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

Reakcje substytucji rodnikowej są przydatne do halogenowania alkanów i alkenów.
Podobnie, podstawienia rodnikowe mogą usuwać halogen z halogenków alkilowych za pomocą wodorku tributylocyny, znanego jako hydrogenoliza halogenków alkilowych.
Warto zauważyć, że wodorek cyny ma słabe wiązanie Sn-H i może reagować z halogenkami alkilu, zastępując halogen wodorem, tworząc alkan i halogenek cyny.
Reakcja jest energetycznie faworyzowana, ponieważ nowo powstałe wiązania są silniejsze niż zerwane wiązania.
Mechanizm hydrogenolizy następuje po reakcji łańcuchowej.
Początkowo wodorek cyny wytwarza rodnik tributylocyny w obecności światła, który następnie odciąga halogen od halogenku organicznego, wytwarzając rodnik pośredni.
Następnie rodnik pośredni pobiera wodór z wodorku cyny, aby ostatecznie utworzyć alkan i rodnik tributylocyny, który propaguje reakcję. 
Reaktywność halogenków alkilowych w kierunku hydrogenolizy zmniejsza się od jodku do fluoru, gdzie fluorki alkilowe są niereaktywne.
W szczególności redukcja chlorków alkilowych wymaga wyższego stężenia rodników tributylocyny, co osiąga się poprzez dodanie do reakcji inicjatora, takiego jak AIBN.

podstawienie rodnikowe: hydrogenoliza halogenków alkilowych wodorkiem tributylocyny

Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride

W celu usunięcia grup funkcyjnych z cząsteczek można zastosować rodnikowe reakcje podstawienia. Jedną z takich reakcji jest wodoroliza halogenków alkilu, podczas której słabe wiązanie Sn–H w wodorku tributylocyny reaguje z halogenkami alkilu, tworząc alkany. Tutaj odczynnik Bu_3SnH daje halogenek tributylocyny jako produkt uboczny.

Wiązania powstałe w tej reakcji są silniejsze niż wiązania zrywane, co czyni ją energetycznie korzystną. Reakcja przebiega według rodnikowego mechanizmu łańcuchowego podobnego do reakcji rodnikowego halogenowania, w którym Bu_3SnH ulega rozszczepieniu homolitycznemu w obecności światła, tworząc rodnik tributylocyny. Ten rodnik oddziela halogen od halogenku alkilu, tworząc pośredni rodnik alkilowy i halogenek tributylocyny. Rodnik pośredni dodatkowo oddziela wodór od wodorku tributylocyny i wytwarza alkan i rodnik tributylocyny, który przyspiesza reakcję.

Ponieważ wiązania C – Br i C – I są słabsze niż wiązania C – Cl, światło dzienne wystarczy do zainicjowania wodorolizy bromków i jodków alkilu. Natomiast fluorki alkilu są niereaktywne ze względu na silne wiązania C – F. Wodoroliza chlorków alkilu przy użyciu Bu_3SnH wymaga wysokiego stężenia rodników tributylocyny, co osiąga się poprzez dodanie do mieszaniny reakcyjnej inicjatora. AIBN jest najpowszechniej stosowanym inicjatorem, ponieważ ulega homolizie termicznej w temperaturze powyżej 60 ° C, tworząc rodniki stabilizowane nitrylem. Rodniki te oddzielają wodór od wodorku tributylocyny, tworząc rodnik tributylocyny, który przyspiesza reakcję.

<img alt="Figure1" src="/files/ftp_upload/12901/PT_12901_Figure%201.svg" style="text-align: center; height: 190px;">

Nadtlenków nie stosuje się jako inicjatorów, ponieważ rodniki nadtlenkowe są wysoce reaktywne i mogą odrywać wodór od halogenków organicznych, prowadząc do niepożądanych reakcji ubocznych.

Podstawienie rodnikowe: hydrogenoliza halogenków alkilowych wodorkiem tributylocyny

Reakcje substytucji rodnikowej mogą być stosowane do usuwania grup funkcyjnych z cząsteczek. Uwodornienie halogenków alkilowych jest jedną z takich reakcji, w której ...

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

Education

JoVE Core

Organic Chemistry

Chemia organiczna

Unit 1

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Radical Chemistry

Chemia rodnikowa

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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