Ch. 16 - Aromatic Compounds

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

Wade 9th Edition

Ch. 16 - Aromatic Compounds

Problem 36a(2)

Chapter 16, Problem 36a(2)

The benzene ring alters the reactivity of a neighboring group in the benzylic position much as a double bond alters the reactivity of groups in the allylic position.

Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates.
a. Use resonance forms to show the delocalization (over four carbon atoms) of the unpaired electron of the benzyl radical.

Verified step by step guidance

Step 1: Recognize that the benzyl radical is stabilized by resonance. The unpaired electron on the benzylic carbon can delocalize into the conjugated π-system of the benzene ring, spreading the electron density over multiple atoms.

Step 2: Draw the initial structure of the benzyl radical. The benzyl radical consists of a benzene ring attached to a CH2 group, where the CH2 group has one unpaired electron.

Step 3: Begin the resonance process by moving the unpaired electron from the benzylic carbon into the π-system of the benzene ring. This creates a double bond between the benzylic carbon and one of the carbons in the benzene ring, while breaking the double bond in the benzene ring to form a new lone electron on another carbon.

Step 4: Continue the resonance process by shifting the lone electron and double bonds around the benzene ring. This results in multiple resonance structures where the unpaired electron is delocalized over four carbon atoms in the benzene ring.

Step 5: Summarize the resonance forms. The delocalization of the unpaired electron stabilizes the benzyl radical, making it more stable than a simple alkyl radical. The resonance forms demonstrate how the electron density is spread across the benzylic carbon and the conjugated π-system of the benzene ring.

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Key Concepts

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

Benzyl Group and Position

The benzyl group consists of a phenyl group (a benzene ring) attached to a methylene (-CH2-) group. The benzylic position refers to the carbon atom directly adjacent to the benzene ring. This position is significant in organic chemistry because it exhibits unique reactivity patterns due to the influence of the aromatic ring.

Stability of Benzylic Intermediates

Benzylic cations, anions, and radicals are more stable than their non-aromatic counterparts due to resonance stabilization. The unpaired electron or charge can be delocalized over the aromatic system, which lowers the energy of these intermediates and makes reactions involving them more favorable.

Resonance and Delocalization

Resonance is a concept in organic chemistry that describes the delocalization of electrons across multiple atoms, particularly in conjugated systems like benzene. In the case of the benzyl radical, the unpaired electron can be shared among the carbon atoms in the benzene ring, leading to several resonance forms that enhance the stability of the radical through electron delocalization.

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Textbook Question

The benzene ring alters the reactivity of a neighboring group in the benzylic position much as a double bond alters the reactivity of groups in the allylic position.

Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates.

b. Toluene reacts with bromine in the presence of light to give benzyl bromide. Propose a mechanism for this reaction.

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Textbook Question

Some of the following compounds show aromatic properties, and others do not.

1. Predict which ones are likely to be aromatic, and explain why they are aromatic.

(n)

(o)

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Textbook Question

The benzene ring alters the reactivity of a neighboring group in the benzylic position much as a double bond alters the reactivity of groups in the allylic position.

Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates.

c. Which of the following reactions will have the faster rate and give the better yield? Use a drawing of the transition state to explain your answer.

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Textbook Question

The benzene ring alters the reactivity of a neighboring group in the benzylic position much as a double bond alters the reactivity of groups in the allylic position.

Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates.

a. Use resonance forms to show the delocalization (over four carbon atoms) of the positive charge of the benzyl cation.

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Textbook Question

Some of the following compounds show aromatic properties, and others do not.

1. Predict which ones are likely to be aromatic, and explain why they are aromatic.

2. Predict which nitrogen atoms are more basic than water and which are less basic.

(m)

Textbook Question

The benzene ring alters the reactivity of a neighboring group in the benzylic position much as a double bond alters the reactivity of groups in the allylic position.

Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates.

a. Use resonance forms to show the delocalization (over four carbon atoms) of the negative charge of the benzyl anion.

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