Textbook Question
What stereoisomers are obtained from the following reactions?
c. (E)-3-methyl-2-pentene + HBr
d. cis-3-hexene + HBr
1052
views
Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions
Bruice8th EditionOrganic ChemistryISBN: 9780135213711
Not the one you use?Change textbookSelect a different textbook
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
All textbooks
Bruice 8th EditionCh. 6 - The Reactions of Alkenes • The Stereochemistry of Addition ReactionsProblem 44
Bruice 8th EditionCh. 6 - The Reactions of Alkenes • The Stereochemistry of Addition ReactionsProblem 44Chapter 7, Problem 44
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?
Verified step by step guidance1
Step 1: Understand the reaction mechanism. The addition of Br₂ to an alkene like trans-2-pentene can proceed via two possible intermediates: a cyclic bromonium ion or a carbocation. The goal is to determine which intermediate is formed during the reaction.
Step 2: Analyze the stereochemical outcome of the reaction. If a cyclic bromonium ion intermediate is formed, the addition of Br₂ will result in an anti-addition (the two bromine atoms will add to opposite sides of the double bond). This is due to the backside attack of the nucleophile (Br⁻) on the bromonium ion.
Step 3: Perform the reaction and isolate the product. React trans-2-pentene with Br₂ in an inert solvent like CCl₄. After the reaction, isolate the dibromide product formed.
Step 4: Determine the stereochemistry of the product. Use techniques like NMR spectroscopy or X-ray crystallography to analyze the stereochemistry of the dibromide product. If the product is exclusively or predominantly anti (with bromines on opposite sides of the former double bond), it supports the formation of a cyclic bromonium ion intermediate.
Step 5: Consider the alternative mechanism. If a carbocation intermediate were formed, the addition of Br₂ could lead to a mixture of syn- and anti-products due to free rotation around the single bond in the planar carbocation. The absence of syn-products in the reaction strongly supports the bromonium ion mechanism.
Verified video answer for a similar problem:
This video solution was recommended by our tutors as helpful for the problem above.
Video duration:
7mWas this helpful?
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Bromonium Ion Mechanism
The bromonium ion mechanism involves the formation of a cyclic intermediate when bromine (Br2) adds to an alkene. In this process, one bromine atom forms a bond with one of the carbon atoms in the double bond, while the other bromine atom is temporarily bonded to the other carbon, creating a three-membered ring structure. This intermediate is more stable than a carbocation due to the positive charge being shared between the two carbons.
Recommended video:
3:39
Metal Ion Catalysis Concept 1
Carbocation Stability
Carbocations are positively charged species that can form during electrophilic addition reactions. Their stability is influenced by the degree of substitution; tertiary carbocations are more stable than secondary or primary ones due to hyperconjugation and inductive effects. In the context of bromine addition, the formation of a carbocation would be less favorable compared to the formation of a bromonium ion, especially in less substituted alkenes like trans-2-pentene.
Recommended video:
Guided course
05:58Determining Carbocation Stability
Stereochemistry of Addition Reactions
The stereochemistry of addition reactions is crucial for understanding the outcome of reactions involving alkenes. The formation of a bromonium ion leads to a specific stereochemical outcome, as the bromine atoms add to the same face of the double bond, resulting in anti-addition. This can be experimentally verified by analyzing the stereochemical configuration of the product, which would differ if a carbocation intermediate were formed.
Recommended video:
1:38Polymer Stereochemistry Concept 1
Related Practice
Textbook Question
What stereoisomers would you expect to obtain from each of the following reactions?
c.
d.
1103
views
Textbook Question
What stereoisomers are obtained from hydroboration–oxidation of the following compounds? Assign an R or S configuration to each asymmetric center.
d. (Z)-3,4-dimethyl-3-hexene
1581
views
Textbook Question
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.
1965
views
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.
1378
views
Textbook Question
What stereoisomers are obtained from hydroboration–oxidation of the following compounds? Assign an R or S configuration to each asymmetric center.
c. cis-2-butene
1242
views