Textbook Question
Draw the important resonance forms of the following free radicals.
a.
1. A Review of General Chemistry
Resonance Structures
5:15 minutesProblem 24b
Textbook Question
Benzyl bromide is a primary halide. It undergoes SN1 substitution about as fast as most tertiary halides. Use resonance structures to explain this enhanced reactivity.
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Begin by identifying the structure of benzyl bromide. Benzyl bromide consists of a benzene ring attached to a CH2 group, which is further bonded to a bromine atom.
Understand the concept of resonance. Resonance involves the delocalization of electrons across adjacent atoms, which can stabilize certain intermediates in a reaction.
When benzyl bromide undergoes SN1 substitution, the first step is the formation of a carbocation intermediate after the bromine leaves. This carbocation is formed at the benzylic position (the CH2 group attached to the benzene ring).
Examine the resonance stabilization of the benzylic carbocation. The positive charge on the carbocation can be delocalized into the benzene ring, creating multiple resonance structures. This delocalization stabilizes the carbocation, making the SN1 reaction more favorable.
Compare the stability of the benzylic carbocation to other carbocations. The resonance stabilization in benzyl bromide allows the carbocation to be as stable as those formed from tertiary halides, which typically have more stable carbocations due to hyperconjugation and inductive effects.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
SN1 Reaction Mechanism
The SN1 reaction mechanism involves a two-step process where the leaving group departs first, forming a carbocation intermediate, followed by the nucleophile attacking the carbocation. This mechanism is typically favored by tertiary halides due to the stability of the carbocation formed, which is stabilized by hyperconjugation and inductive effects.
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Carbocation Stability
Carbocation stability is crucial in SN1 reactions, as the rate-determining step involves the formation of a carbocation. Tertiary carbocations are generally more stable than primary ones due to greater hyperconjugation and inductive effects. However, benzyl carbocations are exceptionally stable due to resonance stabilization, which allows the positive charge to be delocalized over the aromatic ring.
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Resonance Structures
Resonance structures are different Lewis structures for a molecule that depict the delocalization of electrons within the molecule. In benzyl bromide, the carbocation formed after the bromide leaves can be stabilized by resonance, as the positive charge is delocalized over the benzene ring. This delocalization enhances the stability of the carbocation, making the SN1 reaction more favorable despite benzyl bromide being a primary halide.
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