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

Aromatic Hydrocarbons

Organic Chemistry

18. Aromaticity

Aromatic Hydrocarbons

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8:40 minutes

Problem 8

Textbook Question

One of the following compounds is much more stable than the other two. Classify each as aromatic, antiaromatic, or nonaromatic.

Verified step by step guidance

Step 1: Recall the criteria for aromaticity. A compound is aromatic if it satisfies the following conditions: (1) It is cyclic, (2) It is planar, (3) It has a conjugated π-electron system, and (4) It follows Hückel's rule, which states that the molecule must have (4n + 2) π-electrons, where n is a non-negative integer.

Step 2: Analyze heptalene. Heptalene is cyclic and conjugated, but it does not satisfy Hückel's rule because it has 8 π-electrons, which is not of the form (4n + 2). Instead, it falls under the category of antiaromatic compounds because it has (4n) π-electrons and is planar.

Step 3: Analyze azulene. Azulene is cyclic, planar, and conjugated. It has 10 π-electrons, which satisfies Hückel's rule (4n + 2, where n = 2). Therefore, azulene is aromatic and much more stable due to aromatic stabilization.

Step 4: Analyze pentalene. Pentalene is cyclic and conjugated, but it has 8 π-electrons, which does not satisfy Hückel's rule. Additionally, it is antiaromatic because it has (4n) π-electrons and is planar, leading to instability.

Step 5: Conclude the classification. Heptalene and pentalene are antiaromatic and unstable, while azulene is aromatic and much more stable due to aromatic stabilization.

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Key Concepts

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

Aromaticity

Aromatic compounds are cyclic, planar molecules with a ring of resonance that follow Hückel's rule, which states they must have 4n + 2 π electrons (where n is a non-negative integer). This unique electron configuration leads to increased stability due to delocalization of electrons across the ring, making aromatic compounds particularly stable compared to nonaromatic and antiaromatic compounds.

Antiaromaticity

Antiaromatic compounds are also cyclic and planar but contain 4n π electrons, which leads to instability due to the unfavorable electron delocalization. This instability arises because the electron configuration does not satisfy Hückel's rule, resulting in increased energy and reactivity compared to both aromatic and nonaromatic compounds.

Nonaromaticity

Nonaromatic compounds do not meet the criteria for aromaticity or antiaromaticity. They may be acyclic, lack planarity, or have an insufficient number of π electrons. As a result, nonaromatic compounds typically exhibit stability levels that fall between aromatic and antiaromatic compounds, depending on their specific structural features.

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