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

Реакции радикального замещения полезны для галогенирования алканов и алкенов.
Аналогичным образом, радикальные замены могут удалять галогены из алкилгалогенидов с помощью гидрида трибутилолова, что известно как гидрогенолиз алкилгалогенидов.
Примечательно, что гидрид олова имеет слабую связь Sn–H и может реагировать с алкилгалогенидами с заменой галогена водородом, образуя галогениды алканов и олова.
Реакция энергетически благоприятна, так как образующиеся новые связи сильнее разорванных.
Механизм гидрогенолиза следует за цепной реакцией.
Первоначально гидрид олова в присутствии света образует радикал трибутилолова, который затем абсорбирует галоген из органического галогенида с образованием промежуточного радикала.
Затем промежуточный радикал абстрагирует водород из гидрида олова, чтобы в конечном итоге образовать радикал алкана и трибутилолова, который распространяет реакцию. 
Реакционная способность алкилгалогенидов к гидрогенолизу снижается от йодида к фториду, где алкилфториды нереакционноспособны.
В частности, восстановление алкилхлоридов требует более высокой концентрации радикалов трибутилолова, что достигается за счет добавления в реакцию такого инициатора, как AIBN.

Радикальное замещение: гидрогенолиз алкилгалогенидов гидридом трибутил-олова

Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride

Реакции радикального замещения можно использовать для удаления функциональных групп из молекул. Гидрогенолиз алкилгалогенидов является одной из таких реакций, при которой слабая связь Sn–H в гидриде трибутил-олова реагирует с алкилгалогенидами с образованием алканов. Здесь реагент Bu_3SnH дает в качестве побочного продукта галогенид трибутил-олова.

Связи, образующиеся в этой реакции, прочнее разорванных, что делает ее энергетически выгодной. Реакция протекает по радикально-цепному механизму, аналогичному реакциям радикального галогенирования, где Bu_3SnH подвергается гомолитическому расщеплению в присутствии света с образованием радикала трибутил-олова. Этот радикал отрывает галоген от алкилгалогенида, образуя промежуточный алкильный радикал и галогенид трибутил-олова. Промежуточный радикал дополнительно отрывает водород от гидрида трибутил-олова и образует алкан и радикал трибутил-олова, которые ускоряют реакцию.

Поскольку связи C–Br и C–I слабее связей C–Cl, дневного света достаточно, чтобы инициировать гидрогенолиз алкилбромидов и иодидов. Напротив, алкилфториды нереакционноспособны из-за сильных связей C–F. Гидрогенолиз алкилхлоридов с использованием Bu_3SnH требует высокой концентрации радикалов трибутил-олова, что достигается добавлением инициатора в реакционную смесь. ДАК является наиболее широко используемым инициатором, поскольку он подвергается термическому гомолизу при температуре выше 60 ° C с образованием стабилизированных нитрилом радикалов. Эти радикалы отрывают водород от гидрида трибутил-олова, образуя радикал трибутил-олова, который способствует распространению реакции.

<img alt="Figure1" src="/files/ftp_upload/12901/PT_12901_Figure%201.svg" style="text-align: center; height: 190px;">

Пероксиды не используются в качестве инициаторов, поскольку пероксидные радикалы обладают высокой реакционной способностью и могут отрывать водород от органических галогенидов, что приводит к нежелательным побочным реакциям.

Реакции радикального замещения могут быть использованы для удаления функциональных групп из молекул. Гидрогенолиз алкилгалогенидов является одной из таких реакций, в которой ...

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

Education

JoVE Core

Organic Chemistry

Органическая химия

Unit 1

Раздел 1

Radical Chemistry

Радикальная химия

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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

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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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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.

Alkyl Halide Reduction with Tributyltin Hydride

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