Name: electrophilic radicals and alkene reactions
Uploaded: 2023-04-30T11:21:14.000Z
Duration: 1 min 2 s
Channel: JoVE

Electrophilic radicals have an electron-withdrawing group attached to the radical center. These radicals readily react with nucleophilic alkenes.
For instance, the malonate radical, formed from diethyl choloromalonate, reacts quickly with vinyl ether, which possesses an electron-donating oxygen substituent.
This reaction is favored because the electron-deficient, electrophilic radical has a low-energy SOMO, which readily interacts with the high-energy HOMO of the electron-rich, nucleophilic alkene.
Similar SOMO-HOMO interactions are also observed for non-carbon-centered radicals.
For instance, when a chlorine radical reacts with propionic acid, it abstracts a hydrogen from the terminal carbon.
This happens because the low-energy SOMO of the electrophilic chlorine radical interacts well with the high-energy HOMO of the C&#8211;H bond of the terminal methyl group.

radical reactivity: electrophilic radicals

Radical Reactivity: Electrophilic Radicals

Radicals adjacent to electron&#8208;withdrawing groups are called electrophilic radicals. These radicals readily react with nucleophilic alkenes. For example, the malonate radical, in which the radical center is flanked by two electron&#8208;withdrawing groups, reacts readily with butyl vinyl ether, which consists of an electron&#8208;donating oxygen substituent. The reaction between electrophilic malonate radical and nucleophilic vinyl ether is favored because the radical has a low&#8208;energy SOMO, which interacts best with the high&#8208;energy HOMO of the alkene.
The non&#8208;carbon&#8208;centered electrophilic radicals also exhibit similar SOMO&#8211;HOMO interactions. For instance, the non&#8208;carbon&#8208;centered chlorine radical abstracts a hydrogen atom from the terminal methyl group of propionic acid. This is because chlorine radical has a low&#8208;energy SOMO, and the C&#8211;H bond of the terminal methyl group has a high&#8208;energy HOMO. Therefore, interactions between low&#8208;energy SOMO of the chlorine radical and high&#8208;energy HOMO of the terminal C&#8211;H bond favor chlorine attack on the terminal carbon of propionic acid.

Radicals adjacent to electron&#8208;withdrawing groups are called electrophilic radicals. These radicals readily react with nucleophilic alkenes. For ...

radical-reactivity-electrophilic-radicals

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JoVE Core

Organic Chemistry

Unit 1

Radical Chemistry

KEY TERMS AND CONCEPTS

electrophilic radicals and alkene reactions

Electrophilic radicals are radicals next to electron-withdrawing groups. These radicals react readily with nucleophilic alkenes. A key example is the malonate radical, where the radical center is flanked by two electron-withdrawing groups. It reacts easily with butyl vinyl ether, which has an electron-donating oxygen substituent.

This reaction is favored because the malonate radical has a low-energy SOMO, or singly occupied molecular orbital. That low-energy orbital matches best with the high-energy HOMO, or highest occupied molecular orbital, of the alkene. In this case, the SOMO-HOMO interaction helps the electrophilic radical add to the nucleophilic alkene.

Similar orbital effects also appear with non-carbon-centered electrophilic radicals. Chlorine radical is one example. It abstracts a hydrogen atom from the terminal methyl group of propionic acid because the chlorine radical has a low-energy SOMO, while the terminal C-H bond has a high-energy HOMO. This interaction favors chlorine attack at the terminal carbon of propionic acid.

Electrophilic Radicals and Alkene Reactions

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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

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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...

radical formation: homolysis

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radical formation: abstraction

Radical Abstraction in Chain Reactions

Radical abstraction helps drive radical chain reactions. In this process, a relatively unstable radical removes an atom from a compound and forms a new radical that is more stable. The atom taken in this step can be hydrogen or a halogen. A radical initiator can first form radicals by homolysis, which is the breaking of a spin-paired molecule into two radicals. That is different from abstraction. In abstraction, a radical reacts with a spin-paired molecule and produces one new radical plus one...

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Radical Addition and Spin Conservation

Radical addition can form a new radical when an unpaired electron adds to a spin-paired molecule. This is often seen with unsaturated species. In these reactions, the radical adds across a pi bond and breaks that bond to make a new radical. A bromine radical adding to an alkene is a clear example. The product is a carbon-centered radical. Radical reactions also follow spin conservation, much like chemical reactions follow charge conservation. If the starting material has an unpaired electron,...

radical formation: elimination

Radicals from Elimination Reactions

Radicals can form through elimination reactions. This route is the opposite of the addition route. It is driven by the instability of the radical. Dibenzoyl peroxide is one example of this process. It first undergoes homolysis, which is an even split of a bond, to give a pair of unstable radicals. Because the radical is unstable, it then undergoes elimination on its own. During elimination, a C-C bond breaks. The cleavage occurs between the alpha and beta positions relative to the radical...

radical reactivity: overview

Radical Reaction Pathways and Stability

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Radical reaction selectivity can change when the concentrations of the reactants change. In this chemistry example, an alkyl halide reacts with an alkene in the presence of tin hydride and AIBN, an initiator that helps start the radical process. The reaction begins when a tin radical is formed. The tin radical removes the halogen from the alkyl halide. This step creates an alkyl radical. That radical then has two possible paths. It can react with tin hydride to form an alkane, or it can add to...

Electrophilic radicals are radicals next to electron-withdrawing groups. These radicals react readily with nucleophilic alkenes. A key example is the malonate radical, where the radical center is flanked by two electron-withdrawing groups. It reacts easily with butyl vinyl ether, which has an electron-donating oxygen substituent. This reaction is favored because the malonate radical has a low-energy SOMO, or singly occupied molecular orbital. That low-energy orbital matches best with the...

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Nucleophilic radicals are electron-rich radicals that form next to electron-donating groups. They react readily with electrophilic alkenes, which are electron-poor double bonds. The key driving force is a SOMO–LUMO interaction. The high-energy SOMO, or singly occupied molecular orbital, of the radical overlaps with the low-energy LUMO of the alkene. These SOMO–LUMO interactions also explain radical traps and their effect on reaction selectivity. In radical chemistry, a trap is a molecule that...

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Radical Cyclization in One Molecule

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Radical Halogenation: Chiral Products

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Allylic Chlorination by Radical Chain Reaction

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radical substitution: allylic bromination

Allylic Bromination with NBS

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Alkyl Halide Reduction with Tributyltin Hydride

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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...

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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

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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...

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Pinacol Coupling and Solvent Effects

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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 electrophilic radicals react with nucleophilic alkenes, including the malonate radical and chlorine radical examples, through SOMO-HOMO interactions.

Electrophilic Radicals and Alkene Reactions | 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.

Electrophilic Radicals and Alkene Reactions

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