Name: allylic bromination with nbs
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 27 s
Channel: JoVE

הלוגנציה אלילית של אלקן דורשת ריכוז נמוך של הלוגן וטמפרטורה גבוהה.
כדי לבצע ברום אלילי בטמפרטורת החדר, N-bromosuccinimide, או NBS, משמש כמגיב. הוא שומר על ריכוז ברום נמוך ופועל כמקור לרדיקלים של ברום.
לדוגמה, פרופן מגיב עם NBS בנוכחות אור או חמצן כדי ליצור 3-bromopropene.
המנגנון של ברום אלילי כולל שלבי התחלה, ריבוי וסיום.
במהלך שלב החניכה, NBS עובר שסע הומוליטי המושרה באור של הקשר N-Br כדי ליצור רדיקל ברום.
בשלב ההתפשטות הראשון, רדיקל הברום מגיב עם פרופן ומופשט מימן אלילי, ויוצר רדיקל אלילי ו- HBr.
HBr שנוצר מגיב עם NBS כדי לייצר את מולקולת הברום הדרושה להמשך התפשטות התגובה.
בשלב ההתפשטות השני, מולקולת הברום מגיבה עם רדיקל אלילי כדי לתת אליל ברומיד ואטום ברום.
לבסוף, בשלב הסיום, הרדיקלים השונים חוברים זה לזה כדי לתת מוצרים לא רדיקליים, ועוצרים תגובה נוספת.

﻿התמרה רדיקלית: ברומין אלילי

Radical Substitution: Allylic Bromination

<div style="direction:rtl"> בסינתזה אורגנית, ניתן לשנות את היווצרות התוצרים על ידי שינוי תנאי התגובה. לדוגמה, תוצר סיפוח דיברומו נוצר כאשר פרופן מטופל בברום בטמפרטורת החדר. לעומת זאת, פרופן עובר התמרה אלילית בממסים לא קוטביים בטמפרטורות גבוהות כדי לתת 3-ברומופרופן. על מנת להימנע מתגובת הסיפוח, יש לשמור על ריכוז הברום נמוך ככל האפשר לאורך כל התגובה. ניתן להשיג זאת באמצעות N-ברומוסוקסינימיד (NBS) כמגיב במקום ברום מולקולרי. פרופן מגיב עם NBS בנוכחות אור או מי חמצן באמצעות התמרה רדיקלית ליצירת אליל ברומיד או 3-ברומופרופן. בדומה לתגובות רדיקליות, מנגנון הברומינציה האלילי כולל שלושה שלבים: התחלה, התפשטות וסיום. בשלב ההתחלה, NBS עובר פירוק הומוליטי של קשרי N-Br חלשים בנוכחות אור או מי חמצן ליצירת רדיקל ברום. במהלך שלב ההתפשטות הראשון, רדיקל הברום שנוצר מוציא את המימן האלילי כדי לתת רדיקל אלילי מיוצב רזוננס ו-HBr. ה-HBr שנוצר מגיב מיד עם NBS בתגובה יונית המייצרת Br_2, המשתתף בשלב ההתפשטות השני. בסופו של דבר, בשלב הסיום, מתחברים רדיקלים שונים, מה שגורם להיווצרות תוצרים לא רדיקליים המובילים להפסקת התגובה. לאורך כל התגובה, ריכוזי HBr ו_Br_2 נשמרים במינימום. בתנאים אלו, כלומר בממס לא קוטבי עם ריכוז נמוך מאוד של ברום, הסיפוח היוני של Br_2 אינה מתחרה בהצלחה עם ברום רדיקלי. ברומינציה רדיקלית של אלקנים מותמרים אלילית יוצר תערובת של תוצרים. הסיבה לכך היא ייצוב הרזוננס של תוצר ביניים רדיקלי אלילי שנוצר, שיכול להסיר הלוגן מכל אחד מהאתרים (איור 1). <img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;"> </div>

בסינתזה אורגנית, היווצרות מוצרים יכולה להשתנות על ידי שינוי תנאי התגובה. לדוגמה, מוצר תוספת dibromo הוא ...

Education

JoVE Core

Organic Chemistry

כימיה אורגנית

Unit 1

יחידה 1

Radical Chemistry

כימיה רדיקלית

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

allylic bromination with nbs

Allylic bromination with NBS changes propene into allyl bromide under radical conditions. In this reaction, the goal is to favor substitution at the allylic position instead of bromine addition across the double bond. Using N-bromosuccinimide, or NBS, helps keep the bromine concentration low so the addition product is less likely to form.

The reaction conditions matter. Propene gives a dibromo addition product with bromine at room temperature. In non-polar solvents at high temperature, propene instead undergoes allylic substitution to form 3-bromopropene. When propene reacts with NBS in the presence of light or peroxide, it forms allyl bromide, which is also called 3-bromopropene.

The mechanism follows three familiar radical steps: initiation, propagation, and termination. During initiation, NBS undergoes homolytic cleavage of its weak N–Br bond when exposed to light or peroxide. This forms a bromine radical. In the first propagation step, that radical removes an allylic hydrogen and leaves behind a resonance-stabilized allylic radical plus HBr.

HBr does not build up. It reacts right away with NBS in an ionic step that produces Br2. The Br2 then takes part in the second propagation step. In the termination step, radicals combine to form nonradical products, which stops the chain reaction.

Keeping both HBr and Br2 at very low levels is important throughout the reaction. Under these conditions, the ionic addition of Br2 does not compete well with radical bromination. For allylic substituted alkenes, the reaction can give a mixture of products because the resonance-stabilized allylic radical can abstract halogen from either site.

Allylic Bromination with NBS

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Allylic bromination with NBS changes propene into allyl bromide under radical conditions. In this reaction, the goal is to favor substitution at the allylic position instead of bromine addition across the double bond. Using N-bromosuccinimide, or NBS, helps keep the bromine concentration low so the addition product is less likely to form. The reaction conditions matter. Propene gives a dibromo addition product with bromine at room temperature. In non-polar solvents at high temperature, propene...

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key terms and concepts

See how NBS helps propene undergo allylic bromination instead of addition. Learn the radical steps, low bromine conditions, and product mix.

Allylic Bromination with NBS | JoVE K12

organic chemistry

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Allylic Bromination with NBS

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