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

Hidroperoksitler veya peroksitler kullanılarak alkenlere hidrojen bromürün radikal ilavesi, bromun daha az ikame edilmiş karbona yönlendirildiği ve alkil bromürler oluşturduğu bir anti-Markovnikov ilavesi yoluyla ilerler.
Gözlenen bölgesel seçicilik, radikal stabilite ve sterik etki dikkate alınarak açıklanabilir.
Radikal stabilite peroksit etkisinden anlaşılabilir. Peroksit, brom radikalinin önce daha az ikame edilmiş karbona eklenmesine izin vererek daha kararlı bir üçüncül radikal ara maddesi oluşturan bir radikal reaksiyonu başlatır.
Benzer şekilde sterik etkinin etkisi de vardır. Büyük bir brom radikali, daha az engellenmiş karbona katkıda bulunur, van der Waals itmelerini en aza indirir ve daha kararlı bir radikal oluşturur, böylece gözlemlenen anti-Markovnikov ilavesini haklı çıkarır.
Özellikle, bazı alkenler için hidrojen bromürün radikal ilavesi, yeni bir kiral merkez üretir.
Bununla birlikte, bir brom radikali alkenin her iki yüzünden de eklenebildiğinden, rasemik bir enantiyomer karışımı verdiğinden, ekleme stereoselektif değildir.
İlginç bir şekilde, peroksit ile teşvik edilen anti-Markovnikov ilavesi sadece hidrojen bromür ile gözlenir. Diğer hidrojen halojenürler bu ilaveyi göstermez.

alkenlere radikal anti-markovnikov İlavesi: genel bakış

Radical Anti-Markovnikov Addition to Alkenes: Overview

Hidroperoksitlerin veya peroksitlerin varlığında alkenlere hidrojen bromürün eklenmesi, bir anti-Markovnikov yolu üzerinden ilerler ve alkil bromürler verir.

<img alt="Figure1" src="/files/ftp_upload/12902/12902_PT_Figure_1.svg" style="text-align: center; height: 50px;">

Gözlenen bölgesel seçicilik, radikal stabilite ve sterik etkiye dayanarak açıklanabilir. Radikal stabilite perspektifinden bakıldığında, peroksit varlığında hidrojen bromürün eklenmesi, brom radikalini daha stabil bir tersiyer radikal ara ürünü yoluyla daha az ikame edilmiş karbona yönlendirir. Benzer şekilde, sterik çerçevede, daha büyük brom radikali daha az ikame edilmiş karbona bağlanır, bu da daha kararlı bir radikal oluşturur ve sterik etkiyi en aza indirir. Bu nedenle, istikrarlı bir radikal ara maddenin oluşumu, gözlemlenen Markovnikov karşıtı yönelimi haklı çıkararak bölge seçici sonucu belirler.

Hidrojen bromürün radikal ilavesi stereoselektif değildir. Yeni bir kiral merkez üretilirse, alkenin her iki yüzünden de brom radikali ilavesi meydana gelebilir. Böylece yeni bir kiral merkez oluşturmak, enantiyomerik ürünlerin rasemik bir karışımına yol açar. Üstelik anti-Markovnikov ilavesi yalnızca hidrojen bromür ile mümkündür. Diğer hidrojen halojenürlerle reaksiyon gözlenmez.

Alkenlere Radikal Anti-Markovnikov İlavesi: Genel Bakış

Hidroperoksitler veya peroksitler varlığında alkenlere hidrojen bromür ilavesi, bir anti-Markovnikov yolu yoluyla ilerler ve alkil ...

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

Education

JoVE Core

Organic Chemistry

Organik Kimya

Unit 1

Ünite 1

Radical Chemistry

Radikal Kimya

KEY TERMS AND CONCEPTS

TEMEL TERİMLER VE KAVRAMLAR

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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Radical Geometry: Planarity and Inversion

Radical geometry depends on the electronic structure around the radical center. An unpaired electron, like a lone pair, affects the shape of the molecule. For a trivalent radical, the structure falls between the geometries of carbocations and carbanions. A carbocation with sp2 hybridization is trigonal planar. A carbanion with sp3 hybridization is trigonal pyramidal. The difference comes from the number of nonbonding electrons. The carbocation has none, while the carbanion has two. A carbon...

electron paramagnetic resonance (epr) spectroscopy: organic radicals

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EPR Hyperfine Splitting in Organic Radicals

Electron paramagnetic resonance (EPR) spectroscopy can show how an unpaired electron behaves in an organic radical. A simple radical may give a single peak because the electron moves between two spin energy states. In many radicals, the unpaired electron also interacts with nearby spin-active nuclei. This interaction is called hyperfine coupling. It causes hyperfine splitting, which divides the EPR signal into multiplets, or groups of peaks. The number of peaks follows the pattern 2nI + 1,...

radical formation: overview

How Radicals Are Generated

Radicals are generated when bonds break in specific ways or when one radical leads to another. In homolytic bond cleavage, a bond splits evenly and forms radicals. In heterolytic bond cleavage, the bond breaks to form ions instead. A fishhook arrow shows the movement of a single electron during homolytic cleavage. Radicals can come from spin-paired molecules by homolysis or by electron transfer. Homolysis forms two radicals. Electron transfer adds an electron to the molecule, and this process...

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Radical Abstraction in Chain Reactions

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radical reactivity: overview

Radical Reaction Pathways and Stability

Radical reactions follow a small set of pathways that help highly reactive species become more stable. A radical can react with another radical, with a spin-paired molecule, or through unimolecular decomposition, which means one radical species breaks down on its own. In radical-radical coupling, two radicals join to form a spin-paired molecule. In a radical reaction with a spin-paired molecule, a new radical and a new spin-paired molecule are formed. In radical decomposition, one radical...

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Selective MnO2 Oxidation of Alcohols

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Allylic Chlorination by Radical Chain Reaction

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Alkyl Halide Reduction with Tributyltin Hydride

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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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vicinal diols via reductive coupling of aldehydes or ketones: pinacol coupling overview

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Pinacol Coupling and Solvent Effects

Pinacol coupling is a radical dimerization reaction that joins aldehydes or ketones to form vicinal diols. Wilhelm Rudolph Fittig first discovered this reaction in 1859. It is a reductive coupling, which means the molecules are joined after they gain electrons. The reaction begins with a single electron transfer from metals such as sodium or magnesium to an aldehyde or ketone. This forms a ketyl, a radical anion. In a ketyl, the carbon atom can carry radical character while the oxygen atom...

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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Acyloin Condensation: Ester Coupling Pathway

Acyloin condensation is a reaction that joins two ester molecules to form an alpha-hydroxy ketone. The product is also called an acyloin. In this reaction, sodium metal is used in an aprotic solvent, which supports the reductive coupling step. The reaction is related to the pinacol and McMurry reactions, which also involve reductive coupling of ketones or aldehydes. Here, the ester first becomes a ketyl. These ketyl species undergo radical dimerization and form an unstable tetrahedral...

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 Organik Kimya, kısa ve anlaşılması kolay animasyonlu video dersleri kullanarak stereokimya, reaksiyonlar ve mekanizmaların temel kavramlarını açıklar. Buna ek olarak, bilim insanlarının yer aldığı videolar, ilgili kavramların günümüz laboratuvarlarında gerçekleştirilen klasik ve orijinal araştırma deneylerinde nasıl uygulandığını göstermektedir.

Peroxide Effect in HBr Addition to Alkenes

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