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 24a
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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Identify the structure of 3-bromocyclohexene, which consists of a cyclohexene ring with a bromine atom attached to the third carbon.
Recognize that the presence of the double bond in the cyclohexene ring allows for resonance stabilization of the carbocation formed during the SN1 reaction.
Draw the resonance structures: When the bromine leaves, a carbocation is formed at the third carbon. The positive charge can be delocalized to the adjacent carbon atoms through resonance with the double bond.
Illustrate the movement of electrons: The pi electrons from the double bond can move to form a new double bond with the carbocation, shifting the positive charge to the other carbon atom in the ring.
Explain that this resonance stabilization of the carbocation intermediate makes the SN1 reaction more favorable, enhancing the reactivity of 3-bromocyclohexene compared to other secondary halides without such stabilization.
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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 nucleophilic attack. The rate of SN1 reactions depends on the stability of the carbocation; more stable carbocations lead to faster reactions. This mechanism is common in tertiary halides due to their ability to stabilize positive charges.
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Drawing the SN1 Mechanism
Carbocation Stability
Carbocation stability is crucial in determining the rate of SN1 reactions. Stability is enhanced by factors such as hyperconjugation and resonance. Tertiary carbocations are generally more stable than secondary or primary ones due to greater hyperconjugation and inductive effects. In 3-bromocyclohexene, resonance stabilization plays a key role in enhancing carbocation stability.
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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. In 3-bromocyclohexene, the positive charge on the carbocation can be delocalized over the double bond, creating resonance structures that stabilize the intermediate. This delocalization increases the carbocation's stability, making the SN1 reaction proceed more rapidly, similar to tertiary halides.
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03:04Drawing Resonance Structures
Related Practice
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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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
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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Textbook Question
Choose the member of each pair that will react faster by the SN1 mechanism.
a. 1-bromopropane or 2-bromopropane
b. 2-bromo-2-methylbutane or 2-bromo-3-methylbutane
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