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

תגובות החלפה רדיקליות שימושיות להלוגנציה של אלקנים ואלקנים.
באופן דומה, החלפות רדיקליות יכולות להסיר הלוגן מאלקיל הלידים באמצעות טריבוטילטין הידריד, הידוע בשם הידרוגנוליזה של אלקיל הלידים.
יש לציין כי להידריד בדיל יש קשר Sn-H חלש והוא יכול להגיב עם אלקיל הלידים כדי להחליף את ההלוגן במימן, וליצור אלקן ובדיל הליד.
התגובה מועדפת אנרגטית מכיוון שהקשרים החדשים שנוצרו חזקים יותר מהקשרים שנשברו.
מנגנון ההידרוגנוליזה עוקב אחר תגובת שרשרת.
בתחילה, הידריד בדיל מייצר רדיקל טריבוטילטין בנוכחות אור, אשר לאחר מכן מופשט הלוגן מהליד אורגני המייצר ביניים רדיקלי.
לאחר מכן, הביניים הרדיקליים מפשטים מימן מהידריד בדיל כדי ליצור לבסוף אלקן ורדיקל טריבוטילטין, אשר מפיץ את התגובה. 
התגובתיות של אלקיל הלידים כלפי הידרוגנוליזה פוחתת מיודיד לפלואוריד, שם הפלואורידים האלקיל אינם תגובתיים.
בפרט, הפחתת כלורידים אלקיל דורשת ריכוז גבוה יותר של רדיקלים tributyltin, אשר מושגת על ידי הוספת יוזם כגון AIBN לתגובה.

﻿התמרה רדיקלית: הידרוגנוליזה של אלקיל הלידים עם טריבוטילטין הידריד

Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride

<div style="direction:rtl"> ניתן להשתמש בתגובות התמרה רדיקליות כדי להסיר קבוצות פונקציונליות ממולקולות. ההידרוגנוליזה של אלקיל הלידים היא תגובה אחת כזו, שבה הקשר Sn-H החלש בטריבוטילטין הידריד מגיב עם אלקיל הלידים ליצירת אלקנים. כאן, המגיב Bu_3SnH מניב טריבוטילטין הליד כתוצר לוואי. הקשרים הנוצרים בתגובה זו חזקים יותר מהקשרים שנשברו, מה שהופך אותה לטובה מבחינה אנרגטית. התגובה עוקבת אחר מנגנון שרשרת רדיקלי הדומה לתגובות הלוגנציה רדיקלית, שבה Bu_3SnH עובר פירוק הומליטי בנוכחות אור ליצירת רדיקל טריבוטילטין. רדיקל זה מוציא את ההלוגן מאלקיל הליד, ויוצר רדיקל אלקיל ביניים וטרוביטילטין הליד. תוצר הביניים הרדיקלי מוציא עוד יותר מימן מטריבטילטין הידריד ומייצר רדיקל אלקן ורדיקל טריבוטילטין, המפיץ את התגובה. מכיוון שקשרי C–Br ו-C–I חלשים יותר מקשרים C–Cl, אור יום מספיק כדי להתחיל את ההידרוגנוליזה של אלקיל ברומידים ויודידים. לעומת זאת, אלקיל פלואורידים אינם מגיבים בגלל קשרי C-F החזקים. הידרוגנוליזה של אלקיל כלורידים באמצעות Bu_3SnH דורשת ריכוז גבוה של רדיקלים טריבוטילטין, אשר מושגת על ידי הוספת יוזם לתערובת התגובה. AIBN הוא היוזם הנפוץ ביותר, מכיוון שהוא עובר הוומליזה תרמית מעל 60 מעלות צלזיוס ליצירת רדיקלים מיוצבים בניטריל. רדיקלים אלה מוציאים מימן מטריבטילטין הידריד, ויוצרים את רדיקל הטריבטילטין, המפיץ את התגובה. <img alt="Figure1" src="/files/ftp_upload/12901/PT_12901_Figure%201.svg" style="text-align: center; height: 190px;"> פרוקסידים אינם משמשים כיוזמים מכיוון שרדיקלי פרוקסיד הם מאוד תגובתיים ויכולים להפיק מימן מהלידים אורגניים המובילים לתגובות לוואי לא רצויות. </div>

ניתן להשתמש בתגובות החלפה רדיקליות כדי להסיר קבוצות פונקציונליות ממולקולות. הידרוגנוליזה של אלקיל הלידים היא תגובה אחת כזו, שבה ...

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

Education

JoVE Core

Organic Chemistry

כימיה אורגנית

Unit 1

יחידה 1

Radical Chemistry

כימיה רדיקלית

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

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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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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Homolysis and Radical Bonds

Homolysis is a bond-breaking process that splits a shared electron pair evenly. When a bond is broken with enough energy, the two electrons can also move to one atom in heterolysis, which forms ions. Homolysis gives one electron to each atom and creates radicals, which are highly reactive particles with an unpaired electron. HCl shows the difference between these bond cleavages. In solution, HCl can break into H+ and Cl− ions, and chlorine takes both bonding electrons, leaving a naked proton.

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Radical Abstraction in Chain Reactions

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Radical Addition and Spin Conservation

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Radicals from Elimination Reactions

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Radical Reaction Pathways and Stability

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Steric Shielding in Stable Radicals

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How Concentration Steers Radical Additions

Radical reaction selectivity can change when the concentrations of the reactants change. In this chemistry example, an alkyl halide reacts with an alkene in the presence of tin hydride and AIBN, an initiator that helps start the radical process. The reaction begins when a tin radical is formed. The tin radical removes the halogen from the alkyl halide. This step creates an alkyl radical. That radical then has two possible paths. It can react with tin hydride to form an alkane, or it can add to...

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Electrophilic Radicals and Alkene Reactions

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Nucleophilic Radicals and Alkene Traps

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Alkane Halogenation: Stability and Reaction Rates

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Allylic Chlorination by Radical Chain Reaction

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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...

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Hydrogen Bromide Addition to Alkenes by Radicals

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HBr Selectivity in Alkene Addition Thermodynamics

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Pinacol Coupling and Solvent Effects

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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

organic chemistry

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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