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

4. Alkanes and Cycloalkanes

Equatorial Preference

Organic Chemistry

4. Alkanes and Cycloalkanes

Equatorial Preference

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Problem 43

Textbook Question

Draw the most stable chair conformation for the following trisubstituted cyclohexane.

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Analyze the given cyclohexane structure and identify the substituents attached to the ring. In this case, there are three substituents: two isopropyl groups and one methyl group.

Determine the stereochemistry of each substituent based on the wedge (solid) and dash (dotted) bonds. Wedge bonds indicate substituents pointing out of the plane (up), while dash bonds indicate substituents pointing into the plane (down).

Draw the basic chair conformation of cyclohexane, which alternates between axial and equatorial positions for substituents. Axial positions are perpendicular to the plane of the ring, while equatorial positions are roughly parallel to the plane.

Place the substituents in the chair conformation based on their stereochemistry. Substituents in the 'up' position should be placed in equatorial or axial positions depending on stability, and substituents in the 'down' position should follow the same rule. Larger groups (like isopropyl) prefer equatorial positions to minimize steric hindrance.

Verify the stability of the conformation by ensuring that the largest substituents occupy equatorial positions, as this reduces steric strain and makes the conformation more stable.

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Key Concepts

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

Chair Conformation

The chair conformation is the most stable three-dimensional arrangement of cyclohexane, minimizing steric strain and torsional strain. In this conformation, the carbon atoms are arranged in a staggered manner, allowing for optimal bond angles and distances. Understanding this conformation is crucial for analyzing the stability of substituted cyclohexanes.

Substituent Positioning

In cyclohexane derivatives, substituents can occupy axial or equatorial positions. Axial substituents are oriented perpendicular to the plane of the ring, while equatorial substituents extend outward, parallel to the ring. The equatorial position is generally more stable due to reduced steric hindrance, making it essential to determine the most favorable positioning for substituents in chair conformations.

Steric Hindrance

Steric hindrance refers to the repulsion between atoms or groups that are in close proximity, which can destabilize a molecular conformation. In the context of cyclohexane, bulky substituents prefer equatorial positions to minimize interactions with other axial substituents or hydrogen atoms. Recognizing the impact of steric hindrance is vital for predicting the stability of different conformations.

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Related Practice

Textbook Question

Draw the two chair conformers for each of the following and indicate which conformer is more stable:

d. cis-1,2-diethylcyclohexane

Textbook Question

Draw the more stable chair conformer of cis-1-ethyl-2-methylcyclohexane.

Textbook Question

Which compound is more stable: cis-1-ethyl-2-methylcyclohexane or trans-1-ethyl-2-methylcyclohexane?

Textbook Question

Which compound is more stable: cis-1-ethyl-2-methylcyclohexane or trans-1-ethyl-2-methylcyclohexane?

Textbook Question

In contrast to ethane and other alkanes studied in this chapter, there is no free rotation around any bonds in cyclopentane (shown below). Why?

Textbook Question

For each of the following structures, draw the most stable chair conformer.

Textbook Question

For each pair, indicate which conformer is more stable.

Textbook Question

When a pyranose is in the chair conformation in which the CH2OH group and the C-1 OH group are both in axial positions, the two groups can react to form an acetal. This is called the anhydro form of the sugar (it has 'lost water'). The anhydro form of d-idose is shown here. Explain why about 80% of d-idose exists in the anhydro form in an aqueous solution at 100 °C, but only about 0.1% of d-glucose exists in the anhydro form under the same conditions.

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