Textbook Question
What stereoisomers are obtained from the following reactions?
c. (E)-3-methyl-2-pentene + HBr
d. cis-3-hexene + HBr
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Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions
Bruice8th EditionOrganic ChemistryISBN: 9780135213711
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Table of contents
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 1)31
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 2)65
- Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry84
- Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure183
- Ch. 4 - Isomers: The Arrangement of Atoms in Space121
- Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics83
- Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions175
- Ch. 7 - The Reactions of Alkynes • An Introduction to Multistep Synthesis119
- Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene148
- Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides212
- Ch. 10 - Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds131
- Ch. 11 - Organometallic Compounds60
- Ch. 12 - Radicals90
- Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy62
- Ch. 14 - NMR Spectroscopy95
- Ch. 15 - Reactions of Carboxylic Acids and Carboxylic Acid Derivatives123
- Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives141
- Ch. 17 - Reactions at the Alpha-Carbon101
- Ch. 18 - Reactions of Benzene and Substituted Benzenes144
- Ch. 19 - More About Amines • Reactions of Heterocyclic Compounds49
- Ch. 20 - The Organic Chemistry of Carbohydrates80
- Ch. 21 - Amino Acids, Peptides, and Proteins57
- Ch. 22 - Catalysis in Organic Reactions and in Enzymatic Reactions35
- Ch. 23 - The Organic Chemistry of the Coenzymes—Compounds Derived from Vitamins1
- Ch. 28 - Pericyclic Reactions58
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Bruice 8th EditionCh. 6 - The Reactions of Alkenes • The Stereochemistry of Addition ReactionsProblem 46
Bruice 8th EditionCh. 6 - The Reactions of Alkenes • The Stereochemistry of Addition ReactionsProblem 46Chapter 7, Problem 46
When Br2 adds to a cis alkene that has different substituents attached to each of the two sp2 carbons, such as cis-2-heptene, identical amounts of the two threo enantiomers are obtained even though Br- is more likely to add to the less sterically hindered carbon of the bromonium ion. Explain why identical amounts of the two enantiomers are obtained.
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The addition of Br₂ to a cis-alkene, such as cis-2-heptene, proceeds via an anti-addition mechanism. This means that the two bromine atoms will add to opposite sides of the double bond plane.
The reaction begins with the formation of a bromonium ion intermediate. When Br₂ approaches the double bond, one bromine atom forms a three-membered cyclic bromonium ion with the two sp² carbons, while the other bromine atom becomes a bromide ion (Br⁻).
The bromonium ion is symmetrical in terms of its geometry, but the substituents on the sp² carbons of the alkene are different. The bromide ion (Br⁻) will attack the more sterically accessible carbon of the bromonium ion, leading to the opening of the three-membered ring.
Because the attack by Br⁻ occurs from the opposite side of the bromonium ion (anti-addition), the stereochemistry of the product is determined by the relative positions of the substituents on the original alkene. For cis-2-heptene, this results in the formation of two threo enantiomers.
The two enantiomers are formed in equal amounts because the bromonium ion intermediate is planar and symmetrical, allowing the bromide ion to attack either carbon with equal probability. This leads to a racemic mixture of the two threo enantiomers.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Bromonium Ion Formation
When bromine (Br2) adds to an alkene, it first forms a cyclic bromonium ion intermediate. This occurs when one bromine atom forms a bond with one of the sp2 carbons, creating a three-membered ring. The formation of this ion is crucial because it influences the regioselectivity and stereochemistry of the addition reaction, leading to the formation of different products depending on how the bromonium ion is attacked by the nucleophile.
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Stereochemistry of Alkene Addition
The stereochemistry of alkene addition reactions is determined by the orientation of the reactants and the intermediates formed during the reaction. In the case of cis-2-heptene, the addition of bromine leads to the formation of two threo enantiomers due to the symmetrical nature of the bromonium ion. This symmetry allows for the nucleophile to attack from either side of the bromonium ion, resulting in equal amounts of both enantiomers despite the steric hindrance considerations.
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Enantiomers and Optical Activity
Enantiomers are pairs of molecules that are non-superimposable mirror images of each other, often differing in their optical activity. In the context of the bromination of cis-2-heptene, the formation of two threo enantiomers means that the reaction produces equal amounts of both forms. This is significant because it highlights how the reaction conditions and the nature of the intermediates can lead to racemic mixtures, where the optical activities of the enantiomers cancel each other out.
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Related Practice
Textbook Question
What stereoisomers would you expect to obtain from each of the following reactions?
c.
d.
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Textbook Question
What is the product of the reaction of maleate and H2O when H2SO4 is used as a catalyst instead of fumarase?
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Textbook Question
a. What is the major product obtained from the reaction of propene and Br2 plus excess Cl-?
b. Indicate the relative amounts of the stereoisomers that are obtained.
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Textbook Question
The reaction of 2-ethyl-1-pentene with Br2, with H2 + Pd/C, or with R2BH/THF followed by aqueous HO- + H2O2 leads to a racemic mixture. Explain why a racemic mixture is obtained in each case.
Textbook Question
Using a sample of trans-2-pentene, how could you prove that the addition of Br2 forms a cyclic bromonium ion intermediate rather than a carbocation intermediate?
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