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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3:02 minutes

Problem 69b

Textbook Question

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

Verified step by step guidance

Step 1: Analyze the structures of the two ions. The first ion is a cycloheptatrienyl cation (left structure), and the second ion is a cycloheptatrienyl anion (right structure). Both structures have conjugated π-systems.

Step 2: Determine the aromaticity of each ion. Aromatic compounds follow Huckel's rule, which states that a molecule is aromatic if it has (4n + 2) π-electrons, where n is an integer.

Step 3: For the cycloheptatrienyl cation, count the π-electrons. The cation has 6 π-electrons (from the three double bonds), which satisfies Huckel's rule (4n + 2, where n = 1). This makes the cation aromatic and highly stable.

Step 4: For the cycloheptatrienyl anion, count the π-electrons. The anion has 8 π-electrons (from the three double bonds and the lone pair on the negatively charged carbon). This does not satisfy Huckel's rule, making the anion non-aromatic and less stable.

Step 5: Conclude that the cycloheptatrienyl cation is more stable than the cycloheptatrienyl anion due to its aromaticity, which provides extra stabilization through delocalization of π-electrons.

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Key Concepts

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

Aromatic Stability

Aromatic compounds are particularly stable due to their cyclic structure and delocalized π electrons, which follow Hückel's rule (4n + 2 π electrons). This stability is a key factor when comparing the stability of ions derived from aromatic systems, as the presence of a charge can disrupt this delocalization.

Charge Distribution

The stability of charged species (cations and anions) is influenced by how well the charge can be distributed across the molecule. In the case of the provided structures, the negative charge on the anion can be stabilized by resonance, while the positive charge on the cation may be less effectively stabilized, affecting overall stability.

Resonance Structures

Resonance structures are different ways of drawing a molecule that illustrate the delocalization of electrons. For the ions in the question, the ability of the negative ion to delocalize its charge through resonance can enhance its stability compared to the positive ion, which may have fewer resonance contributors to stabilize the positive charge.

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Related Practice

Textbook Question

Classify the following compounds as aromatic, antiaromatic, or nonaromatic.

(c)

(d)

Textbook Question

The following molecules and ions are grouped by similar structures. Classify each as aromatic, antiaromatic, or nonaromatic. For the aromatic and antiaromatic species, give the number of pi electrons in the ring.

(a)

Textbook Question

(c)

Textbook Question

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

Textbook Question

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

Textbook Question

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

Textbook Question

Biphenyl has the following structure.

a. Is biphenyl a (fused) polynuclear aromatic hydrocarbon?

b. How many pi electrons are there in the two aromatic rings of biphenyl? How does this number compare with that for naphthalene?

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.

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

Key Concepts

Aromatic Stability

Charge Distribution

Resonance Structures

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