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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3:07 minutes

Problem 69c

Textbook Question

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

Verified step by step guidance

Step 1: Analyze the chemical structures provided. The left structure represents a positively charged ion (carbocation), while the right structure represents a negatively charged ion (carbanion). Both ions are based on a cyclopentadienyl ring system.

Step 2: Consider the concept of aromaticity. The cyclopentadienyl ring can achieve aromatic stability if it follows Huckel's rule, which states that a molecule is aromatic if it has (4n + 2) π-electrons, where n is an integer.

Step 3: Evaluate the π-electron count for each ion. The carbocation (left structure) has 4 π-electrons because the positive charge removes one electron from the conjugated system. The carbanion (right structure) has 6 π-electrons because the negative charge adds one electron to the conjugated system.

Step 4: Determine aromatic stability. The carbanion with 6 π-electrons satisfies Huckel's rule (4n + 2, where n = 1), making it aromatic and highly stable. The carbocation with 4 π-electrons does not satisfy Huckel's rule and is not aromatic, making it less stable.

Step 5: Conclude that the carbanion (right structure) is more stable than the carbocation (left structure) due to its aromaticity and adherence to Huckel's rule.

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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 the delocalization of π electrons across the ring. This stability arises from fulfilling Huckel's rule, which states that a compound must have 4n + 2 π electrons (where n is a non-negative integer) to be aromatic. The stability of aromatic systems is crucial when comparing the stability of ions derived from them.

Charge Distribution

The stability of charged species, such as cations and anions, is influenced by how well the charge is distributed across the molecule. In the context of aromatic compounds, a positive charge (cation) can be stabilized by resonance, while a negative charge (anion) can also benefit from resonance but may be less stable if it disrupts the aromaticity. Understanding charge distribution helps in predicting which ion is more stable.

Resonance Structures

Resonance structures are different ways of drawing a molecule that illustrate the delocalization of electrons. For ions in aromatic systems, resonance can significantly affect stability; cations may have resonance forms that distribute the positive charge, enhancing stability, while anions may have resonance forms that can either stabilize or destabilize the negative charge depending on the overall structure. Analyzing resonance is key to determining ion stability.

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Related Practice

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.

Textbook Question

How would you convert the following compounds to aromatic compounds?

(a)

(b)

(c)

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