Name: alkyl halide reduction with tributyltin hydride
Uploaded: 2023-04-30T11:22:27.000Z
Duration: 1 min 26 s
Channel: JoVE

Radical substitution reactions are useful for the halogenation of alkanes and alkenes.
Similarly, radical substitutions can remove halogen from alkyl halides using tributyltin hydride, known as the hydrogenolysis of alkyl halides.
Notably, tin hydride has a weak Sn&#8211;H bond and can react with alkyl halides to replace the halogen with hydrogen, forming alkane and tin halide.
The reaction is energetically favored as the new bonds formed are stronger than the bonds broken.
The mechanism of hydrogenolysis follows a chain reaction.
Initially, tin hydride produces a tributyltin radical in the presence of light, which then abstracts halogen from an organic halide producing a radical intermediate.
Next, the radical intermediate abstracts hydrogen from tin hydride to finally form an alkane and a tributyltin radical, which propagates the reaction. 
The reactivity of alkyl halides towards hydrogenolysis decreases from iodide to fluoride, where the alkyl fluorides are unreactive.
In particular, the reduction of alkyl chlorides requires a higher concentration of tributyltin radicals, which is achieved by adding an initiator such as AIBN to the reaction.

radicale substitutie: hydrogenolyse van alkylhalogeniden met tributyltinhydride

Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride

Radicale substitutiereacties kunnen worden gebruikt om functionele groepen uit moleculen te verwijderen. De hydrogenolyse van alkylhalogeniden is zo'n reactie, waarbij de zwakke Sn-H-binding in tributyltinhydride reageert met alkylhalogeniden om alkanen te vormen. Hier levert het reagens Bu_3SnH tributyltinhalogenide op als bijproduct.


 De bindingen die bij deze reactie worden gevormd, zijn sterker dan de verbroken bindingen, waardoor deze energetisch gunstig is. De reactie volgt een radicaalketenmechanisme dat vergelijkbaar is met radicaalhalogeneringsreacties, waarbij Bu_3SnH homolytische splitsing ondergaat in aanwezigheid van licht om een tributyltinradicaal te vormen. Dit radicaal onttrekt het halogeen aan een alkylhalogenide en vormt een alkylradicaal tussenproduct en tributyltinhalogenide. Het radicaaltussenproduct onttrekt verder waterstof uit tributyltinhydride en produceert een alkaan en een tributyltinradicaal, die de reactie vermenigvuldigt.


 Omdat C-Br- en C-I-bindingen zwakker zijn dan C-Cl-bindingen, is daglicht voldoende om de hydrogenolyse van alkylbromiden en jodiden op gang te brengen. Daarentegen zijn alkylfluoriden niet reactief vanwege de sterke C-F bindingen. De hydrogenolyse van alkylchloriden met behulp van Bu_3SnH vereist een hoge concentratie tributyltinradicalen, die wordt bereikt door een initiator aan het reactiemengsel toe te voegen. AIBN is de meest gebruikte initiator, omdat het thermische homolyse ondergaat boven 60 °C om nitril-gestabiliseerde radicalen te vormen. Deze radicalen onttrekken waterstof aan tributyltinhydride en vormen het tributyltinradicaal, dat de reactie vermenigvuldigingt.


 <img alt="Figure1" src="/files/ftp_upload/12901/PT_12901_Figure 1.svg" style="text-align: center; height: 190px;" />


 Peroxiden worden niet als initiatoren gebruikt omdat peroxideradicalen zeer reactief zijn en waterstof kunnen onttrekken aan organische halogeniden, wat leidt tot ongewenste nevenreacties.

Radicale substitutie: Hydrogenolyse van alkylhalogeniden met tributyltinhydride

Radical substitution reactions can be used to remove functional groups from molecules. The hydrogenolysis of alkyl halides is one such reaction, where the ...

radical-substitution-hydrogenolysis-alkyl-halides-with-tributyltin

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Organic Chemistry

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Onderdeel 1

Radical Chemistry

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KEY TERMS AND CONCEPTS

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alkyl halide reduction with tributyltin hydride

Alkyl halide reduction with tributyltin hydride removes a halogen and replaces it with hydrogen. This reaction turns alkyl halides into alkanes. The reagent Bu3SnH has a weak Sn–H bond, and the reaction forms tributyltin halide as a byproduct. Because the new bonds are stronger than the bonds that are broken, the reaction is energetically favorable.

The reaction proceeds by a radical chain mechanism, which is similar to radical halogenation. Light causes Bu3SnH to split homolytically, meaning the bond breaks evenly to form a tributyltin radical. That radical removes the halogen from the alkyl halide. This step creates an alkyl radical intermediate and tributyltin halide.

Next, the alkyl radical takes hydrogen from tributyltin hydride. This forms the alkane product and a new tributyltin radical. The new radical then continues the chain reaction.

Bond strength affects how easily the reaction starts. C–Br and C–I bonds are weaker than C–Cl bonds, so daylight can start the hydrogenolysis of alkyl bromides and iodides. Alkyl fluorides do not react because C–F bonds are very strong.

Alkyl chlorides need more tributyltin radicals, so an initiator is added to the reaction mixture. AIBN is the most common initiator. Above 60 °C, it undergoes thermal homolysis and forms nitrile-stabilized radicals. These radicals remove hydrogen from tributyltin hydride and create the tributyltin radical that keeps the reaction going.

Peroxides are not used as initiators in this reaction. Their radicals are very reactive and can remove hydrogen from organic halides. That can lead to unwanted side reactions.

Alkyl Halide Reduction with Tributyltin Hydride

alkyl-halide-reduction-with-tributyltin-hydride

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key terms and concepts

Learn how tributyltin hydride removes halogens from alkyl halides through a radical chain reaction, and why light, AIBN, and bond strength matter.

Alkyl Halide Reduction with Tributyltin Hydride | JoVE K12

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Alkyl Halide Reduction with Tributyltin Hydride

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