Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Problem 37g

Chapter 15, Problem 37g

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?

Verified step by step guidance

Step 1: Understand the structure of the pentadienyl radical. It consists of a conjugated system with alternating double and single bonds: H₂C=CH-CH=CH-CH₂·. The unpaired electron is delocalized over the three central carbon atoms (C2, C3, and C4) due to resonance.

Step 2: Removing the highest-energy electron from the pentadienyl radical results in the formation of the pentadienyl cation. This means the unpaired electron is removed, leaving a positive charge that will also be delocalized over the conjugated system.

Step 3: Draw the resonance structures of the pentadienyl cation. The positive charge will be shared among the three central carbon atoms (C2, C3, and C4) due to the delocalization of π-electrons in the conjugated system. Use curved arrows to show the movement of electrons in each resonance structure.

Step 4: Analyze the resonance structures to confirm that the positive charge is distributed over C2, C3, and C4. This delocalization stabilizes the cation and agrees with the resonance picture of the pentadienyl radical, where the unpaired electron was also delocalized over the same three carbon atoms.

Step 5: Conclude that the positive charge in the pentadienyl cation is shared among C2, C3, and C4, and this distribution is consistent with the resonance picture of the original pentadienyl radical. This demonstrates the importance of resonance in stabilizing both radicals and cations in conjugated systems.

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Key Concepts

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

Radicals and Cations

Radicals are species that contain an unpaired electron, making them highly reactive. In contrast, cations are positively charged species formed when an electron is removed from a neutral atom or molecule. In the case of the pentadienyl radical, removing the highest-energy electron results in a pentadienyl cation, which has a positive charge that can be delocalized over adjacent carbon atoms.

Delocalization of Charge

Delocalization refers to the spreading of electrons or charge over multiple atoms rather than being localized on a single atom. In the pentadienyl cation, the positive charge can be shared among the carbon atoms involved in the double bonds, which stabilizes the cation. This delocalization is a key factor in understanding the stability and reactivity of the cation.

Resonance Structures

Resonance structures are different ways of drawing a molecule that represent the same arrangement of atoms but differ in the placement of electrons. For the pentadienyl cation, resonance structures illustrate how the positive charge can be distributed across different carbon atoms. This concept helps explain the stability of the cation, as the actual structure is a hybrid of these resonance forms.

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Drawing Resonance Structures

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Textbook Question

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?

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.

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

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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.

Textbook Question

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

a. Use resonance forms to show which three carbon atoms bear the unpaired electron.