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Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet SpectroscopyProblem 37h
Chapter 15, Problem 37h

The pentadienyl radical, H2C=CH–CH=CH–CH2•, has its unpaired electron delocalized over three carbon atoms.
h. Add an electron to the pentadienyl radical to give the pentadienyl anion. Which carbon atoms share the negative charge? Does this picture agree with the resonance picture?

Verified step by step guidance
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Step 1: Understand the structure of the pentadienyl radical. The pentadienyl radical, H₂C=CH-CH=CH-CH₂·, is a conjugated system with alternating double and single bonds. The unpaired electron is delocalized over the three central carbon atoms (C2, C3, and C4) due to resonance.
Step 2: Add an electron to the pentadienyl radical. Adding an electron to the radical will pair with the unpaired electron, resulting in a pentadienyl anion. This anion now has a negative charge that is delocalized over the conjugated system.
Step 3: Draw the resonance structures of the pentadienyl anion. The negative charge is delocalized over the three central carbon atoms (C2, C3, and C4) through resonance. Write out the resonance structures to visualize how the negative charge shifts between these carbons.
Step 4: Analyze the resonance picture. The resonance structures show that the negative charge is shared among C2, C3, and C4. This delocalization stabilizes the anion and agrees with the resonance picture of the pentadienyl radical.
Step 5: Conclude the analysis. The negative charge in the pentadienyl anion is shared over the three central carbon atoms (C2, C3, and C4), and this picture is consistent with the resonance structures. This delocalization is a key feature of conjugated systems and contributes to the stability of the anion.

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Key Concepts

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

Radicals and Anions

Radicals are species with unpaired electrons, making them highly reactive. In contrast, anions are negatively charged species that have gained an electron. The pentadienyl radical has an unpaired electron that can be stabilized by delocalization across multiple carbon atoms. When an electron is added to form the pentadienyl anion, the negative charge is distributed among the carbon atoms involved in the double bonds.
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Delocalization of Electrons

Delocalization refers to the spreading of electrons across multiple atoms rather than being localized between two. In the case of the pentadienyl radical and anion, the unpaired electron and the added electron can be delocalized over the carbon atoms involved in the conjugated system. This delocalization stabilizes the radical and anion, allowing for resonance structures that depict the distribution of the negative charge.
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Resonance Structures

Resonance structures are different ways of drawing a molecule that illustrate the delocalization of electrons. For the pentadienyl anion, resonance structures show how the negative charge can be shared among different carbon atoms. This concept helps in understanding the stability of the anion, as the actual structure is a hybrid of these resonance forms, reflecting the distribution of the negative charge across the molecule.
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Related Practice
Textbook Question

A student was studying terpene synthesis, and she wanted to make the compound shown here. First she converted 3-bromo-6-methylcyclohexene to alcohol A. She heated alcohol A with sulfuric acid and purified one of the components (compound B) from the resulting mixture. Compound B has the correct molecular formula for the desired product.

a. Suggest how 3-bromo-6-methylcyclohexene might be converted to alcohol A.

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Textbook Question

The pentadienyl radical, H2C=CH–CH=CH–CH2•, has its unpaired electron delocalized over three carbon atoms.

g. Remove the highest-energy electron from the pentadienyl radical to give the pentadienyl cation. Which carbon atoms share the positive charge? Does this picture agree with the resonance picture?

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Textbook Question

Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous sp2 hybridized atoms.

(a)

(b)

(c)

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Textbook Question

The pentadienyl radical, H2C=CH–CH=CH–CH2•, has its unpaired electron delocalized over three carbon atoms.

b. How many MOs are there in the molecular orbital picture of the pentadienyl radical?

c. How many nodes are there in the lowest-energy MO of the pentadienyl system? How many in the highest-energy MO?

d. Draw the MOs of the pentadienyl system in order of increasing energy

Textbook Question

A student was studying terpene synthesis, and she wanted to make the compound shown here. First she converted 3-bromo-6-methylcyclohexene to alcohol A. She heated alcohol A with sulfuric acid and purified one of the components (compound B) from the resulting mixture. Compound B has the correct molecular formula for the desired product.

(c) Propose a mechanism for the dehydration of alcohol A to compound B.

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Textbook Question

The pentadienyl radical, H2C=CH–CH=CH–CH2•, has its unpaired electron delocalized over three carbon atoms.

f. Show how your molecular orbital picture agrees with the resonance picture showing delocalization of the unpaired electron onto three carbon atoms.