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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1:34 minutes

Problem 16c

Textbook Question

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

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Identify the type of strain energy present in the molecule. Strain energy can arise from torsional strain (eclipsing interactions), steric strain (atoms or groups being too close to each other), or angle strain (bond angles deviating from their ideal values).

Analyze the conformations in the given pair. For example, if the molecule is a cyclohexane derivative, determine whether the conformation is in a chair, boat, or twist-boat form, as these have different strain energies.

If the molecule is a cyclohexane derivative, evaluate the placement of substituents in axial and equatorial positions. Substituents in the equatorial position generally reduce steric strain compared to axial positions.

Compare the torsional strain in the conformations. For example, staggered conformations (e.g., anti or gauche) typically have lower torsional strain than eclipsed conformations.

Select the conformation with the least strain energy by considering all the factors (torsional, steric, and angle strain) and determining which conformation minimizes these interactions.

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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 atoms in a molecule that can be interconverted by rotation around single bonds. This analysis helps in understanding the stability of various conformations, as certain arrangements can lead to steric hindrance or torsional strain, affecting the overall energy of the molecule.

Strain Energy

Strain energy refers to the energy stored in a molecule due to the distortion of its normal bond angles and lengths. In organic molecules, factors such as steric strain (from repulsion between atoms) and torsional strain (from eclipsing interactions) contribute to the overall strain energy, influencing the stability and reactivity of different conformations.

Staggered vs. Eclipsed Conformations

In conformational analysis, staggered and eclipsed conformations are two key arrangements of atoms around a single bond. Staggered conformations, where atoms are positioned at maximum distance from each other, generally have lower energy and less strain compared to eclipsed conformations, where atoms are aligned and experience increased steric and torsional strain.

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Related Practice

Textbook Question

Calculate the dihedral angle (θ) for the conformations shown.

(c)

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

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).

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)

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