization process more favorable.

Energetic Favorability

Energetic favorability refers to the tendency of a chemical reaction to proceed because it results in a lower energy state. Cyclohexa-1,3-diene is more stable than cyclohexa-1,4-diene due to the conjugation of double bonds, which lowers the overall energy of the molecule. This increased stability makes the tautomerization energetically favorable in acidic conditions.

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Related Practice

Textbook Question

Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.

(f)

Textbook Question

Which of the following carbocations would you expect to rearrange?

Textbook Question

Which of the following carbocations would you expect to undergo rearrangement?

Textbook Question

Show a reaction coordinate diagram for the two processes in Figure 13.41 that rationalizes pathway B as the one that gives the major product.

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Textbook Question

The molecular orbital picture of H₂ can be represented by the following diagram. Label σ and σ* on the diagram—that is, which is (a) and which is (b)? Which is lower in energy? Why?

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Textbook Question

Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.

(a)

Textbook Question

Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.

(b)

Textbook Question

Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.

(e)

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