Textbook Question
How does the carbonyl in mCPBA weaken the O―O σ bond (i.e., make a better leaving group)?
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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 12
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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Understand the structure of the bromonium ion: The bromonium ion is a three-membered cyclic intermediate formed during the reaction of an alkene with bromine (Br₂). It consists of a positively charged bromine atom bonded to two carbons of the former double bond, creating significant ring strain and partial positive charges on the carbons.
Recognize the role of water as a nucleophile: Water is a polar molecule with lone pairs of electrons on the oxygen atom, making it a good nucleophile. It is also present in excess in the reaction medium, increasing the likelihood of its participation in the reaction.
Analyze the electrophilic sites in the bromonium ion: The carbon atoms in the bromonium ion are partially positive due to the electron-withdrawing nature of the bromine atom. These carbons are more electrophilic than the bromine atom itself, making them the preferred site for nucleophilic attack.
Consider steric and electronic factors: The bromonium ion is a strained three-membered ring, and breaking the ring by attacking one of the carbons relieves this strain. Additionally, attacking the bromine atom directly would not relieve the strain and is less favorable.
Conclude why water attacks the carbon: Water attacks the carbon of the bromonium ion because it is more electrophilic and breaking the ring relieves strain. This leads to the formation of a halohydrin, where the hydroxyl group (-OH) is added to one carbon and the bromine atom to the other.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Bromonium Ion
A bromonium ion is a cyclic intermediate formed during the electrophilic addition of bromine to an alkene. It features a positively charged bromine atom that is bonded to two carbon atoms, creating a three-membered ring. This structure is highly reactive and stabilizes the positive charge through resonance, making it a key player in the mechanism of halohydrin formation.
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Nucleophilic Attack
Nucleophilic attack refers to the process where a nucleophile, such as water, donates a pair of electrons to an electrophile, leading to the formation of a new bond. In the context of halohydrin formation, water acts as a nucleophile that attacks the more electrophilic carbon of the bromonium ion. This step is crucial as it determines the final structure of the halohydrin product.
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Regioselectivity
Regioselectivity is the preference of a chemical reaction to yield one structural isomer over others when multiple products are possible. In the case of halohydrin formation, water preferentially attacks the more substituted carbon of the bromonium ion due to greater stability and steric factors. This selectivity influences the final product's configuration and properties.
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Related Practice
Textbook Question
Suggest an alkene that could be used to make each of the following halohydrins.
(a)
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Textbook Question
Suggest an alkene that could be used to make each of the following halohydrins.
(c)
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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 contrast to the addition of Br₂ the addition of HBr [Section 8.3] is not stereospecific. Why?
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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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