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Radical Substitution: Allylic Bromination

Radical Substitution: Allylic Bromination

In organic synthesis, the formation of products can be altered by changing the reaction conditions. For example, a dibromo addition product is formed when propene is treated with bromine at room temperature. In contrast, propene undergoes allylic substitution in non-polar solvents at high temperatures to give 3-bromopropene. In order to avoid the addition reaction, the bromine concentration must be kept as low as possible throughout the reaction. This can be achieved using N-bromosuccinimide...

Regioselectivity of Electrophilic Additions-Peroxide Effect

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In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.

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The reaction of hydrogen bromide with alkenes in the presence of hydroperoxides or peroxides proceeds via anti-Markovnikov addition. The radical chain reaction comprises initiation, propagation, and termination steps.
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Halogenation of Alkenes

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Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.

Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride

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Radical substitution reactions can be used to remove functional groups from molecules. The hydrogenolysis of alkyl halides is one such reaction, where the weak Sn–H bond in tributyltin hydride reacts with alkyl halides to form alkanes. Here, the reagent Bu3SnH yields tributyltin halide as a byproduct.
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Radical Formation: Addition

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Radicals can be formed by adding a radical to a spin-paired molecule. This is typically observed with unsaturated species, where the addition of a radical across the π bond leads to the production of a new radical by dissolving the π bond. For example, the addition of a Br radical to an alkene yields a carbon-centered radical.
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Updated: May 11, 2026

Efficient Synthesis of All-Carbon Quaternary Centers via the Conjugate Addition of Functionalized Monoorganozinc Bromides

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Published on: May 26, 2019

Reductive bromine atom-transfer reaction.

Shuhei Sumino¹, Akira Fusano, Ilhyong Ryu

¹Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan.

Organic Letters

|May 24, 2013

Summary

This summary is machine-generated.

Photochemical reactions involving atom-transfer radical (ATR) addition to alkenes yield unique addition/reduction products. Hydrogen bromide, generated during the process, is crucial for this reductive pathway.

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Area of Science:

Organic Chemistry
Photochemistry
Radical Reactions

Background:

Atom-transfer radical (ATR) reactions typically form C-C and C-X bonds at adjacent carbons in alkenes.
Conventional ATR reactions with alkenes and R-X do not typically yield addition/reduction products.

Purpose of the Study:

To investigate the outcome of ATR reactions on alkenes under photochemical conditions.
To identify the mechanism and key intermediates in the observed reductive ATR pathway.

Main Methods:

Carrying out atom-transfer radical reactions of alkenes with R-X.
Utilizing irradiation with a low-pressure mercury (Hg) lamp to initiate the reaction.
Analyzing reaction products to determine yield and structure.

Main Results:

Irradiation of ATR reactions with alkenes using a low-pressure Hg lamp resulted in addition/reduction products.
These reductive products were obtained in good yields.
Evidence suggests hydrogen bromide plays a key role in the reductive ATR pathway.

Conclusions:

Photochemical activation alters the typical outcome of ATR reactions on alkenes.
The formation of hydrogen bromide is critical for achieving addition/reduction products via a reductive ATR mechanism.
This study reveals a novel reductive pathway for ATR reactions under photochemical conditions.