Textbook Question
Propose a mechanism for each reaction, showing explicitly how the observed mixtures of products are formed.
(e) 3-chlorobut-1-ene + AgNO3, H2O → but-2-en-1-ol + but-3-en-2-ol
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Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy
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
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Wade 9th EditionCh. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet SpectroscopyProblem 7b
Wade 9th EditionCh. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet SpectroscopyProblem 7bChapter 15, Problem 7b
Propose a mechanism for each reaction, showing explicitly how the observed mixtures of products are formed.
(b) 2-methylbut-3-en-2-ol + HBr → 1-bromo-3-methylbut-2-ene + 3-bromo-3-methylbut-1-ene
Verified step by step guidance1
Step 1: Protonation of the alcohol group - The hydroxyl group (-OH) in 4-methylpent-3-en-2-ol is protonated by HBr, forming a good leaving group (water). This step generates a carbocation intermediate at the 2-position.
Step 2: Formation of the carbocation - The loss of water from the protonated alcohol leads to the formation of a carbocation at the 2-position. This carbocation is stabilized by hyperconjugation and resonance effects due to the adjacent double bond.
Step 3: Rearrangement of the carbocation - The carbocation undergoes a hydride shift or methyl shift to form a more stable carbocation. This rearrangement explains the formation of different products.
Step 4: Nucleophilic attack by bromide ion - The bromide ion (Br⁻) attacks the carbocation, leading to the formation of the brominated products. Depending on the position of the carbocation after rearrangement, different regioisomers are formed.
Step 5: Explanation of product mixture - The observed products (1-bromo-3-methylbut-2-ene, 3-bromo-3-methylbut-1-ene, and others) result from the combination of direct bromide attack and carbocation rearrangements. The mixture reflects the regioselectivity and possible rearrangements during the reaction.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electrophilic Addition Reactions
Electrophilic addition reactions involve the addition of an electrophile to a nucleophile, typically across a double bond. In this case, HBr acts as the electrophile, adding to the double bond of 2-methylbut-3-en-2-ol. The mechanism results in the formation of a carbocation intermediate, which can lead to different product outcomes based on the stability of the carbocation formed.
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Features of Addition Mechanisms.
Carbocation Stability
Carbocation stability is crucial in determining the outcome of reactions involving electrophilic addition. More stable carbocations, such as tertiary carbocations, are favored over less stable ones. In the reaction of 2-methylbut-3-en-2-ol with HBr, the formation of a tertiary carbocation leads to the production of multiple bromoalkene products, as the carbocation can rearrange or react with Br- in different ways.
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Regioselectivity
Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others. In the context of the reaction between 2-methylbut-3-en-2-ol and HBr, regioselectivity is observed as the reaction can produce different bromo products depending on the site of electrophilic attack. The stability of the resulting carbocation influences which product is formed preferentially.
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Related Practice
Textbook Question
Propose a mechanism for each reaction, showing explicitly how the observed mixtures of products are formed.
c. cyclopenta-1,3-diene + Br2 → 3,4-dibromocyclopent-1-ene + 3,5-dibromocyclopent-1-ene
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Textbook Question
When Br2 is added to buta-1,3-diene at –15 °C, the product mixture contains 60% of product A and 40% of product B. When the same reaction takes place at 60 °C, the product ratio is 10% A and 90% B.
a. Propose structures for products A and B. (Hint: In many cases, an allylic carbocation is more stable than a bromonium ion.)
b. Propose a mechanism to account for formation of both A and B.
c. Show why A predominates at –15 °C and B predominates at 60 °C.
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Textbook Question
When 3-bromo-1-methylcyclohexene undergoes solvolysis in hot ethanol, two products are formed. Propose a mechanism that accounts for both of these products.
Textbook Question
Draw another resonance form for each of the substituted allylic cations shown in the preceding figure, showing how the positive charge is shared by another carbon atom. In each case, state whether your second resonance form is a more important or less important resonance contributor than the first structure. (Which structure places the positive charge on the more-substituted carbon atom?)
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Textbook Question
Treatment of an alkyl halide with AgNO3 in alcohol often promotes ionization.
Ag+ + R–Cl → AgCl + R+
When 4-chloro-2-methylhex-2-ene reacts with AgNO3 in ethanol, two isomeric ethers are formed. Suggest structures, and propose a mechanism for their formation
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