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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 16a,b
Chapter 16, Problem 16a,b

a. Explain how pyrrole is isoelectronic with the cyclopentadienyl anion.
b. Specifically, what is the difference between the cyclopentadienyl anion and pyrrole?

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Step 1: Understand the term 'isoelectronic'. Isoelectronic species have the same number of electrons and similar electronic structures. In this case, pyrrole (Structure 2) and the cyclopentadienyl anion (Structure 1) are being compared.
Step 2: Analyze the cyclopentadienyl anion (Structure 1). It is a five-membered ring with a negative charge on one of the carbons. The negative charge contributes two electrons to the π-system, and the remaining carbons each contribute one electron, resulting in a total of 6 π-electrons. This satisfies the Hückel rule for aromaticity (4n+2 π-electrons, where n=1).
Step 3: Analyze pyrrole (Structure 2). Pyrrole is a five-membered ring containing a nitrogen atom. The nitrogen atom has a lone pair of electrons that are delocalized into the π-system, contributing two electrons. The remaining carbons each contribute one electron, resulting in a total of 6 π-electrons, making pyrrole aromatic and isoelectronic with the cyclopentadienyl anion.
Step 4: Compare the structural difference between the cyclopentadienyl anion and pyrrole. The cyclopentadienyl anion has a negative charge on one of its carbons, while pyrrole has a nitrogen atom in place of one of the carbons. The nitrogen atom contributes its lone pair to the π-system, whereas the negative charge on the cyclopentadienyl anion contributes electrons to the π-system.
Step 5: Summarize the key difference. The cyclopentadienyl anion achieves aromaticity through a negative charge on a carbon atom, while pyrrole achieves aromaticity through the lone pair of electrons on the nitrogen atom. Both structures are aromatic and isoelectronic, having 6 π-electrons in their conjugated systems.

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Key Concepts

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

Isoelectronic Species

Isoelectronic species are atoms, ions, or molecules that have the same number of electrons and, consequently, the same electronic structure. In this case, pyrrole and the cyclopentadienyl anion both have a total of 6 valence electrons, which allows them to exhibit similar bonding characteristics despite differing in structure and overall charge.
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Aromaticity

Aromaticity refers to the enhanced stability of certain cyclic compounds due to the delocalization of π electrons across the ring structure. Both pyrrole and the cyclopentadienyl anion are aromatic, which contributes to their stability and unique chemical properties, despite the presence of different atoms (nitrogen in pyrrole and a negative charge in the anion).
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Structural Differences

The cyclopentadienyl anion (Structure 1) has a negative charge localized on one of the carbon atoms, while pyrrole (Structure 2) contains a nitrogen atom that contributes to the π system. This difference in atom type and charge distribution affects their reactivity and interactions with other chemical species, despite their isoelectronic nature.
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Related Practice
Textbook Question

When 3-chlorocyclopropene is treated with AgBF4, AgCl precipitates. The organic product can be obtained as a crystalline material, soluble in polar solvents such as nitromethane but insoluble in hexane. When the crystalline material is dissolved in nitromethane containing KCl, the original 3-chlorocyclopropene is regenerated. Determine the structure of the crystalline material, and write equations for its formation and its reaction with chloride ion.

Textbook Question

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

(a)

(b)

(c)

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.

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Textbook Question

The following hydrocarbon has an unusually large dipole moment. Explain how a large dipole moment might arise.

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Textbook Question

The polarization of a carbonyl group can be represented by a pair of resonance structures:

Cyclopropenone and cycloheptatrienone are more stable than anticipated. Cyclopentadienone, however, is relatively unstable and rapidly undergoes a Diels–Alder dimerization. Explain.

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Textbook Question

Draw resonance forms to show the charge distribution on the pyrrole structure.

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