Textbook Question
Draw the important resonance contributors for the following cations, anions, and radicals.
(c)
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16. Conjugated Systems
Stability of Conjugated Intermediates
6:31 minutesProblem 36a(1)
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.
Verified step by step guidance1
Step 1: Begin by identifying the benzyl cation structure. The benzyl cation consists of a positively charged carbon atom directly attached to a benzene ring. This positive charge can be delocalized due to the conjugation with the π-electrons of the benzene ring.
Step 2: Draw the first resonance structure. The positive charge on the benzylic carbon can be delocalized into the benzene ring by shifting one of the π-electron pairs from the double bond adjacent to the benzylic carbon. This creates a double bond between the benzylic carbon and the benzene ring, while the positive charge moves to the ortho position of the benzene ring.
Step 3: Draw the second resonance structure. From the first resonance structure, shift another π-electron pair from the adjacent double bond in the benzene ring. This moves the positive charge to the para position of the benzene ring, while maintaining conjugation.
Step 4: Draw the third resonance structure. From the second resonance structure, shift another π-electron pair from the adjacent double bond in the benzene ring. This moves the positive charge to the other ortho position of the benzene ring.
Step 5: Summarize the resonance stabilization. The benzyl cation is stabilized by resonance because the positive charge is delocalized over four carbon atoms (the benzylic carbon and three carbons in the benzene ring). This delocalization reduces the energy of the cation, making it more stable than a simple alkyl cation.
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Video duration:
6mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Resonance
Resonance is a concept in organic chemistry that describes the delocalization of electrons in molecules where the bonding cannot be expressed by a single Lewis structure. Instead, multiple structures, known as resonance forms, are used to represent the molecule's electron distribution. This delocalization stabilizes the molecule, as seen in the benzyl cation, where the positive charge can be spread over several atoms, enhancing stability.
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Benzylic Position
The benzylic position refers to the carbon atom directly attached to a benzene ring, specifically the carbon adjacent to the aromatic system. This position is significant because it exhibits unique reactivity patterns due to the influence of the benzene ring, which can stabilize intermediates like cations, anions, and radicals through resonance, making benzylic intermediates more stable than their non-aromatic counterparts.
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Stability of Intermediates
In organic chemistry, the stability of intermediates such as cations, anions, and radicals is crucial for understanding reaction mechanisms. Benzylic cations, anions, and radicals are more stable than simple alkyl intermediates due to resonance stabilization provided by the adjacent benzene ring. This increased stability affects the reactivity and pathways of chemical reactions involving these intermediates.
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