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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5:45 minutes

Problem 19g,h

Textbook Question

Explain why each compound is aromatic, antiaromatic, or nonaromatic.
(g)
(h)

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 an integer.

Step 2: Analyze compound (g), cytosine. Cytosine is a six-membered ring containing nitrogen and oxygen atoms. The ring is cyclic and planar due to sp2 hybridization of the atoms in the ring. The conjugated π-electron system includes the double bonds and lone pairs on the heteroatoms. Count the π-electrons to determine if it satisfies Huckel's rule.

Step 3: For compound (g), cytosine, consider the lone pairs on the nitrogen and oxygen atoms. Not all lone pairs contribute to aromaticity; only those in conjugation with the π-system are considered. Verify if the total π-electron count is (4n + 2). If it satisfies Huckel's rule, the compound is aromatic; if not, it is nonaromatic or antiaromatic.

Step 4: Analyze compound (h), a six-membered ring with oxygen. The ring is cyclic and planar, and it has alternating double bonds, indicating a conjugated π-electron system. Count the π-electrons, including the lone pair on oxygen, to determine if it satisfies Huckel's rule.

Step 5: For compound (h), verify if the lone pair on oxygen contributes to the conjugated π-system. If the total π-electron count satisfies Huckel's rule, the compound is aromatic. If it does not satisfy Huckel's rule, determine if the compound is antiaromatic (4n π-electrons) or nonaromatic (does not meet the criteria for aromaticity).

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Key Concepts

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

Aromaticity

Aromatic compounds are cyclic, planar molecules with a ring of resonance that follow Hückel's rule, which states they must have 4n + 2 π electrons (where n is a non-negative integer). This delocalization of electrons contributes to their stability and unique chemical properties. Common examples include benzene and its derivatives.

Antiaromaticity

Antiaromatic compounds are also cyclic and planar but contain 4n π electrons, leading to destabilization due to the presence of electron-electron repulsion in the ring. This instability often results in higher reactivity compared to nonaromatic compounds. An example of an antiaromatic compound is cyclobutadiene.

Nonaromaticity

Nonaromatic compounds do not meet the criteria for aromaticity or antiaromaticity. They may be acyclic, lack planarity, or have an insufficient number of π electrons. These compounds do not exhibit the special stability associated with aromatic systems and typically behave like aliphatic compounds. An example is cyclohexane.

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