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 16d

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)

Verified step by step guidance

Identify the molecule structure: The image shows 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 4 π electrons in total.

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 the molecule fits this rule.

Evaluate antiaromaticity: If the molecule has 4n π electrons, it is considered antiaromatic. Check if the molecule fits this condition.

Classify the molecule: Based on the number of π electrons and the application of Hückel's rule, classify the molecule as aromatic, nonaromatic, or antiaromatic. In this case, the cyclopentadienyl cation with 4 π electrons is antiaromatic.

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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 the special stability and unique properties of cyclic compounds that follow Hückel's rule, which states that a molecule is aromatic if it has (4n + 2) π electrons, where n is a non-negative integer. Aromatic compounds are typically planar, fully conjugated, and exhibit resonance, leading to lower energy and increased stability compared to non-aromatic compounds.

Anti-aromaticity

Anti-aromaticity describes compounds that are cyclic, planar, and fully conjugated but contain 4n π electrons, which leads to instability. These compounds are less stable than their non-aromatic counterparts due to the presence of destabilizing interactions among the π electrons, making them highly reactive and less favorable in chemical reactions.

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. Conversely, if it contains 4n π electrons, it is classified as anti-aromatic. This rule is essential for classifying the stability and reactivity of cyclic compounds in organic chemistry.

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Related Practice

Textbook Question

Biphenyl has the following structure.

c. The heat of hydrogenation for biphenyl is about 418 kJ/mol (100 kcal/mol). Calculate the resonance energy of biphenyl.

d. Compare the resonance energy of biphenyl with that of naphthalene and with that of two benzene rings. Explain the difference in the resonance energies of naphthalene and biphenyl.

Textbook Question

How would you convert the following compounds to aromatic compounds?

(a)

(b)

(c)

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

(c)

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)

Textbook Question

Which of the following are aromatic?

Textbook Question