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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5:24 minutes

Problem 48a

Textbook Question

Which of the following are aromatic?
a.
b.
c.

Verified step by step guidance

Step 1: Recall the criteria for aromaticity. A molecule is aromatic if it satisfies the following conditions: (1) It is cyclic, (2) It is planar, (3) It has a conjugated π-electron system, and (4) It follows Huckel's rule, which states that the molecule must have (4n + 2) π-electrons, where n is a non-negative integer.

Step 2: Analyze structure A. Structure A is a five-membered ring with two double bonds and a positive charge. Count the π-electrons: each double bond contributes 2 π-electrons, so there are 4 π-electrons. The positive charge does not contribute any π-electrons. Check if it satisfies Huckel's rule (4n + 2 = 6 π-electrons). Since it does not, structure A is not aromatic.

Step 3: Analyze structure B. Structure B is a six-membered ring with three double bonds. Count the π-electrons: each double bond contributes 2 π-electrons, so there are 6 π-electrons. Check if it satisfies Huckel's rule (4n + 2 = 6 π-electrons). Since it does, structure B is aromatic.

Step 4: Analyze structure C. Structure C is a five-membered ring with two double bonds and a negative charge. Count the π-electrons: each double bond contributes 2 π-electrons, and the negative charge contributes 2 π-electrons, so there are 6 π-electrons. Check if it satisfies Huckel's rule (4n + 2 = 6 π-electrons). Since it does, structure C is aromatic.

Step 5: Summarize the findings. Structure B and structure C are aromatic, while structure A is not 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 compounds that exhibit enhanced stability due to resonance. For a compound to be considered aromatic, it must be cyclic, planar, and follow Hückel's rule, which states that it must have a total of 4n + 2 π electrons, where n is a non-negative integer. This unique electron configuration allows for delocalization, contributing to the compound's stability.

Hückel's Rule

Hückel's Rule is a criterion used to determine the aromaticity of a compound. It states that a planar, cyclic molecule is aromatic if it contains 4n + 2 π electrons in its conjugated system, where n is an integer (0, 1, 2, ...). This rule helps in identifying which compounds can exhibit aromatic characteristics based on their electron count.

Planarity and Conjugation

For a compound to be aromatic, it must be both planar and have a continuous overlap of p-orbitals, allowing for conjugation. Planarity ensures that the π electrons can delocalize across the entire ring structure, while conjugation refers to the alternating single and double bonds that facilitate this electron delocalization. Without these features, a compound cannot achieve the stability associated with aromaticity.

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Related Practice

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?

Textbook Question

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

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)

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