Textbook Question
(••) Predict the product(s) that would result when the alkenes are allowed to react under the following conditions: (iii) Br₂, H₂O(h)
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Ch. 9 - Alkenes II: Oxidation and Reduction
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471
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Table of contents
- Ch. 2 - General Chemistry Translated: Finding the Electrons114
- Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis109
- Ch. 4 - Acids and Bases: Electron Flow144
- Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics118
- Ch. 6 - Stereoisomerism: Arrangement of Atoms in Space112
- Ch. 7 - Principles of Green (Organic) Chemistry3
- Ch. 8 - Alkenes I: Properties and Electrophilic Additions158
- Ch. 9 - Alkenes II: Oxidation and Reduction150
- Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions101
- Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions108
- Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes172
- Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination239
- Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry54
- Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy93
- Ch. 16 - Metals in Organic Chemistry48
- Ch. 17 - Carbonyl Addition Reactions: Aldehydes and Ketones45
- Ch. 18 - Nucleophilic Acyl Substitution I: Carboxylic Acids18
- Ch. 19 - Nucleophilic Acyl Substitution II: Carboxylic Acid Derivatives39
- Ch. 20 - Enolates: Carbonyl Addition and Substitution13
- Ch. 21 - Conjugated Systems I: Stability and Addition Reactions56
- Ch. 22 - Conjugated Systems II: Pericyclic Reactions54
- Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions64
- Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring62
- Ch. 25 - Amines: Structure, Reactions, and Synthesis27
- Ch. 26 - Amino Acids, Proteins, and Peptide Synthesis9
- Ch. 27 - Carbohydrates, Nucleic Acids, and Lipids54
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
Chapter 8, Problem 9a
In contrast to the addition of Br₂ the addition of HBr [Section 8.3] is not stereospecific. Why?
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The addition of Br₂ to an alkene is stereospecific because it proceeds through a bromonium ion intermediate. This intermediate is a three-membered ring structure that forces the incoming nucleophile (Br⁻) to attack from the opposite side of the ring, leading to anti-addition. This ensures a specific stereochemical outcome.
In contrast, the addition of HBr to an alkene is not stereospecific because it proceeds through a carbocation intermediate. The formation of the carbocation is planar, allowing the nucleophile (Br⁻) to attack from either side of the plane, leading to a mixture of stereoisomers.
The image shows that the addition of Br₂ results in a single stereochemical outcome (anti-addition), while the addition of HBr results in a mixture of stereoisomers due to the non-stereospecific nature of the reaction.
The key difference lies in the intermediates: the bromonium ion in the Br₂ addition restricts the approach of the nucleophile, while the planar carbocation in the HBr addition allows for attack from both sides.
Thus, the addition of HBr is not stereospecific because the planar carbocation intermediate does not impose any stereochemical constraints on the reaction.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Stereospecificity
Stereospecificity refers to a reaction where the stereochemistry of the reactants influences the stereochemistry of the products. In stereospecific reactions, a specific stereoisomer of a reactant will yield a specific stereoisomer of the product. This is crucial in understanding reactions like the addition of Br₂, where the formation of a cyclic bromonium ion leads to a specific stereochemical outcome.
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Electrophilic Addition Reactions
Electrophilic addition reactions involve the addition of an electrophile to a nucleophile, typically across a double bond. In the case of HBr addition, the hydrogen atom acts as the electrophile, while the double bond acts as the nucleophile. Unlike Br₂, which forms a cyclic intermediate leading to stereospecificity, HBr does not form such intermediates, resulting in a non-stereospecific addition.
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Reaction Mechanism
The reaction mechanism describes the step-by-step process by which reactants are converted into products. For HBr addition, the mechanism involves the protonation of the alkene followed by nucleophilic attack by bromide. This mechanism does not favor a specific stereochemical pathway, leading to a mixture of products, unlike the more defined pathway seen in Br₂ addition.
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Related Practice
Textbook Question
Predict the product(s) of each of the following reactions, making sure to indicate the relative stereochemical outcome. Indicate any racemic mixtures by drawing both enantiomers.
(a)
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Textbook Question
In which of the two steps in the alkene halogenation mechanism does a redox reaction occur?
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Textbook Question
Provide arrow-pushing mechanisms for Assessments 9.10(b) and 9.10(c) that rationalize the regioselective and stereospecific formation of each halohydrin.
(b)
(c)
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Textbook Question
Explain why water attacks the carbon of the bromonium ion as opposed to the bromonium ion itself in the second step of halohydrin formation.
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Textbook Question
Provide an arrow-pushing mechanism that rationalizes the stereospecific formation of each dihalide in Assessment 9.6.
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