Name: steric shielding in stable radicals
Uploaded: 2023-04-30T11:20:59.000Z
Duration: 1 min 10 s
Channel: JoVE

Recall that the presence of electron-donating, electron-withdrawing, and conjugating groups adjacent to a radical center results in electronically-stabilized radicals.
Some of these electronically-stabilized radicals are known as persistent radicals. These radicals are remarkably stable; some can even be isolated and purified.
In addition to the electronic factors, steric factors also contribute to the stability of these persistent radicals.
For example, consider the triphenylmethyl radical. The exceptionally high stability of this radical is due to the presence of three phenyl rings.
The three surrounding rings are twisted out of the plane by 30&#176; in a propeller-like conformation.
As a result, the central carbon bearing the radical character is sterically shielded by the twisted phenyl rings, making the radical highly stable and unavailable for reactions.
Overall, steric effects make a radical more stable and less reactive.

radicale reactiviteit: sterische effecten

Radical Reactivity: Steric Effects

De aanwezigheid van elektronendonerende, elektronenzuigende of conjugerende groepen naast een radicaalcentrum verleent elektronische stabilisatie aan de radicalen. Voorbeelden van zulke elektronisch gestabiliseerde radicalen zijn trifenylmethyl, tetramethylpiperidine-N-oxide en 2,2-difenyl-1-picrylhydrazyl. Deze radicalen zijn opmerkelijk stabiel en staan bekend als persistente radicalen. Sommige van de persistente radicalen kunnen zelfs worden geïsoleerd en gezuiverd.

Naast elektronische factoren zijn sterische factoren ook verantwoordelijk voor de stabiliteit van deze persistente radicalen. De uitzonderlijk hoge stabiliteit van het trifenylmethylradicaal is bijvoorbeeld te danken aan de aanwezigheid van drie omringende fenylringen die 30° uit het vlak zijn gedraaid in een propellerconformatie. Deze gedraaide fenylringen beschermen sterisch de centrale koolstof, die het grootste deel van het radicale karakter draagt. Als gevolg hiervan wordt het radicaal zeer stabiel en niet meer beschikbaar voor moleculen om te reageren. Daarom verleent sterische hindering stabiliteit aan de radicalen, waardoor ze minder reactief worden.

Radicale reactiviteit: sterische effecten

The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the ...

Education

JoVE Core

Organic Chemistry

Organische chemie

Unit 1

Onderdeel 1

Radical Chemistry

Radicale chemie

KEY TERMS AND CONCEPTS

BELANGRIJKE TERMEN EN CONCEPTEN

steric shielding in stable radicals

Steric shielding helps make some radicals more stable and less reactive. A radical is a species with an unpaired electron at its center. In many cases, nearby groups can also give electronic stabilization, including electron-donating, electron-withdrawing, or conjugating groups.

Several radicals are known as persistent radicals because they are remarkably stable. Examples include triphenylmethyl, tetramethylpiperidine-N-oxide, and 2,2-diphenyl-1-picrylhydrazyl. Some of these persistent radicals can even be isolated and purified.

Steric factors also play a major role in that stability. The triphenylmethyl radical is especially stable because three phenyl rings surround the radical center. These rings are twisted out of the plane by 30° in a propeller shape.

This twisted shape shields the central carbon, which holds most of the radical character. As a result, the radical is harder for other molecules to reach and react with. Steric hindrance therefore makes radicals more stable and less reactive.

Steric Shielding in Stable Radicals

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

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,...

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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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Homolysis and Radical Bonds

Homolysis is a bond-breaking process that splits a shared electron pair evenly. When a bond is broken with enough energy, the two electrons can also move to one atom in heterolysis, which forms ions. Homolysis gives one electron to each atom and creates radicals, which are highly reactive particles with an unpaired electron. HCl shows the difference between these bond cleavages. In solution, HCl can break into H+ and Cl− ions, and chlorine takes both bonding electrons, leaving a naked proton.

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Radical Abstraction in Chain Reactions

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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

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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Radical Cyclization in One Molecule

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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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radical substitution: allylic bromination

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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...

radical anti-markovnikov addition to alkenes: mechanism

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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...

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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key terms and concepts

Explore how steric hindrance and surrounding phenyl rings stabilize persistent radicals such as triphenylmethyl, TMNO, and DPPH.

Steric Shielding in Stable Radicals | 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.

Steric Shielding in Stable Radicals

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