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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Problem 61e

Textbook Question

(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic.
(ii) For aromatic molecules, solve for n in Hückel’s rule. For all other molecules, explain which rule of aromaticity is being broken.
(e)

Verified step by step guidance

Identify the molecule: The structure shown is a cyclopentadienyl cation, which is a five-membered ring with two double bonds and a positive charge.

Determine the number of π electrons: Count the π electrons in the molecule. Each double bond contributes two π electrons, so there are a total of 4 π electrons in the cyclopentadienyl cation.

Apply Hückel's rule: Hückel's rule states that a molecule is aromatic if it has (4n + 2) π electrons, where n is a non-negative integer. Check if 4 π electrons fit this rule.

Evaluate aromaticity: Since 4 π electrons do not satisfy the (4n + 2) rule, the molecule cannot be classified as aromatic. Instead, it fits the 4n rule, which is characteristic of antiaromatic compounds.

Conclude the classification: The cyclopentadienyl cation is antiaromatic because it has 4 π electrons, which fits the 4n rule, leading to instability due to the lack of aromatic stabilization.

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Key Concepts

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

Aromaticity

Aromaticity refers to a property of cyclic compounds that exhibit enhanced stability due to the delocalization of π electrons across the ring structure. For a molecule to be classified as aromatic, it must be cyclic, planar, and follow Hückel's rule, which states that it must have a specific number of π electrons (4n + 2, where n is a non-negative integer).

Hückel's Rule

Hückel's rule is a criterion used to determine the aromaticity of a compound. It states that a planar, cyclic molecule is aromatic if it contains 4n + 2 π electrons in its conjugated system. This rule helps in identifying aromatic compounds and is essential for solving problems related to their stability and reactivity.

Anti-aromaticity

Anti-aromaticity is a property of cyclic compounds that have 4n π electrons, leading to increased instability compared to non-aromatic compounds. Such molecules do not fulfill Hückel's rule and often exhibit significant strain or reactivity due to their electron configuration. Understanding anti-aromaticity is crucial for classifying molecules correctly in organic chemistry.

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Related Practice

Textbook Question

Some of the following compounds show aromatic properties, and others do not.

1. Predict which ones are likely to be aromatic, and explain why they are aromatic.

(n)

(o)

Textbook Question

(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic.

(ii) For the aromatic and antiaromatic molecules, solve for n in Hückel’s/Breslow’s rule. For the other molecules, explain which of the rules of aromaticity is being broken.

(b)

Textbook Question

(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic.

(ii) For the aromatic and antiaromatic molecules, solve for n in Hückel’s/Breslow’s rule. For the other molecules, explain which of the rules of aromaticity is being broken.

(d)

Textbook Question

(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic.

(ii) For aromatic molecules, solve for n in Hückel’s rule. For all other molecules, explain which rule of aromaticity is being broken.

(c)

Textbook Question

Which of the following are aromatic?

Textbook Question

Which of the following are aromatic?

Textbook Question

Which ion in each of the following pairs is more stable?

Textbook Question