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

Die radikalische Addition von Bromwasserstoff an Alkene unter Verwendung von Hydroperoxiden oder Peroxiden erfolgt über eine Anti-Markovnikov-Zugabe, bei der das Brom auf den weniger substituierten Kohlenstoff geleitet wird, wodurch Alkylbromide gebildet werden.
Die beobachtete Regioselektivität kann unter Berücksichtigung der Radikalstabilität und des sterischen Effekts erklärt werden.
Die Radikalstabilität lässt sich an der Peroxidwirkung ablesen. Das Peroxid löst eine Radikalreaktion aus, die es dem Bromradikal ermöglicht, zuerst an den weniger substituierten Kohlenstoff anzuhängen, wodurch ein stabileres tertiäres Radikalzwischenprodukt entsteht.
Ähnlich ist der Einfluß der sterischen Wirkung. Ein großer Bromrest fügt dem weniger behinderten Kohlenstoff hinzu, minimiert die Van-der-Waals-Abstoßungen und bildet ein stabileres Radikal, wodurch die beobachtete Anti-Markovnikov-Zugabe gerechtfertigt wird.
Insbesondere die radikale Zugabe von Bromwasserstoff für einige Alkene erzeugt ein neues chirales Zentrum.
Die Zugabe ist jedoch nicht stereoselektiv, da ein Bromradikal von beiden Seiten des Alkens hinzufügen kann, wodurch ein racemisches Gemisch von Enantiomeren entsteht.
Interessanterweise wird die peroxidgeförderte Anti-Markovnikov-Zugabe nur bei Wasserstoffbromid beobachtet. Andere Halogenwasserstoffe zeigen diesen Zusatz nicht.

radikale anti-markownikow-addition an alkene: Überblick

Radical Anti-Markovnikov Addition to Alkenes: Overview

Die Addition von Bromwasserstoff an Alkene in Gegenwart von Hydroperoxiden oder Peroxiden verläuft über einen Anti-Markownikow-Weg und ergibt Alkylbromide.

<img alt="Figure1" src="/files/ftp_upload/12902/12902_PT_Figure_1.svg" style="text-align: center; height: 50px;">

Die beobachtete Regioselektivität lässt sich mit der Radikalstabilität und dem sterischen Effekt erklären. Unter dem Gesichtspunkt der Radikalstabilität führt die Zugabe von Bromwasserstoff in Gegenwart von Peroxid dazu, dass das Bromradikal über ein stabileres tertiäres Radikalzwischenprodukt zum weniger substituierten Kohlenstoff gelenkt wird. In ähnlicher Weise ist im sterischen Gerüst das größere Bromradikal an den weniger substituierten Kohlenstoff gebunden, wodurch ein stabileres Radikal entsteht und der sterische Effekt minimiert wird. Daher bestimmt die Bildung eines stabilen radikalischen Zwischenprodukts das regioselektive Ergebnis und rechtfertigt die beobachtete Anti-Markownikow-Orientierung.

Die radikalische Addition von Bromwasserstoff ist nicht stereoselektiv. Wenn ein neues Chiralitätszentrum entsteht, kann die Addition des Bromradikals von beiden Seiten des Alkens aus erfolgen. Die Bildung eines neuen Chiralitätszentrums führt also zu einer racemischen Mischung enantiomerer Produkte. Darüber hinaus ist die Anti-Markownikow-Addition nur mit Bromwasserstoff möglich. Bei anderen Halogenwassersubstanzenn wird die Reaktion nicht beobachtet.

Radikale Anti-Markownikow-Addition an Alkene: Überblick

Die Zugabe von Bromwasserstoff zu Alkenen in Gegenwart von Hydroperoxiden oder Peroxiden erfolgt über einen Anti-Markovnikov-Weg und liefert Alkylgehalt ...

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

Education

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JoVE Core

Organic Chemistry

Organische Chemie

Unit 1

Einheit 1

Radical Chemistry

Radikalchemie

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

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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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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 Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Peroxide Effect in HBr Addition to Alkenes

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