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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Problem 24

Textbook Question

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

Verified step by step guidance

Step 1: Understand the concept of dihedral angle. A dihedral angle is the angle between two planes formed by four atoms or groups in a molecule. It is measured by looking at the spatial relationship between two substituents on adjacent carbons in a molecule.

Step 2: Analyze the image provided. The molecule shown is a staggered conformation of a substituted ethane derivative. The dihedral angle in question is between the bromine atom (Br) and the methyl group (CH₃) as viewed along the C-C bond axis.

Step 3: Recognize the geometry of the molecule. The molecule adopts a tetrahedral geometry around each carbon atom due to sp³ hybridization. The bond angles in a tetrahedral geometry are approximately 109.5°, not 120°.

Step 4: Clarify why 120° is incorrect. A dihedral angle of 120° would imply a planar geometry, which is characteristic of sp² hybridization (e.g., trigonal planar structures). However, the molecule in the image is clearly staggered and tetrahedral, not planar.

Step 5: Explain why 109° is also incorrect. While the bond angles in a tetrahedral geometry are close to 109.5°, the dihedral angle specifically refers to the spatial relationship between substituents on adjacent carbons. In the staggered conformation shown, the dihedral angle between Br and CH₃ is 180°, not 109°.

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Key Concepts

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

Dihedral Angle

The dihedral angle is the angle between two intersecting planes, specifically the angle formed by two bonds in a molecule. In organic chemistry, it is crucial for understanding the spatial arrangement of atoms and the conformational stability of molecules. The dihedral angle can influence steric interactions and the overall energy of a molecule.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can accommodate bonding in molecules. For example, sp3 hybridization leads to tetrahedral geometry with bond angles of approximately 109.5°, while sp2 hybridization results in trigonal planar geometry with bond angles of about 120°. Understanding hybridization helps explain the geometry and angles in molecular structures.

Steric Strain

Steric strain occurs when atoms in a molecule are forced closer together than their preferred distance, leading to increased energy and instability. This strain can affect the dihedral angles between bonds, as certain conformations may be energetically unfavorable. Recognizing steric strain is essential for predicting the most stable conformations of a molecule.

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Related Practice

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

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

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)

Textbook Question

Construct a graph, similar to Figure 3-11, of the torsional energy of 3-methylpentane along the C2―C3 bond.

a. Place C2 in front, represented by three bonds coming together in a Y shape, and C3 in back, represented by a circle with three bonds pointing out from it.

b. Define the dihedral angle as the angle between the methyl group on the front carbon and the ethyl group on the back carbon.

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