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

12. Alcohols, Ethers, Epoxides and Thiols

Leaving Group Conversions Summary

Organic Chemistry

12. Alcohols, Ethers, Epoxides and Thiols

Leaving Group Conversions Summary

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14:52 minutes

Problem 45

Textbook Question

Both cis- and trans-2-methylcyclohexanol undergo dehydration in warm sulfuric acid to give 1-methylcyclohexene as the major alkene product. These alcohols can also be converted to alkenes by tosylation using TsCl and pyridine, followed by elimination using KOC(CH₃)₃ as a strong base. Under these basic conditions, the tosylate of cis-2-methylcyclohexanol eliminates to give mostly 1-methylcyclohexene, but the tosylate of trans-2-methylcyclohexanol eliminates to give only 3-methylcyclohexene. Explain how this stereochemical difference in reactants controls a regiochemical difference in the products of the basic elimination, but not in the acid-catalyzed elimination.

Verified step by step guidance

Step 1: Understand the difference between acid-catalyzed and base-promoted elimination reactions. Acid-catalyzed elimination (E1 mechanism) proceeds through a carbocation intermediate, which allows for rearrangements and does not depend on the stereochemistry of the starting material. In contrast, base-promoted elimination (E2 mechanism) is a concerted process that requires specific stereochemical alignment of the leaving group and β-hydrogen (anti-periplanar geometry).

Step 2: Analyze the stereochemistry of cis- and trans-2-methylcyclohexanol. In the chair conformation of cis-2-methylcyclohexanol, the hydroxyl group and the β-hydrogen on the adjacent carbon can adopt an anti-periplanar arrangement, which is favorable for E2 elimination. For trans-2-methylcyclohexanol, the hydroxyl group and the β-hydrogen on the adjacent carbon cannot adopt this anti-periplanar geometry in the same chair conformation, leading to a different elimination pathway.

Step 3: Consider the tosylation step. Tosylation replaces the hydroxyl group with a tosyl group (Ts), which is a better leaving group. This step does not alter the stereochemistry of the molecule, so the stereochemical differences between cis- and trans-2-methylcyclohexanol are preserved in their respective tosylates.

Step 4: Examine the E2 elimination of the tosylates under basic conditions. For the tosylate of cis-2-methylcyclohexanol, the anti-periplanar geometry allows elimination to occur at the β-hydrogen that leads to the formation of 1-methylcyclohexene as the major product. For the tosylate of trans-2-methylcyclohexanol, the anti-periplanar geometry aligns with a different β-hydrogen, leading to the formation of 3-methylcyclohexene as the product.

Step 5: Explain why acid-catalyzed elimination does not show this regiochemical difference. In the acid-catalyzed E1 mechanism, the formation of a planar carbocation intermediate allows for free rotation and rearrangement, which eliminates the stereochemical constraints seen in the E2 mechanism. As a result, both cis- and trans-2-methylcyclohexanol give the same major product, 1-methylcyclohexene, under acidic conditions.

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Key Concepts

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

Stereochemistry

Stereochemistry refers to the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the case of cis- and trans-2-methylcyclohexanol, the different spatial arrangements of the methyl group relative to the hydroxyl group influence the pathways and products of elimination reactions. Understanding stereochemistry is crucial for predicting the outcomes of reactions based on the orientation of substituents.

Regioselectivity

Regioselectivity is the preference of a chemical reaction to yield one structural isomer over others when multiple products are possible. In the elimination reactions of the tosylates derived from cis- and trans-2-methylcyclohexanol, the orientation of the leaving group and the base's approach determines which alkene is formed. This concept is essential for understanding why different stereoisomers lead to different major products in elimination reactions.

Acid-Catalyzed vs. Base-Catalyzed Elimination

Acid-catalyzed elimination typically involves the formation of a carbocation intermediate, which can lead to more stable products regardless of the stereochemistry of the starting alcohol. In contrast, base-catalyzed elimination proceeds via a concerted mechanism that is more sensitive to the stereochemistry of the reactants, resulting in different products based on the spatial arrangement of substituents. This distinction is key to understanding why the stereochemical differences in reactants affect the products in base-catalyzed reactions but not in acid-catalyzed ones.

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