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

1. A Review of General Chemistry

Molecular Orbitals

Organic Chemistry

1. A Review of General Chemistry

Molecular Orbitals

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5:11 minutes

Problem 79

Textbook Question

There is free rotation around the C―C bond in ethane. There is an extremely high barrier to rotation around the C=C bond in in ethene. Explain.

Verified step by step guidance

In ethane (C₂H₆), the carbon-carbon bond is a single bond (sigma bond, denoted as σ). A sigma bond is formed by the head-on overlap of atomic orbitals, and it allows for free rotation around the bond axis because the electron density is symmetrically distributed along the bond axis.

In ethene (C₂H₄), the carbon-carbon bond is a double bond, consisting of one sigma bond (σ) and one pi bond (π). The sigma bond is formed by the head-on overlap of orbitals, while the pi bond is formed by the side-by-side overlap of p orbitals above and below the plane of the atoms.

The pi bond in ethene restricts rotation because the side-by-side overlap of the p orbitals would be disrupted if the carbon atoms were to rotate relative to each other. This disruption would break the pi bond, which requires significant energy to overcome.

The energy barrier to rotation around the C=C bond in ethene is extremely high due to the strength of the pi bond. In contrast, the single sigma bond in ethane does not have this restriction, allowing for free rotation.

In summary, the free rotation in ethane is due to the single sigma bond, while the restricted rotation in ethene is due to the presence of the pi bond in the double bond, which prevents rotation without breaking the bond.

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Key Concepts

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

C―C Bond Rotation

In ethane, the C―C bond is a single bond, allowing for free rotation around the bond axis. This rotation occurs because single bonds have sigma (σ) bonds formed by the head-on overlap of orbitals, which do not have any significant energy barriers to rotation. As a result, the spatial arrangement of the atoms can change freely without breaking the bond.

C=C Bond Characteristics

In contrast, the C=C bond in ethene is a double bond, consisting of one sigma (σ) bond and one pi (π) bond. The pi bond is formed by the side-to-side overlap of p orbitals, which restricts rotation because breaking the pi bond requires significant energy. This results in a high barrier to rotation around the C=C bond, leading to fixed geometries in the molecule.

Energy Barriers in Bond Rotation

The energy barrier to rotation around a bond is influenced by the type of bond and the presence of substituents. In ethene, the pi bond creates a substantial energy barrier that must be overcome to rotate the molecule, while in ethane, the absence of such a barrier allows for unrestricted rotation. Understanding these energy dynamics is crucial for predicting molecular behavior and reactivity.

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a. <IMAGE>

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