Textbook Question
In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction:
d. Explain why cyclohexene reacts with bromine much faster than cyclohexane, which must be heated to react.
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Ch.6 - Alkyl Halides; Nucleophilic Substitution
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 6, Problem 24b
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.
Verified step by step guidance1
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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Drawing the SN1 Mechanism
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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05:58Determining Carbocation Stability
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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03:04Drawing Resonance Structures
Related Practice
Textbook Question
Propose a mechanism involving a hydride shift or an alkyl shift for each solvolysis reaction. Explain how each rearrangement forms a more stable intermediate.
Hint: Most rearrangements convert 2° (or incipient 1°) carbocations to 3° or resonance-stabilized carbocations.
(b)
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Textbook Question
Choose the member of each pair that will react faster by the SN1 mechanism.
c. n-propyl bromide or allyl bromide
d. 1-bromo-2,2-dimethylpropane or 2-bromopropane
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Textbook Question
Give the SN1 mechanism for the formation of 2-ethoxy-3-methylbutane, the unrearranged product in this reaction.
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Textbook Question
3-Bromocyclohexene is a secondary halide. It undergoes SN1 substitution about as fast as most tertiary halides. Use resonance structures to explain this enhanced reactivity.
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Textbook Question
Choose the member of each pair that will react faster by the SN1 mechanism.
e. 2-iodo-2-methylbutane or tert-butyl chloride
f. 2-bromo-2-methylbutane or ethyl iodide
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