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Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 23 - Benzene I: Aromatic Stability and Substitution ReactionsProblem 16b
Chapter 22, Problem 16b

(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic.
(ii) For the aromatic and antiaromatic molecules, solve for n in Hückel’s/Breslow’s rule. For the other molecules, explain which of the rules of aromaticity is being broken.
(b) Chemical structure of a pentagonal ring with a negative charge, used for classifying molecular aromaticity.

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1
Identify the molecule: The structure shown is a cyclopentadienyl anion, which is a five-membered carbon ring with two double bonds and a negative charge.
Determine the number of π electrons: Each double bond contributes 2 π electrons, and the negative charge contributes 2 π electrons, totaling 6 π electrons.
Apply Hückel's rule: For a molecule to be aromatic, it must have (4n + 2) π electrons, where n is a non-negative integer. Set up the equation 4n + 2 = 6 and solve for n.
Solve for n: Subtract 2 from both sides to get 4n = 4, then divide by 4 to find n = 1.
Conclude the classification: Since the molecule has 6 π electrons and satisfies Hückel's rule with n = 1, it is classified as aromatic.

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Key Concepts

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

Aromaticity

Aromaticity is a property of cyclic, planar structures with a ring of resonance bonds that leads to enhanced stability. For a molecule to be aromatic, it must follow Hückel's rule, which states that it should have (4n + 2) π electrons, where n is a non-negative integer. This rule helps determine the electron count that allows for delocalization and stability.
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Hückel's Rule

Hückel's rule is a criterion used to determine if a planar ring molecule is aromatic. According to this rule, a molecule is aromatic if it contains a total of (4n + 2) π electrons, where n is a non-negative integer. This electron count allows for the delocalization of electrons, contributing to the molecule's stability and unique chemical properties.
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Antiaromaticity

Antiaromaticity occurs in cyclic, planar molecules that have (4n) π electrons, leading to instability due to electron delocalization that does not favor resonance. Unlike aromatic compounds, antiaromatic compounds are less stable than their non-cyclic or non-planar counterparts. Identifying antiaromaticity involves checking the electron count and ensuring the molecule is planar and cyclic.
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Related Practice
Textbook Question

(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic.

(ii) For the aromatic and antiaromatic molecules, solve for n in Hückel’s/Breslow’s rule. For the other molecules, explain which of the rules of aromaticity is being broken.

(d)

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Textbook Question

Cyclopentane has pKₐ = 50, whereas cyclopentadiene has pKₐ = 16. Explain this difference.

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Textbook Question

Which resonance structure, A or B, is most contributing?

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Textbook Question

Draw a Frost circle for the cyclopropenyl anion and compare it to the Frost circle for the cyclopropenyl cation. What has changed?

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Textbook Question

Draw a Frost circle for the cyclopentadienyl cation and compare it to the Frost circle for the cyclopentadienyl anion. What has changed?

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Textbook Question

Which of the following concerted reactions would have a more stable transition state? Why?

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