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Ch. 16 - Aromatic Compounds
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 16 - Aromatic CompoundsProblem 3
Chapter 16, Problem 3

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.

Verified step by step guidance
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Step 1: Begin with benzene (C6H6). Draw its resonance forms by alternating the double bonds in the six-membered ring. Show all carbon and hydrogen atoms explicitly. Benzene has two resonance forms where the positions of the double bonds shift around the ring.
Step 2: For cyclobutadiene (C4H4), draw its resonance forms. Cyclobutadiene is a four-membered ring with alternating double bonds. Show the two resonance forms where the double bonds switch positions, and include all carbon and hydrogen atoms.
Step 3: For cyclooctatetraene (C8H8), draw its resonance forms. Cyclooctatetraene is an eight-membered ring with alternating double bonds. Show the resonance forms where the double bonds shift positions around the ring, and include all carbon and hydrogen atoms.
Step 4: To illustrate the p orbital overlap for benzene, cyclobutadiene, and cyclooctatetraene, assume all molecules are planar. Show the p orbitals on each sp2 hybridized carbon atom extending above and below the plane of the ring. Indicate how these p orbitals overlap to form continuous π systems.
Step 5: Highlight the differences in aromaticity among the three molecules. Benzene has a fully delocalized π system and follows Hückel's rule (4n+2 π electrons, where n=1). Cyclobutadiene does not follow Hückel's rule (4 π electrons, n=0) and is antiaromatic. Cyclooctatetraene, if planar, would also not follow Hückel's rule (8 π electrons, n=2) and would be antiaromatic, but in reality, it adopts a non-planar tub-shaped conformation to avoid antiaromaticity.

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Key Concepts

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

Resonance Structures

Resonance structures are different ways of drawing a molecule that illustrate the delocalization of electrons. In compounds like benzene, resonance forms show how electrons are shared among multiple bonds, leading to stability. Each resonance form contributes to the overall hybrid structure, which is more stable than any individual form.

Planarity and Hybridization

Planarity in organic molecules refers to the arrangement of atoms in a flat, two-dimensional structure, which is crucial for effective orbital overlap. In benzene and similar compounds, carbon atoms are sp2 hybridized, allowing for a trigonal planar geometry that facilitates the formation of π bonds through overlapping p orbitals.

Conjugation and Aromaticity

Conjugation occurs when p orbitals overlap across adjacent atoms, allowing for the delocalization of π electrons. Aromatic compounds, like benzene, exhibit special stability due to this delocalization, which is a result of their cyclic structure and planarity, fulfilling Huckel's rule of having (4n + 2) π electrons.
Related Practice
Textbook Question

Show the product of the Diels–Alder dimerization of cyclobutadiene. (This reaction is similar to the dimerization of cyclopentadiene, discussed in Section 15-11.)

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Textbook Question

Make a model of cyclooctatetraene in the tub conformation. Draw this conformation, and estimate the angle between the p orbitals of adjacent pi bonds.

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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

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

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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:

(c)

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