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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5:22 minutes

Problem 32a

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)

Verified step by step guidance

Step 1: Analyze the structure of each molecule or ion. All three structures are cyclopropene derivatives, with one containing a neutral cyclopropene ring, one containing a cyclopropene ring with a negative charge, and one containing a cyclopropene ring with a positive charge.

Step 2: Determine the number of pi electrons in the ring for each structure. The neutral cyclopropene has 2 pi electrons from the double bond. The negatively charged cyclopropene has 4 pi electrons (2 from the double bond and 2 from the lone pair on the negatively charged carbon). The positively charged cyclopropene has 2 pi electrons from the double bond.

Step 3: Apply Huckel's rule to classify aromaticity. Huckel's rule states that a molecule is aromatic if it has (4n + 2) pi electrons, where n is a non-negative integer. Anti-aromatic molecules have 4n pi electrons, and non-aromatic molecules do not meet the criteria for aromaticity or anti-aromaticity.

Step 4: Classify each structure. The neutral cyclopropene has 2 pi electrons, which fits Huckel's rule for aromaticity (n=0). The negatively charged cyclopropene has 4 pi electrons, which fits the criteria for anti-aromaticity. The positively charged cyclopropene has 2 pi electrons, which fits Huckel's rule for aromaticity (n=0).

Step 5: Summarize the classification. The neutral cyclopropene and positively charged cyclopropene are aromatic with 2 pi electrons each. The negatively charged cyclopropene is anti-aromatic with 4 pi electrons.

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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 that follow Hückel's rule, which states they must have 4n + 2 π electrons (where n is a non-negative integer). This electron configuration allows for delocalization, contributing to the stability and unique reactivity of aromatic compounds. Common examples include benzene and its derivatives.

Antiaromaticity

Antiaromatic compounds are also cyclic and planar but contain 4n π electrons, leading to instability due to the unfavorable electron configuration. This results in increased reactivity compared to nonaromatic compounds. An example of an antiaromatic compound is cyclobutadiene, which has 4 π electrons.

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. Nonaromatic compounds do not exhibit the special stability associated with aromatic compounds, and their reactivity is generally more predictable and less complex.

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