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 Heterocycles

Organic Chemistry

18. Aromaticity

Aromatic Heterocycles

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3:19 minutes

Problem 41a,b,c(iii)

Textbook Question

For each heterocyclic compound,
(iii) are any of the rings aromatic? Explain.
(a)
(b)
(c)

Verified step by step guidance

Step 1: Recall the criteria for aromaticity. A compound is aromatic if it satisfies the following conditions: (a) It is cyclic, (b) It is planar, (c) It has a conjugated π-electron system, and (d) 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 compound (a). The structure contains a fused ring system with one benzene ring and a five-membered ring containing two oxygen atoms. The benzene ring is aromatic because it is cyclic, planar, conjugated, and has 6 π-electrons (4n + 2, where n = 1). The five-membered ring is not aromatic because it does not have a conjugated π-electron system.

Step 3: Analyze compound (b). The structure is a six-membered ring containing sulfur and an oxygen atom. The ring is conjugated and planar, and it has 6 π-electrons (4n + 2, where n = 1). Therefore, this ring is aromatic.

Step 4: Analyze compound (c). The structure is a five-membered ring containing nitrogen and an oxygen atom. The ring is not conjugated because the lone pair on nitrogen is not part of the π-system, and it does not satisfy Huckel's rule. Therefore, this ring is not aromatic.

Step 5: Summarize the findings. Compound (a) has an aromatic benzene ring, compound (b) is aromatic, and compound (c) 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 the delocalization of π electrons across the ring. For a compound to be aromatic, it must follow Hückel's rule, which states that it should have a planar structure and contain a total of 4n + 2 π electrons, where n is a non-negative integer. This delocalization leads to a lower energy state and unique chemical reactivity.

Heterocyclic Compounds

Heterocyclic compounds are cyclic structures that contain at least one atom in the ring that is not carbon, such as nitrogen, oxygen, or sulfur. These compounds can exhibit diverse chemical properties and biological activities, making them significant in pharmaceuticals and organic chemistry. The presence of heteroatoms can influence the aromaticity and stability of the compound.

Criteria for Aromaticity

To determine if a heterocyclic compound is aromatic, one must assess its structure for planarity, cyclic nature, and the presence of 4n + 2 π electrons. Additionally, the compound should not have any sp3 hybridized atoms in the ring, as these do not contribute to π electron delocalization. Evaluating these criteria helps in identifying whether the compound can be classified as aromatic.

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Related Practice

Textbook Question

Explain why each compound is aromatic, antiaromatic, or nonaromatic.

(g)

(h)

Textbook Question

Explain why each compound is aromatic, antiaromatic, or nonaromatic.

(a)

(b)

(c)

Textbook Question

a. Explain how pyrrole is isoelectronic with the cyclopentadienyl anion.

b. Specifically, what is the difference between the cyclopentadienyl anion and pyrrole?

Textbook Question

Determine which of the heterocyclic amines just shown are aromatic. Give the reasons for your conclusions.

Textbook Question

Show which of the nitrogen atoms in purine are basic, and which one is not basic. For the nonbasic nitrogen, explain why its nonbonding electrons are not easily available to become protonated.

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.

(b)

Textbook Question

Anions of hydrocarbons are rare, and dianions of hydrocarbons are extremely rare. The following hydrocarbon reacts with two equivalents of butyllithium to form a dianion of formula [C₈H₆]^2–. Propose a structure for this dianion, and suggest why it forms so readily.

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.

(a)

(b)

(c)

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