Name: selective mno2 oxidation of alcohols
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 21 s
Channel: JoVE

Les alcools allylique et benzylique subissent une oxydation sélective par le dioxyde de manganèse (IV) activé via un intermédiaire radicalaire.
Les alcools allyliques primaires donnent des aldéhydes, et les alcools allyliques secondaires forment des cétones.
Le MnO2 activé est préparé par une réaction d’oxydoréduction lorsqu’un sel de Mn+2, tel que le sulfate de manganèse, réagit avec le permanganate de potassium.
La grande sélectivité du MnO2 vis-à-vis des alcools allylique et benzylique peut être comprise à partir du mécanisme réactionnel.
Le groupe hydroxyle de l’alcool s’ajoute initialement à MnO2 pour former un ester.
Ensuite, le Mn(IV) accepte un électron, et un atome d’hydrogène est transféré du carbone allylique ou benzylique à l’oxygène attaché au manganèse.
Cela réduit le Mn(IV) en Mn(III), ce qui donne un radical carbone allylique ou benzylique, qui est stabilisé par résonance.
Enfin, un réarrangement électronique supplémentaire réduit Mn(III) en Mn(II) avec la formation d’un aldéhyde ou d’un produit cétonique.
Notamment, lors de l’oxydation radicalaire, l’alcool non benzylique n’est pas affecté.

<ol>
	<li>Loudon, M. et Parise, J. (2016). Chimie organique. New York, NY : Macmillan Publishers, 851-852.</li>
	<li>Klein, D. (2017). Chimie organique. New Jersey, NJ : Wiley, 462.</li>
</ol>

oxydation radicale des alcools allyliques et benzyliques

Radical Oxidation of Allylic and Benzylic Alcohols

L'oxyde de manganèse (IV) activé peut oxyder sélectivement les alcools allyliques et benzyliques via un mécanisme intermédiaire radical. Les alcools allyliques primaires sont oxydés en aldéhydes, tandis que les alcools allyliques secondaires donnent des cétones. La réaction redox du permanganate de potassium avec un sel de Mn(II) tel que le sulfate de manganèse (dans des conditions alcalines ou acides), suivie d'un séchage complet, donne l'agent oxydant : le MnO_2 activé. Alors que le MnO_2 est insoluble dans les solvants utilisés pour la réaction, l'oxydation des alcools allyliques et benzyliques se produit à la surface du MnO_2. L’eau étant en compétition avec l’alcool pour les sites actifs du MnO_2, elle doit être éliminée par séchage.

L'oxydation radicalaire par MnO_2 est sélective et le réactif n'affecte pas les alcools non benzyliques ou non allyliques. Initialement, le groupe hydroxyle de l’alcool s’ajoute au MnO_2 pour former un ester. Ensuite, le Mn(IV) accepte un électron et un atome d'hydrogène est transféré du carbone allylique ou benzylique à l'oxygène lié au manganèse. Ainsi, Mn(IV) est réduit en Mn(III), produisant un radical carboné allylique ou benzylique, stabilisé par résonance. La stabilité accrue de cet intermédiaire radicalaire benzylique/allylique, par rapport à la stabilité de l'intermédiaire formé par les alcools non benzyliques ou non allyliques, augmente la vitesse de cette étape et favorise la sélectivité. Un réarrangement électronique supplémentaire entraîne la réduction du Mn(III) en Mn(II) et donne l'aldéhyde ou la cétone.

Oxydation radicale des alcools allyliques et benzyliques

L’oxyde de manganèse (IV) activé peut oxyder sélectivement les alcools allyliques et benzyliques par un mécanisme intermédiaire radicalaire. Les alcools allyliques primaires sont ...

radical-oxidation-of-allylic-and-benzylic-alcohols

Education

Enseignement

JoVE Core

Organic Chemistry

Chimie organique

Unit 1

Unité 1

Radical Chemistry

Chimie radicalaire

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

selective mno2 oxidation of alcohols

Activated manganese(IV) oxide, or MnO2, is a selective oxidizing agent for allylic and benzylic alcohols. It changes primary allylic alcohols into aldehydes and secondary allylic alcohols into ketones. The reaction happens by a radical pathway, so the carbon next to the alcohol group plays a key role.

