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Which is more highly regioselective: reaction of an alkene with BH3 or with 9-BBN?
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Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions
Bruice8th EditionOrganic ChemistryISBN: 9780135213711
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TOC.tableOfContents
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding31
- 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 ReactionsProblema 21a
Bruice 8th EditionCh. 6 - The Reactions of Alkenes • The Stereochemistry of Addition ReactionsProblema 21aCapitolo 6, Problema 21a
How does the first step in the reaction of propene with Br2 differ from the first step in the reaction of propene with HBr?
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The first step in the reaction of propene with Br₂ involves the formation of a bromonium ion intermediate. This occurs through an electrophilic addition mechanism where the π-electrons of the double bond in propene attack one of the bromine atoms in Br₂, leading to the formation of a cyclic bromonium ion and a bromide ion (Br⁻).
In contrast, the first step in the reaction of propene with HBr involves the formation of a carbocation intermediate. The π-electrons of the double bond in propene attack the hydrogen atom in HBr, resulting in the addition of a proton (H⁺) to one of the carbon atoms in the double bond, leaving behind a carbocation on the other carbon atom.
The key difference lies in the intermediates formed: the reaction with Br₂ forms a bromonium ion, which is a three-membered cyclic structure, while the reaction with HBr forms a carbocation, which is a planar, positively charged species.
The driving force for the reaction with Br₂ is the polarization of the Br-Br bond due to the electron-rich double bond in propene, whereas the reaction with HBr is driven by the electrophilic nature of the proton (H⁺) in HBr.
Understanding these differences is crucial because the nature of the intermediate (bromonium ion vs. carbocation) determines the subsequent steps and the stereochemistry of the final product in each reaction.
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Electrophilic Addition Reactions
Electrophilic addition reactions are a fundamental type of reaction in organic chemistry where an electrophile reacts with a nucleophile, resulting in the formation of a more saturated product. In the case of alkenes like propene, the double bond acts as a nucleophile, attacking the electrophile, which can be a halogen or a hydrogen halide. Understanding this concept is crucial for analyzing how different electrophiles interact with alkenes.
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05:39
Features of Addition Mechanisms.
Mechanism of Bromination
The bromination of alkenes involves the formation of a cyclic bromonium ion intermediate, which is a key step in the reaction mechanism. This intermediate is formed when Br2 approaches the double bond, leading to a three-membered ring structure. The subsequent attack by a bromide ion results in the formation of a vicinal dibromide. This mechanism differs from that of hydrogen halides, where no such cyclic intermediate is formed.
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00:54Mechanism of Allylic Bromination.
Hydrogen Halide Addition Mechanism
The addition of hydrogen halides, such as HBr, to alkenes follows a different mechanism compared to halogen addition. In this case, the hydrogen atom from HBr acts as the electrophile, leading to the formation of a carbocation intermediate. The stability of this carbocation influences the regioselectivity of the reaction, which is not a factor in the bromination mechanism. Understanding these differences is essential for predicting the products of these reactions.
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05:39Features of Addition Mechanisms.
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Each of the following reactions has two nucleophiles that could add to the intermediate formed by the reaction of the alkene with an electrophile. What is the major product of each reaction?
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What will be the product of the preceding reaction if HBr is used in place of Br2?
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Why are Na+ and K+ unable to form covalent bonds?
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