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

Huckel's Rule

Organic Chemistry

18. Aromaticity

Huckel's Rule

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3:26 minutes

Problem 61h

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.
(h)

Verified step by step guidance

Identify the structure: The molecule shown is a cyclic compound with alternating single and double bonds, forming a ring structure.

Count the number of π-electrons: In this structure, each double bond contributes two π-electrons. Count the total number of π-electrons in the ring.

Apply Hückel's rule: Aromatic compounds must satisfy Hückel's rule, which states that a molecule is aromatic if it has (4n + 2) π-electrons, where n is a non-negative integer. Check if the number of π-electrons fits this formula.

Check planarity and cyclic nature: For a molecule to be aromatic, it must be cyclic, planar, and fully conjugated. Ensure that the molecule meets these criteria.

Determine aromaticity: Based on the π-electron count and structural criteria, classify the molecule as aromatic, nonaromatic, or antiaromatic. If aromatic, solve for n in Hückel's rule. If not, explain which rule of aromaticity is being broken.

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Key Concepts

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

Aromaticity

Aromaticity refers to a property of cyclic compounds that exhibit enhanced stability due to the delocalization of π electrons across the ring structure. For a molecule to be classified as aromatic, it must be cyclic, planar, fully conjugated, and follow Hückel's rule, which states that the number of π electrons must equal 4n + 2, where n is a non-negative integer.

Hückel's Rule

Hückel's rule is a criterion used to determine the aromaticity of a cyclic compound. It states that a planar, cyclic molecule with a continuous overlap of p-orbitals is aromatic if it contains 4n + 2 π electrons, where n is an integer (0, 1, 2, ...). This rule helps in identifying stable aromatic compounds and differentiating them from nonaromatic and antiaromatic compounds.

Anti-aromaticity

Anti-aromaticity is a property of cyclic compounds that leads to instability due to the presence of 4n π electrons, which results in a destabilizing effect. Unlike aromatic compounds, anti-aromatic compounds do not satisfy Hückel's rule and are typically less stable, often leading to higher reactivity. Understanding the distinction between aromatic, nonaromatic, and anti-aromatic compounds is crucial for predicting their chemical behavior.

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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.

(b)

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.

(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.

(f)

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.

(g)

Textbook Question

The ribonucleosides that make up ribonucleic acid (RNA) are composed of D-ribose (a sugar) and four heterocyclic “bases.” The general structure of a ribonucleoside is shown here.

The four heterocyclic bases are cytosine, uracil, guanine, and adenine. Cytosine and uracil are called pyrimidine bases because their structures resemble pyrimidine. Guanine and adenine are called purine bases because their structures resemble purine.

a. Determine which rings of these bases are aromatic.

Textbook Question

Classify the following conjugated systems as having 4n or 4n + 2 π electrons.

(c)

Textbook Question

Classify the following conjugated systems as having 4n or 4n + 2 π electrons.

(f)

Textbook Question

Which compound in each set is aromatic? Explain your choice.

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