The oxidizing agent is made from the redox reaction of potassium permanganate with an Mn(II) salt such as manganese sulfate. This can be done under alkaline or acidic conditions. After that, the material must be dried well to give activated MnO2. Water competes with alcohol for the active sites on MnO2, so removing water is important.

MnO2 is insoluble in the solvents used for this reaction. Even so, the oxidation takes place on the MnO2 surface. The reagent is selective and does not affect non-benzylic or non-allylic alcohols. First, the alcohol hydroxyl group adds to MnO2 to form an ester. Then Mn(IV) accepts an electron, and a hydrogen atom moves from the allylic or benzylic carbon to the oxygen attached to manganese.

This step reduces Mn(IV) to Mn(III) and forms a carbon radical. A radical is a species with an unpaired electron. The allylic or benzylic radical is resonance-stabilized, which means its electrons can spread out over more than one structure. That extra stability makes the reaction faster than for non-benzylic or non-allylic alcohols and helps explain the selectivity. Further electron rearrangement reduces Mn(III) to Mn(II) and gives the aldehyde or ketone.

Selective MnO2 Oxidation of Alcohols

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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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Radical Addition and Spin Conservation

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Electrophilic Radicals and Alkene Reactions

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Radical Cyclization in One Molecule

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Activated manganese(IV) oxide, or MnO2, is a selective oxidizing agent for allylic and benzylic alcohols. It changes primary allylic alcohols into aldehydes and secondary allylic alcohols into ketones. The reaction happens by a radical pathway, so the carbon next to the alcohol group plays a key role. The oxidizing agent is made from the redox reaction of potassium permanganate with an Mn(II) salt such as manganese sulfate. This can be done under alkaline or acidic conditions. After that, the...

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Alkane Halogenation: Stability and Reaction Rates

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Radical Halogenation: Chiral Products

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radical substitution: allylic chlorination

radical-substitution-allylic-chlorination

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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radical anti-markovnikov addition to alkenes: overview

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Peroxide Effect in HBr Addition to Alkenes

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radical anti-markovnikov addition to alkenes: mechanism

radical-anti-markovnikov-addition-to-alkenes-mechanism

Hydrogen Bromide Addition to Alkenes by Radicals

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radical anti-markovnikov addition to alkenes: thermodynamics

radical-anti-markovnikov-addition-to-alkenes-thermodynamics

HBr Selectivity in Alkene Addition Thermodynamics

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vicinal diols via reductive coupling of aldehydes or ketones: pinacol coupling overview

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Pinacol Coupling and Solvent Effects

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alkenes via reductive coupling of aldehydes or ketones: mcmurry reaction

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Titanium-Coupled Alkenes from Carbonyls

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&alpha;-hydroxy ketones via reductive coupling of esters: acyloin condensation overview

hydroxy-ketones-via-reductive-coupling-esters-acyloin-condensation

Acyloin Condensation: Ester Coupling Pathway

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key terms and concepts

See how activated MnO2 selectively oxidizes allylic and benzylic alcohols to aldehydes or ketones through a radical surface mechanism.

Selective MnO2 Oxidation of Alcohols | JoVE K12

organic chemistry

JoVE Core Chimie Organique explique les concepts de base de la stéréochimie, des réactions et des mécanismes à l'aide de leçons vidéo animées concises et faciles à comprendre. De plus, les vidéos de scientifiques à l'action présentent l'application de ces concepts grâce à des expériences de recherche classiques et originales réalisées dans des laboratoires modernes.

Selective MnO2 Oxidation of Alcohols

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