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 16b

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)

Verified step by step guidance

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

Determine the number of π electrons: Each double bond contributes 2 π electrons, and the negative charge contributes 2 π electrons, totaling 6 π electrons.

Apply Hückel's rule: For a molecule to be aromatic, it must have (4n + 2) π electrons, where n is a non-negative integer. Set up the equation 4n + 2 = 6 and solve for n.

Solve for n: Subtract 2 from both sides to get 4n = 4, then divide by 4 to find n = 1.

Conclude the classification: Since the molecule has 6 π electrons and satisfies Hückel's rule with n = 1, it is classified as aromatic.

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Key Concepts

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

Aromaticity

Aromaticity is a property of cyclic, planar structures with a ring of resonance bonds that leads to enhanced stability. For a molecule to be aromatic, it must follow Hückel's rule, which states that it should have (4n + 2) π electrons, where n is a non-negative integer. This rule helps determine the electron count that allows for delocalization and stability.

Hückel's Rule

Hückel's rule is a criterion used to determine if a planar ring molecule is aromatic. According to this rule, a molecule is aromatic if it contains a total of (4n + 2) π electrons, where n is a non-negative integer. This electron count allows for the delocalization of electrons, contributing to the molecule's stability and unique chemical properties.

Antiaromaticity

Antiaromaticity occurs in cyclic, planar molecules that have (4n) π electrons, leading to instability due to electron delocalization that does not favor resonance. Unlike aromatic compounds, antiaromatic compounds are less stable than their non-cyclic or non-planar counterparts. Identifying antiaromaticity involves checking the electron count and ensuring the molecule is planar and cyclic.

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Related Practice

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)

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.

(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

(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?

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