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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3:01 minutes

Problem 69a

Textbook Question

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

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Analyze the structures provided: The left structure represents a cyclopropyl anion (negative charge), while the right structure represents a cyclopropyl cation (positive charge). Cyclopropane is a strained ring system due to its small bond angles, which deviate significantly from the ideal tetrahedral angle of 109.5°.

Consider the electronic effects: The cyclopropyl ring has significant angle strain and torsional strain. The negative charge in the cyclopropyl anion is localized on the carbon atom, which is destabilized due to the strain and lack of resonance stabilization. On the other hand, the cyclopropyl cation benefits from hyperconjugation and inductive effects, which can help delocalize the positive charge to some extent.

Evaluate resonance and stabilization: The cyclopropyl cation is stabilized by the overlap of the σ-bonds in the ring with the empty p-orbital of the positively charged carbon atom. This interaction is known as σ-conjugation, which provides additional stability to the cation. The cyclopropyl anion lacks such stabilization mechanisms.

Compare the stability of the ions: The cyclopropyl cation is more stable than the cyclopropyl anion due to the presence of σ-conjugation and the ability to delocalize the positive charge, whereas the cyclopropyl anion suffers from localized negative charge and ring strain.

Conclude the comparison: Based on the above analysis, the cyclopropyl cation is the more stable ion in this pair.

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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 delocalization leads to a lower energy state, making aromatic systems more stable than non-aromatic or anti-aromatic compounds. The concept of aromaticity is governed by Hückel's rule, which states that a compound must have 4n + 2 π electrons to be considered aromatic.

Resonance

Resonance refers to the phenomenon where a molecule can be represented by two or more valid Lewis structures, known as resonance structures. These structures contribute to the overall stability of the molecule, as the actual structure is a hybrid of these forms. In the context of the question, resonance can help explain the stability of the ions by showing how charge distribution can be stabilized through delocalization.

Charge Distribution

Charge distribution is crucial in determining the stability of ions. Ions with charges that can be delocalized over multiple atoms tend to be more stable than those with localized charges. In the given structures, analyzing how the positive charge is distributed can provide insights into which ion is more stable, as a more favorable charge distribution often leads to lower energy and increased stability.

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Related Practice

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

Which of the following are aromatic?

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.

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