Textbook Question
Predict the major products of treating the following compounds with hot, concentrated potassium permanganate, followed by acidification with dilute HCl.
(a) isopropylbenzene
(b) p-xylene
(c)
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19. Reactions of Aromatics: EAS and Beyond
Side-Chain Halogenation
Multiple Choice
Which of the following does NOT explain why reactive intermediates at the benzylic position are relatively stable?
A
A benzylic C–X bond will break faster to form a carbocation.
B
The benzylic C–H bond has a lower BDE.
C
The benzylic C–H bond has a lower pKa value.
D
The original benzylic C–H bond has a shorter bond length.
Verified step by step guidance1
Understand the concept of benzylic position: The benzylic position refers to the carbon atom directly attached to a benzene ring. Reactive intermediates at this position, such as carbocations, radicals, or carbanions, are often more stable due to resonance stabilization with the aromatic ring.
Examine the first option: A benzylic C–X bond will break faster to form a carbocation. This is true because the resulting carbocation can be stabilized by resonance with the benzene ring, making the bond easier to break.
Consider the second option: The benzylic C–H bond has a lower bond dissociation energy (BDE). A lower BDE indicates that the bond is weaker and easier to break, which can facilitate the formation of reactive intermediates.
Evaluate the third option: The benzylic C–H bond has a lower pKa value. A lower pKa value suggests that the hydrogen is more acidic, meaning it can be removed more easily to form a carbanion, which can also be resonance-stabilized.
Analyze the fourth option: The original benzylic C–H bond has a shorter bond length. Bond length is not directly related to the stability of reactive intermediates. Therefore, this option does not explain the stability of reactive intermediates at the benzylic position.
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