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

Aromaticity

Organic Chemistry

18. Aromaticity

Aromaticity

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Problem 2

Textbook Question

Between 1865 and 1890, other possible structures were proposed for benzene, such as those shown here. Considering what nineteenth-century chemists knew about benzene, which is a better proposal for benzene’s structure: Dewar benzene or Ladenburg benzene? Why?

Verified step by step guidance

Step 1: Begin by understanding the historical context of benzene's structure. Nineteenth-century chemists knew that benzene had the molecular formula C₆H₆ and exhibited unusual stability compared to other unsaturated compounds. This stability was difficult to explain using conventional structures.

Step 2: Analyze Dewar benzene's structure. Dewar benzene consists of two fused cyclobutene rings. While this structure accounts for the molecular formula, it does not explain benzene's exceptional stability or its lack of reactivity typical of alkenes. Additionally, Dewar benzene would have significant ring strain due to the cyclobutene rings.

Step 3: Examine Ladenburg benzene's structure. Ladenburg benzene is a prismane structure, which is highly strained due to its triangular prism shape. This strain would make the molecule highly reactive, contradicting the observed stability of benzene.

Step 4: Compare both structures to the actual properties of benzene. Benzene is known to undergo substitution reactions rather than addition reactions, indicating delocalized π-electrons rather than localized double bonds. Neither Dewar benzene nor Ladenburg benzene accounts for this delocalization.

Step 5: Conclude that neither Dewar benzene nor Ladenburg benzene is a better proposal for benzene’s structure. The actual structure of benzene, as proposed by Kekulé, involves a resonance hybrid of alternating single and double bonds, which better explains its stability and chemical behavior.

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Key Concepts

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

Resonance Theory

Resonance theory explains that certain molecules can be represented by multiple valid Lewis structures, known as resonance structures. For benzene, this means that its actual structure is a hybrid of these forms, leading to equal bond lengths and strengths, which are intermediate between single and double bonds. This concept is crucial for understanding why traditional structures like Dewar and Ladenburg benzene may not accurately represent benzene's true nature.

Aromaticity

Aromaticity is a property of cyclic compounds that exhibit enhanced stability due to delocalized π electrons. For a compound to be aromatic, it must follow Huckel's rule, which states that it should have (4n + 2) π electrons. Benzene is the prototypical aromatic compound, and understanding this concept helps evaluate the proposed structures, as both Dewar and Ladenburg benzene deviate from the classic aromatic characteristics.

Historical Context of Chemical Structures

The historical context of chemical structures involves understanding the scientific knowledge and theories available to chemists in the 19th century. During this period, chemists were exploring the nature of chemical bonding and molecular structure, leading to various proposals for benzene's structure. Recognizing the limitations and prevailing theories of the time is essential for assessing the validity of Dewar and Ladenburg's proposals in relation to benzene's actual structure.

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Related Practice

Textbook Question

a. Draw the resonance forms of benzene, cyclobutadiene, and cyclooctatetraene, showing all the carbon and hydrogen atoms.

b. Assuming that these molecules are all planar, show how the p orbitals on the sp2 hybrid carbon atoms form continuous rings of overlapping orbitals above and below the plane of the carbon atoms.

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Textbook Question

Using the information in Figure 16-2, calculate the values of ∆H° for the following reactions:

(a)

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Textbook Question

Using the information in Figure 16-2, calculate the values of ∆H° for the following reactions:

(b)

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Textbook Question

Using the information in Figure 16-2, calculate the values of ∆H° for the following reactions:

(c)

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Multiple Choice

How does ΔH°_hydr of an aromatic molecule compare to an otherwise identical sample of an analogous nonconjugated molecule?

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Textbook Question

Does the MO energy diagram of cyclooctatetraene (Figure 16-8) appear to be a particularly stable or unstable configuration? Explain.

Molecule A is significantly more water soluble than molecule B. Justify this observation.

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Textbook Question

If Kekulé's original hypothesis had been correct and benzene was really an equilibrium between two structures, how many distinct isomers would exist for 1,2-dichlorobenzene?

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