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

18. Aromaticity

Annulene

Organic Chemistry

18. Aromaticity

Annulene

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3:58 minutes

Problem 64

Textbook Question

Despite having only sp²-hybridized carbons and having 10 electrons (4n + 2) , the annulene shown is not aromatic. Why?

Verified step by step guidance

Identify the structure shown in the image as a 10-membered annulene, which is a cyclic hydrocarbon with alternating single and double bonds.

Recall the criteria for aromaticity: a molecule must be cyclic, planar, fully conjugated, and follow Huckel's rule (4n + 2) π electrons, where n is a non-negative integer.

Verify that the molecule has 10 π electrons, which satisfies Huckel's rule for n=2 (4n + 2 = 10).

Assess the planarity of the molecule. For a molecule to be aromatic, it must be planar to allow for the delocalization of π electrons across the ring.

Consider the size and strain of the 10-membered ring. Larger rings like this one often have difficulty maintaining planarity due to steric strain and flexibility, which can prevent effective π electron delocalization, thus making the molecule non-aromatic.

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Key Concepts

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

Aromaticity

Aromaticity is a property of cyclic compounds that exhibit enhanced stability due to delocalized π electrons. For a compound to be aromatic, it must be cyclic, planar, and follow Hückel's rule, which states that it should have 4n + 2 π electrons, where n is a non-negative integer. This delocalization allows for resonance, contributing to the compound's stability.

Hückel's Rule

Hückel's rule is a criterion used to determine the aromaticity of a compound based on its π electron count. According to this rule, a planar, cyclic molecule is aromatic if it contains 4n + 2 π electrons. This means that while a compound may have the correct number of π electrons, other structural factors must also be considered to confirm its aromatic nature.

Planarity and Hybridization

Planarity refers to the arrangement of atoms in a molecule such that all atoms lie in the same geometric plane, which is crucial for effective overlap of p orbitals in aromatic compounds. In the case of sp²-hybridized carbons, each carbon atom contributes one unhybridized p orbital for π bonding. If the molecule is not fully planar, even with the correct number of π electrons, it may not exhibit aromaticity.

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