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

1. A Review of General Chemistry

Molecular Orbitals

Organic Chemistry

1. A Review of General Chemistry

Molecular Orbitals

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Problem 75

Textbook Question

In this, and previous, chapters, we have seen 1,2-alkyl and 1,2-hydride shifts. If both are possible, as in the carbocation shown, which would you expect to occur? Explain your answer.

Verified step by step guidance

Step 1: Analyze the carbocation structure shown in the image. The carbocation is located on a secondary carbon, and there are two possible shifts: a 1,2-hydride shift (path a) and a 1,2-alkyl shift (path b).

Step 2: Understand the purpose of these shifts. Both shifts aim to stabilize the carbocation by moving the positive charge to a more stable position. Stability increases as we move from secondary to tertiary carbocations.

Step 3: Evaluate the 1,2-hydride shift (path a). This shift involves the migration of a hydrogen atom with its bonding electrons to the carbocation center. If this shift results in a tertiary carbocation, it is likely to occur because tertiary carbocations are more stable than secondary ones.

Step 4: Evaluate the 1,2-alkyl shift (path b). This shift involves the migration of an alkyl group with its bonding electrons to the carbocation center. If this shift also results in a tertiary carbocation, it could compete with the hydride shift. However, hydride shifts are generally faster and more favorable due to lower steric hindrance.

Step 5: Conclude that the 1,2-hydride shift (path a) is expected to occur because it is typically faster and leads to a more stable tertiary carbocation. The 1,2-alkyl shift (path b) is less likely unless it provides an even greater stabilization or is sterically favored.

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Key Concepts

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

Carbocation Stability

Carbocations are positively charged species that are highly reactive due to the electron deficiency at the positively charged carbon atom. Their stability is influenced by the degree of substitution; tertiary carbocations are more stable than secondary or primary ones due to hyperconjugation and inductive effects from adjacent alkyl groups. Understanding the stability of carbocations is crucial for predicting the preferred rearrangement pathway.

1,2-Alkyl and 1,2-Hydride Shifts

1,2-alkyl shifts involve the migration of an alkyl group from one carbon to an adjacent carbocation, while 1,2-hydride shifts involve the migration of a hydrogen atom. These shifts occur to stabilize the carbocation by forming a more stable carbocation. The preference for one shift over the other depends on the resulting carbocation's stability and the steric and electronic factors involved.

Rearrangement Mechanism

Rearrangement mechanisms in organic chemistry describe how carbocations can undergo structural changes to achieve greater stability. This process often involves the migration of groups or atoms, such as hydrides or alkyl groups, to form a more stable carbocation. The mechanism is influenced by factors such as sterics, electronic effects, and the overall energy of the transition state, which ultimately determines the favored pathway.

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