Textbook Question
When 1,2-dimethylcyclopentene undergoes hydroboration–oxidation, one diastereomer of the product predominates. Show why this addition is stereospecific, and predict the stereochemistry of the major product.
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Ch.8 - Reactions of Alkenes
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 8, Problem 17
Give mechanisms to account for the stereochemistry of the products observed from the addition of bromine to cis- and trans-but-2-ene (Figure 8-5). Why are two products formed from the cis isomer but only one from the trans? (Making models will be helpful.)
Verified step by step guidance1
Step 1: Begin by understanding the reaction mechanism for the addition of bromine (Br₂) to alkenes. Bromine adds to the double bond in a two-step process: (1) formation of a bromonium ion intermediate and (2) nucleophilic attack by bromide ion on the bromonium ion.
Step 2: For cis-but-2-ene, the double bond is planar, and the bromonium ion forms with bromine bridging the two carbons of the double bond. The attack by the bromide ion occurs from the opposite side of the bromonium ion due to steric hindrance, leading to anti-addition. This results in two enantiomers because the substituents on the double bond are arranged asymmetrically.
Step 3: For trans-but-2-ene, the double bond is also planar, and the bromonium ion forms similarly. However, due to the symmetry of the trans configuration, the anti-addition of bromide results in a single meso compound. This is because the product has an internal plane of symmetry, making it achiral.
Step 4: The stereochemistry of the products is determined by the anti-addition mechanism, which ensures that the bromine atoms are added to opposite sides of the double bond. In the cis isomer, the asymmetry leads to two enantiomers, while in the trans isomer, the symmetry leads to a meso compound.
Step 5: To visualize this, use molecular models to represent the cis- and trans-but-2-ene structures. Observe how the bromonium ion forms and how the bromide ion attacks from the opposite side. This will help clarify why the cis isomer produces two enantiomers and the trans isomer produces a single meso compound.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Stereochemistry
Stereochemistry is the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the context of alkenes, stereochemistry is crucial for understanding how different isomers (cis and trans) can lead to different products during reactions, such as the addition of bromine. The arrangement of substituents around the double bond influences the formation of stereoisomers, which can result in distinct products.
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Electrophilic Addition Mechanism
The electrophilic addition mechanism describes how alkenes react with electrophiles, such as bromine, to form addition products. In this process, the double bond acts as a nucleophile, attacking the electrophile and leading to the formation of a cyclic bromonium ion intermediate. This intermediate can then open up to yield different products depending on the stereochemistry of the starting alkene, explaining why cis-but-2-ene produces two products while trans-but-2-ene yields only one.
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Formation of Stereoisomers
The formation of stereoisomers occurs when a reaction leads to products that differ in the spatial arrangement of atoms. In the case of cis-but-2-ene, the addition of bromine can lead to both enantiomers due to the possibility of attack from either side of the bromonium ion, resulting in two distinct products. Conversely, trans-but-2-ene leads to a meso compound, which is achiral and has a plane of symmetry, resulting in only one unique product.
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Related Practice
Textbook Question
Propose a mechanism for the addition of bromine water to cyclopentene, being careful to show why the trans product results and how both enantiomers are formed.
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Textbook Question
When HBr adds across the double bond of 1,2-dimethylcyclopentene, the product is a mixture of the cis and trans isomers. Show why this addition is not stereospecific.
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Textbook Question
Show how you would accomplish the following transformations.
(c) 1-methylcycloheptanol → 2-methylcycloheptanol
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Textbook Question
Propose mechanisms and predict the major products of the following reactions. Include stereochemistry where appropriate.
(c) (E)-dec-3-ene + Br2 in CCl4
(d) (Z)-dec-3-ene + Br2 in CCl4
Problem-Solving Hint: Models may be helpful whenever stereochemistry is involved. Write complete structures, including all bonds and charges, when writing mechanisms.
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Textbook Question
The solutions to Solved Problem 8-5 showed only how one enantiomer of the product is formed. For each product, show how an equally probable reaction forms the other enantiomer.
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