Name: radical abstraction in chain reactions
Uploaded: 2023-04-30T11:20:29.000Z
Duration: 47 s
Channel: JoVE

Wenn ein Radikal eine Spezies von einer Verbindung abstrahiert, führt dies zur Bildung eines neuen Radikals. Handelt es sich bei der abstrahierten Spezies um ein Wasserstoffatom, spricht man von Wasserstoffabstraktion.
Dies unterscheidet sich jedoch vom Protonentransferprozess. Ein Protonentransfer ist ionisch, wobei nur der Kern des Wasserstoffatoms eine Verlagerung erfährt.
Bei der Wasserstoffabstraktionsmethode wird das gesamte Wasserstoffatom – das Proton und das Elektron – hinüberbewegt.
Ähnlich wie bei der Wasserstoffentnahme verhält es sich bei der Halogenentnahme. Dabei wird anstelle eines Wasserstoffatoms ein Halogenatom abstrahiert, um ein neues Radikal zu erzeugen.

radikalbildung: abstraktion

Radical Formation: Abstraction

Das Elektron eines Atoms kann durch ein relativ instabiles Radikal aus einer Verbindung abstrahiert werden, um ein neues Radikal mit relativ größerer Stabilität zu erzeugen. Beispielsweise kann ein Initiator, der durch Homolyse Radikale bildet, eine geeignete Spezies wie ein Wasserstoffatom oder ein Halogenatom von einer Verbindung abstrahieren, um ein neues Radikal zu erzeugen. Diese Fähigkeit der Radikale, sich durch Abstraktion fortzupflanzen, ist ein entscheidendes Merkmal der radikalischen Kettenreaktionen.

Auch wenn die Homolyse Radikale erzeugt, unterscheidet sie sich von der radikalen Abstraktion. Bei der Homolyse erzeugt ein spingepaartes Molekül zwei Radikale; In der Abstraktion reagiert ein Radikal mit einem Spinpaar-Molekül und erzeugt ein neues Radikal und ein neues Spinpaar-Molekül.

Am wichtigsten ist, dass es einen grundlegenden Unterschied zwischen den Mechanismen der Wasserstoffabstraktion und des Protonentransfers gibt. Der Protonentransfer ist ionischer Natur, wobei nur der Kern des Wasserstoffatoms bewegt wird. Dadurch entsteht ein Anion des anderen Reaktanten unter Beibehaltung seines Elektrons. Allerdings entsteht durch die Verlagerung des Wasserstoffatoms mit seinem Elektron bei der Wasserstoffabstraktion eine Radikalspezies. Wie die Wasserstoffabstraktion ist auch die Halogenabstraktion möglich, bei der ein Halogenatom mit einem ungepaarten Elektron abstrahiert wird.

Radikalbildung: Abstraktion

Das Elektron eines Atoms kann durch ein relativ instabiles Radikal von einer Verbindung abstrahiert werden, um ein neues Radikal von relativ größerer Stabilität zu erzeugen. Für...

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JoVE Core

Organic Chemistry

Organische Chemie

Unit 1

Einheit 1

Radical Chemistry

Radikalchemie

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

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 new spin-paired molecule.

Hydrogen abstraction is not the same as proton transfer. Proton transfer is ionic and moves only the hydrogen nucleus. It leaves an anion behind with its electron. In hydrogen abstraction, the hydrogen atom moves with its electron, so a radical species is formed. Halogen abstraction works in a similar way, because a halogen atom can be removed with one unpaired electron.

Radical Abstraction in Chain Reactions

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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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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.

radical formation: abstraction

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,...

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Radicals from Elimination Reactions

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Peroxide Effect in HBr Addition to Alkenes

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Hydrogen Bromide Addition to Alkenes by Radicals

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HBr Selectivity in Alkene Addition Thermodynamics

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key terms and concepts

Learn how radical abstraction helps radical chain reactions by forming new radicals through hydrogen or halogen atom removal, and how it differs from homolysis and proton transfer.

Radical Abstraction in Chain 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.

Radical Abstraction in Chain Reactions

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