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

Radykalne dodawanie bromowodoru do alkenów za pomocą wodoronadtlenków lub nadtlenków przebiega poprzez addycję anty-Markovnikova, gdzie brom jest kierowany do mniej podstawionego węgla, tworząc bromki alkilowe.
Obserwowaną regioselektywność można wyjaśnić, biorąc pod uwagę stabilność rodnikową i efekt steryczny.
Stabilność radykalną można zrozumieć na podstawie efektu nadtlenkowego. Nadtlenek inicjuje reakcję rodnikową, pozwalając rodnikowi bromu dodać się najpierw do mniej podstawionego węgla, generując bardziej stabilny trzeciorzędowy rodnik pośredni.
Podobny jest wpływ efektu sterycznego. Duży rodnik bromowy dodaje się do mniej zaburzonego węgla, minimalizując odpychanie van der Waalsa i tworząc bardziej stabilny rodnik, uzasadniając w ten sposób obserwowany dodatek anty-Markownikowa.
Warto zauważyć, że radykalne dodanie bromowodoru do niektórych alkenów tworzy nowe centrum chiralne.
Jednak dodatek nie jest stereoselektywny, ponieważ rodnik bromu może dodawać się z dowolnej powierzchni alkenu, dając racemiczną mieszaninę enancjomerów.
Co ciekawe, promowana nadtlenkiem addycja anty-Markownikowa jest obserwowana tylko w przypadku bromowodoru. Inne halogenki wodoru nie wykazują tego dodatku.

rodnikowa addycja anty-markownikowa do alkenów: przegląd

Radical Anti-Markovnikov Addition to Alkenes: Overview

Dodatek bromowodoru do alkenów w obecności wodoronadtlenków lub nadtlenków przebiega szlakiem anty-Markownikowa i daje bromki alkilu.

<img alt="Figure1" src="/files/ftp_upload/12902/12902_PT_Figure_1.svg" style="text-align: center; height: 50px;">

Zaobserwowaną regioselektywność można wyjaśnić na podstawie stabilności rodnikowej i efektu sterycznego. Z punktu widzenia stabilności rodnikowej dodanie bromowodoru w obecności nadtlenku kieruje rodnik bromowy na mniej podstawiony węgiel poprzez bardziej stabilny trzeciorzędowy rodnik pośredni. Podobnie w strukturze sterycznej większy rodnik bromu jest związany z mniej podstawionym węglem, co tworzy bardziej stabilny rodnik, minimalizując efekt steryczny. Dlatego utworzenie stabilnego rodnikowego związku pośredniego determinuje wynik regioselektywny, uzasadniający obserwowaną orientację anty-Markownikowa.

Rodnikowa addycja bromowodoru nie jest stereoselektywna. Jeśli zostanie wytworzone nowe centrum chiralne, dodanie rodnika bromu może nastąpić z dowolnej strony alkenu. Zatem utworzenie nowego centrum chiralnego prowadzi do racemicznej mieszaniny produktów enancjomerycznych. Ponadto addycja anty-Markownikowa jest możliwa tylko w przypadku bromowodoru. Reakcji nie obserwuje się w przypadku innych halogenowodorów.

Rodnikowa addycja anty-Markownikowa do alkenów: przegląd

Dodawanie bromowodoru do alkenów w obecności wodoronadtlenków lub nadtlenków przebiega szlakiem anty-Markovnikowa i daje alkilo ...

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

Education

JoVE Core

Organic Chemistry

Chemia organiczna

Unit 1

Jednostka 1

Radical Chemistry

Chemia rodnikowa

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...

radical anti-markovnikov addition to alkenes: mechanism

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

Hydrogen Bromide Addition to Alkenes by Radicals

Hydrogen bromide can add to alkenes through a radical chain reaction when hydroperoxides or peroxides are present. This pathway gives anti-Markovnikov addition, which means the bromine ends up on the less substituted side of the double bond. The reaction begins with chain initiation. First, mild heating breaks the weak peroxide bond by homolytic cleavage, which means each atom keeps one electron and two alkoxy radicals form. Next, an alkoxy radical abstracts a hydrogen atom and forms a bromine...

radical anti-markovnikov addition to alkenes: thermodynamics

radical-anti-markovnikov-addition-to-alkenes-thermodynamics

HBr Selectivity in Alkene Addition Thermodynamics

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Pinacol Coupling and Solvent Effects

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alkenes via reductive coupling of aldehydes or ketones: mcmurry reaction

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Titanium-Coupled Alkenes from Carbonyls

The McMurry reaction turns aldehydes or ketones into alkenes with titanium metal. It first forms a diol-like intermediate through a coupling step, and then removes oxygen to make the double bond. In some cases, this process can be compared to pinacol coupling, which also joins carbonyl compounds. Titanium acts as a source of electrons in the first step. It transfers a single electron to the carbonyl group and forms a ketyl intermediate. That intermediate can lead to a vicinal diol, which is a...

&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 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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