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

Radical addition of hydrogen bromide to alkenes using hydroperoxides or peroxides proceeds via an anti-Markovnikov addition, where the bromine is directed to the less substituted carbon, forming alkyl bromides.
The observed regioselectivity can be explained by considering the radical stability and steric effect.
Radical stability can be understood from the peroxide effect. The peroxide initiates a radical reaction, allowing the bromine radical to add first to the less-substituted carbon, generating a more stable tertiary radical intermediate.
Similar is the influence of the steric effect. A large bromine radical adds to the less-hindered carbon, minimizing the van der Waals repulsions and forming a more stable radical, thereby justifying the observed anti-Markovnikov addition.
Notably, the radical addition of hydrogen bromide for some alkenes produces a new chiral center.
However, the addition is not stereoselective, as a bromine radical can add from either face of the alkene, giving a racemic mixture of enantiomers.
Interestingly, the peroxide-promoted anti-Markovnikov addition is only observed with hydrogen bromide. Other hydrogen halides do not show this addition.

radical anti-markovnikov addition to alkenes: overview

Radical Anti-Markovnikov Addition to Alkenes: Overview

The addition of hydrogen bromide to alkenes in the presence of hydroperoxides or peroxides proceeds via an anti-Markovnikov pathway and yields alkyl bromides.
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The observed regioselectivity can be explained based on the radical stability and steric effect. From the radical stability perspective, adding hydrogen bromide in the presence of peroxide directs the bromine radical at the less substituted carbon via a more stable tertiary radical intermediate. Similarly, in the steric framework, the larger bromine radical is bonded to the less substituted carbon, which forms a more stable radical, minimizing the steric effect. Therefore, the formation of a stable radical intermediate determines the regioselective outcome, justifying the observed anti-Markovnikov orientation.
The radical addition of hydrogen bromide is not stereoselective. If a new chiral center is produced, the addition of bromine radical can occur from either face of the alkene. So, forming a new chiral center leads to a racemic mixture of enantiomeric products. Moreover, the anti-Markovnikov addition is only feasible with hydrogen bromide. The reaction is not observed with other hydrogen halides.

The addition of hydrogen bromide to alkenes in the presence of hydroperoxides or peroxides proceeds via an anti-Markovnikov pathway and yields alkyl ...

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

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JoVE Core

Organic Chemistry

Unit 1

Radical Chemistry

KEY TERMS AND CONCEPTS

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...

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EPR Hyperfine Splitting in Organic Radicals

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Radical Halogenation: Chiral Products

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radical substitution: allylic chlorination

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Allylic Chlorination by Radical Chain Reaction

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Alkyl Halide Reduction with Tributyltin Hydride

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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

Anti-Markovnikov addition of hydrogen halides to alkenes depends on thermodynamics. In this radical reaction, only HBr makes the overall process feasible. With HCl and HI, the same addition is thermodynamically unfavorable. Temperature affects whether the reaction is spontaneous. Spontaneity is measured with Gibbs free energy, or ΔG. When ΔG is negative, the reaction can occur on its own. ΔG is determined by two terms: ΔH, which is the enthalpy change, and −TΔS, which is the entropy term. The...

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

alkenes-via-reductive-coupling-aldehydes-or-ketones-mcmurry

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

hydroxy-ketones-via-reductive-coupling-esters-acyloin-condensation

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

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Peroxide Effect in HBr Addition to Alkenes

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