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

Newman Projections

Organic Chemistry

4. Alkanes and Cycloalkanes

Newman Projections

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4:50 minutes

Problem 40e

Textbook Question

Construct a graph, similar to Figure 3-11, of the torsional energy of 3-methylpentane along the C2―C3 bond.
e. Indicate which conformations are the most stable (lowest energy) and the least stable (highest energy).

Verified step by step guidance

Understand the concept of conformations: Conformations are different spatial arrangements of a molecule that result from rotation around single bonds. The stability of a conformation depends on factors such as steric hindrance, torsional strain, and angle strain.

Identify the molecule in question: To determine the most and least stable conformations, you need to know the specific molecule and its substituents. For example, in alkanes like butane, the conformations include anti, gauche, eclipsed, and fully eclipsed.

Analyze steric and torsional strain: The most stable conformation typically minimizes steric hindrance (repulsion between bulky groups) and torsional strain (repulsion between electron clouds in bonds). For example, in butane, the anti conformation (where bulky groups are 180° apart) is the most stable, while the fully eclipsed conformation (where bulky groups are directly aligned) is the least stable.

Use Newman projections to visualize conformations: Draw Newman projections to represent the molecule as viewed along the axis of a single bond. This helps in identifying the relative positions of substituents and assessing their interactions.

Rank the conformations: Based on the analysis of steric and torsional strain, rank the conformations from most stable (lowest energy) to least stable (highest energy). For example, in butane, the order is anti (most stable), gauche, eclipsed, and fully eclipsed (least stable).

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Key Concepts

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

Conformational Analysis

Conformational analysis involves studying the different spatial arrangements of a molecule that can be interconverted by rotation around single bonds. These arrangements, or conformers, can have varying degrees of stability based on steric interactions and torsional strain. Understanding this concept is crucial for determining which conformations of a molecule are more stable or less stable.

Stability Factors

The stability of conformations is influenced by several factors, including steric hindrance, torsional strain, and angle strain. Steric hindrance occurs when atoms are forced too close together, leading to repulsion, while torsional strain arises from eclipsed interactions between atoms. Recognizing how these factors affect energy levels helps in identifying the most and least stable conformations.

Energy Diagrams

Energy diagrams visually represent the relative energies of different conformations of a molecule. These diagrams typically show energy on the vertical axis and the dihedral angle or rotation on the horizontal axis. By analyzing these diagrams, one can easily identify which conformations correspond to local minima (most stable) and maxima (least stable) in energy.

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Related Practice

Textbook Question

For each of the pairs in Assessment 3.29, which conformation would you expect to be more prominent at equilibrium?

(a)

(b)

(c)

Textbook Question

Choose the conformation in each pair that is most stable. If both are equally stable, then write 'no difference.'

(a)

Textbook Question

In Chapter 5, we introduce reaction coordinate diagrams as a plot of potential energy versus the progress of a reaction. Consider the reaction coordinate diagram drawn for the 'reaction' of conformation A becoming conformation B. Which structure is present at the top of the hill?

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Textbook Question

Which conformation in each of the following pairs has the least strain energy?

(c)

Textbook Question

Why is it incorrect to say that the dihedral angle shown is 120° or even 109°?

Textbook Question

For each pair of conformations shown, choose which is most stable. [If both conformations have the same number of gauche interactions, choose the one where the interactions are between smaller groups.]

(e)

Textbook Question

(f)

Textbook Question

Which of the following structures represent the same compound? Which ones represent different compounds?

(f)

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