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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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 25d
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.
Verified step by step guidance1
Cyclohexene contains a double bond, which is an area of high electron density. This makes it more reactive toward electrophiles like bromine compared to cyclohexane, which lacks a double bond and is less reactive.
In the presence of light (hv), bromine undergoes homolytic cleavage to form bromine radicals. These radicals initiate a chain reaction with cyclohexene.
The double bond in cyclohexene allows for the formation of a more stable allylic radical during the reaction. This stability arises from resonance delocalization of the unpaired electron over the π-system of the double bond.
Cyclohexane, on the other hand, does not have a double bond and cannot form a resonance-stabilized radical. As a result, its reaction with bromine requires higher energy input, such as heating, to proceed.
The reaction of cyclohexene with bromine is faster because the allylic radical intermediate is stabilized, lowering the activation energy for the reaction and making it more favorable under light-promoted conditions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electrophilic Addition
Electrophilic addition is a fundamental reaction mechanism in organic chemistry where an electrophile reacts with a nucleophile, leading to the formation of a more saturated product. In the case of cyclohexene, the double bond acts as a nucleophile, readily reacting with bromine, an electrophile, to form bromocyclohexene. This reaction is favored due to the stability of the carbocation intermediate formed during the process.
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1,2 vs 1,4 Addition
Stability of Alkenes vs. Alkanes
Alkenes, such as cyclohexene, are generally more reactive than alkanes like cyclohexane due to the presence of a carbon-carbon double bond. This double bond is a site of high electron density, making alkenes more susceptible to electrophilic attack. In contrast, alkanes are saturated hydrocarbons with only single bonds, requiring higher energy conditions, such as heat, to initiate reactions with electrophiles like bromine.
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Reaction Conditions and Activation Energy
The reaction conditions significantly influence the rate of chemical reactions. Cyclohexene reacts with bromine under mild conditions due to its lower activation energy, allowing the reaction to proceed at room temperature or with light. Conversely, cyclohexane requires heating to provide the necessary energy to overcome the activation barrier for bromination, making it less reactive under standard conditions.
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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
Give the SN1 mechanism for the formation of 2-ethoxy-3-methylbutane, the unrearranged product in this reaction.
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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.
(c)
754
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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.
(d)
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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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