Name: allylic chlorination by radical chain reaction
Uploaded: 2023-04-30T11:22:13.000Z
Duration: 1 min 31 s
Channel: JoVE

Im Allgemeinen durchlaufen Alkene eine elektrophile Additionsreaktion, wenn sie bei niedrigen Temperaturen mit Halogenen behandelt werden.
Allylische Substitutionsreaktionen treten jedoch bei niedrigen Halogenkonzentrationen und unter radikalischen Bedingungen wie hohen Temperaturen oder in Gegenwart von Radikalinitiatoren auf.
Zum Beispiel erfährt Propen eine allylische Chlorierung, wenn es in der Gasphase bei 400 °C mit Chlor reagiert wird.
Allylischer Wasserstoff wird substituiert, da die sp3 allylische C-H-Bindung schwächer ist als die sp2 vinylische C-H-Bindung.
Zusätzlich ist das gebildete allylische radikalische Zwischenprodukt resonanzstabilisiert.
Die allylische Chlorierung folgt einem Kettenmechanismus, der die Schritte Initiierung, Vermehrung und Beendigung umfasst.
Im Initiationsschritt dissoziiert das Chlormolekül in zwei hochreaktive Chloratome.
Im ersten Vermehrungsschritt abstrahiert das Chloratom allylischen Wasserstoff, was zur Bildung eines allylischen Radikal-Zwischenprodukts führt.
Im zweiten Vermehrungsschritt reagiert das Allylradikal mit einem Chlormolekül zu einem Allylchlorid und einem Chloratom, wodurch sich die Reaktion weiter fortsetzt.
Schließlich wird die Reaktion durch Kopplung zweier beliebiger Radikale beendet, wodurch die reaktiven Zwischenprodukte erschöpft werden.

radikalsubstitution: allylische chlorierung

Radical Substitution: Allylic Chlorination

Wenn Alkene bei niedrigen Temperaturen mit Halogenen reagieren, kommt es in der Regel zu einer Additionsreaktion. Bei Erhöhung der Temperatur oder unter Reaktionsbedingungen, die Radikale bilden und eine niedrige, aber konstante Konzentration von Halogenradikalen bieten, wird jedoch eine allylische Substitutionsreaktion begünstigt. Dies liegt daran, dass Allylwassersubstanzen sehr reaktiv sind, da das gebildete Zwischenprodukt resonanzstabilisiert ist. Wenn beispielsweise Propen in der Gasphase bei 400 °C mit Chlor behandelt wird, kommt es zu einer allylischen Chlorierung, bei der 3-Chlorpropen entsteht.

Die radikalische Substitutionsreaktion der allylischen Chlorierung folgt einem Kettenmechanismus, der dem der Alkan-Halogenierungen ähnlich ist und umfasst Initiierungs-, Fortpflanzungs- und Abbruchschritte. Der Einleitungsschritt beinhaltet die Dissoziation des Chlormoleküls in zwei Chloratome. Der Ausbreitungsschritt umfasst zwei Kettenausbreitungsschritte. Im ersten Kettenfortpflanzungsschritt abstrahiert das Chloratom das Allylwasserstoffatom und bildet ein resonanzstabilisiertes Allylradikal-Zwischenprodukt. Im zweiten Kettenfortpflanzungsschritt reagiert das Allylradikal-Zwischenprodukt mit einem Chlormolekül und bildet ein Allylchlorid und ein Chloratom. Das im zweiten Kettenfortsetzungsschritt gebildete Chloratom abstrahiert weiter Allylwasserstoff und setzt die Reaktion fort. Im Abbruchschritt reagieren die Radikale miteinander, um nichtradikalische Produkte zu bilden und die Reaktion zu beenden.

Radikalsubstitution: Allylische Chlorierung

Wenn Alkene bei niedrigen Temperaturen mit Halogenen reagieren, kommt es typischerweise zu einer Additionsreaktion. Bei Erhöhung der Temperatur oder Unterreaktion ...

radical-substitution-allylic-chlorination

Education

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Organic Chemistry

Organische Chemie

Unit 1

Einheit 1

Radical Chemistry

Radikalchemie

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

allylic chlorination by radical chain reaction

Allylic chlorination is a radical chain reaction in alkenes. It happens when halogen reactions move away from simple addition and toward substitution at the allylic position, which is the carbon next to a double bond.

At low temperatures, alkenes usually react with halogens by addition. When the temperature increases, or when the reaction conditions create radicals, allylic substitution becomes favored. This works because allylic hydrogens are very reactive and the intermediate formed is resonance stabilized.

Propene gives a clear example. When propene reacts with chlorine in the gas phase at 400 °C, it forms 3-chloropropene. The reaction follows the same chain pattern seen in alkane halogenation.

The chain mechanism has three parts: initiation, propagation, and termination. In initiation, the chlorine molecule splits into two chlorine atoms. In propagation, one chlorine atom removes an allylic hydrogen and forms a resonance-stabilized allyl radical. That allyl radical then reacts with another chlorine molecule to make allyl chloride and a new chlorine atom.

The new chlorine atom keeps the chain going by abstracting another allylic hydrogen. In termination, radicals react with each other and form non-radical products. This stops the reaction.

Allylic Chlorination by Radical Chain Reaction

allylic-chlorination-by-radical-chain-reaction

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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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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Radical Abstraction in Chain Reactions

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Radical Addition and Spin Conservation

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radical reactivity: overview

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Radical Cyclization in One Molecule

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Selective MnO2 Oxidation of Alcohols

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Radical Halogenation: Chiral Products

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Allylic Bromination with NBS

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Alkyl Halide Reduction with Tributyltin Hydride

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radical anti-markovnikov addition to alkenes: overview

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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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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...

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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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Acyloin Condensation: Ester Coupling Pathway

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key terms and concepts

Explore allylic chlorination in alkenes, including radical chain steps, resonance-stabilized intermediates, and the example of propene forming 3-chloropropene.

Allylic Chlorination by Radical Chain Reaction | JoVE K12

organic chemistry

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Allylic Chlorination by Radical Chain Reaction

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