Name: peroxide effect in hbr addition to alkenes
Uploaded: 2023-04-30T11:22:35.000Z
Duration: 1 min 25 s
Channel: JoVE

L’addition radicale de bromure d’hydrogène aux alcènes à l’aide d’hydroperoxydes ou de peroxydes se fait par l’intermédiaire d’une addition anti-Markovnikov, où le brome est dirigé vers le carbone le moins substitué, formant des bromures d’alkyle.
La régiosélectivité observée peut s’expliquer en considérant la stabilité radicalaire et l’effet stérique.
La stabilité radicalaire peut être comprise à partir de l’effet peroxyde. Le peroxyde initie une réaction radicalaire, permettant au radical brome de s’ajouter d’abord au carbone moins substitué, générant ainsi un radical tertiaire intermédiaire plus stable.
L’influence de l’effet stérique est similaire. Un grand radical bromé s’ajoute au carbone moins entravé, minimisant les répulsions de van der Waals et formant un radical plus stable, justifiant ainsi l’addition anti-Markovnikov observée.
Notamment, l’ajout radical de bromure d’hydrogène pour certains alcènes produit un nouveau centre chiral.
Cependant, l’addition n’est pas stéréosélective, car un radical brome peut s’ajouter à partir de l’une ou l’autre face de l’alcène, ce qui donne un mélange racémique d’énantiomères.
Il est intéressant de noter que l’ajout d’anti-Markovnikov promu par le peroxyde n’est observé qu’avec du bromure d’hydrogène. D’autres halogénures d’hydrogène ne montrent pas cet ajout.

ajout radical anti-markovnikov aux alcènes : aperçu

Radical Anti-Markovnikov Addition to Alkenes: Overview

L'ajout de bromure d'hydrogène aux alcènes en présence d'hydroperoxydes ou de peroxydes se fait via une voie anti-Markovnikov et donne des bromures d'alkyle.



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<img alt="Figure1" src="/files/ftp_upload/12902/12902_PT_Figure_1.svg" style="text-align: center; height: 50px;">



La régiosélectivité observée peut être expliquée sur la base de la stabilité radicale et de l'effet stérique. Du point de vue de la stabilité radicale, l'ajout de bromure d'hydrogène en présence de peroxyde dirige le radical brome vers le carbone le moins substitué via un radical intermédiaire tertiaire plus stable. De même, dans la structure stérique, le radical brome le plus gros est lié au carbone le moins substitué, qui forme un radical plus stable, minimisant l'effet stérique. Par conséquent, la formation d’un intermédiaire radicalaire stable détermine le résultat régiosélectif, justifiant l’orientation anti-Markovnikov observée.

L'addition radicalaire de bromure d'hydrogène n'est pas stéréosélective. Si un nouveau centre chiral est produit, l’ajout de radical brome peut se produire sur l’une ou l’autre face de l’alcène. Ainsi, la formation d’un nouveau centre chiral conduit à un mélange racémique de produits énantiomères. De plus, l'addition anti-Markovnikov n'est réalisable qu'avec du bromure d'hydrogène. La réaction n'est pas observée avec d'autres halogénures d'hydrogène.

Ajout radical anti-Markovnikov aux alcènes : aperçu

L’ajout de bromure d’hydrogène aux alcènes en présence d’hydroperoxydes ou de peroxydes se fait par une voie anti-Markovnikov et donne des alkyles ...

radical-anti-markovnikov-addition-to-alkenes-overview

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Organic Chemistry

Chimie organique

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Radical Chemistry

Chimie radicalaire

KEY TERMS AND CONCEPTS

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peroxide effect in hbr addition to alkenes

Hydrogen bromide can add to alkenes through a peroxide-driven radical pathway. In this reaction, hydroperoxides or peroxides change the usual addition pattern and lead to alkyl bromides with anti-Markovnikov regiochemistry, which means bromine ends up on the less substituted carbon.

This regioselectivity can be explained by radical stability. When the bromine radical adds to the alkene at the less substituted carbon, the reaction forms a more stable tertiary radical intermediate. The same outcome also fits a steric view, because the larger bromine radical attaches in a way that reduces crowding and favors the less substituted carbon.

The reaction is not stereoselective. If the addition creates a new chiral center, the bromine radical can attack from either face of the alkene. As a result, the product is a racemic mixture of enantiomers. This anti-Markovnikov addition is observed with hydrogen bromide, but not with the other hydrogen halides.

Peroxide Effect in HBr Addition to Alkenes

peroxide-effect-in-hbr-addition-to-alkenes

radicals: electronic structure and geometry

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EPR Hyperfine Splitting in Organic Radicals

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radical anti-markovnikov addition to alkenes: overview

Hydrogen bromide can add to alkenes through a peroxide-driven radical pathway. In this reaction, hydroperoxides or peroxides change the usual addition pattern and lead to alkyl bromides with anti-Markovnikov regiochemistry, which means bromine ends up on the less substituted carbon. This regioselectivity can be explained by radical stability. When the bromine radical adds to the alkene at the less substituted carbon, the reaction forms a more stable tertiary radical intermediate. The same...

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radical-anti-markovnikov-addition-to-alkenes-mechanism

Hydrogen Bromide Addition to Alkenes by Radicals

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HBr Selectivity in Alkene Addition Thermodynamics

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Pinacol Coupling and Solvent Effects

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&alpha;-hydroxy ketones via reductive coupling of esters: acyloin condensation overview

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key terms and concepts

Learn how peroxides change HBr addition to alkenes, giving anti-Markovnikov alkyl bromides through a radical pathway and often a racemic mixture.

Peroxide Effect in HBr Addition to Alkenes | JoVE K12

organic chemistry

JoVE Core Chimie Organique explique les concepts de base de la stéréochimie, des réactions et des mécanismes à l'aide de leçons vidéo animées concises et faciles à comprendre. De plus, les vidéos de scientifiques à l'action présentent l'application de ces concepts grâce à des expériences de recherche classiques et originales réalisées dans des laboratoires modernes.

Peroxide Effect in HBr Addition to Alkenes

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