Login-Verarbeitung ...

Chapter 16: Dienes, Conjugated Pi Systems, and Pericyclic Reactions Back To Chapter

16.7: Electrophilic 1,2- and 1,4-Addition of X₂ to 1,3-Butadiene

TABLE OF
CONTENTS

JoVE Core
Organic Chemistry

Ein Abonnement für JoVE ist erforderlich, um diesen Inhalt ansehen zu können. Melden Sie sich an oder starten Sie Ihre kostenlose Testversion.

Education

Electrophilic 1,2- and 1,4-Addition of X₂ to 1,3-Butadiene

Previous Video Next Video

Previous Video
16.6: Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene

Next Video
16.8: Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control

Embed Languages Add to Favorites ADD TO PLAYLIST

TRANSCRIPT

Besides hydrogen halides, halogens are another class of electrophilic reagents that react with conjugated dienes to form 1,2- and 1,4-addition products.

Here, the numbers designate the mode of addition of the electrophile.

When 1,3-butadiene is treated with bromine, one of the π bonds attacks the electrophilic center on the polarized halogen, displacing a bromide ion and forming a bromonium ion intermediate.

Next, the displaced bromide ion attacks the intermediate at one of the carbons, causing the ring to open.

Between C1 and C2, C2 is the secondary carbon that can stabilize the positive charge better. So, it is the preferred site for a nucleophilic attack by the bromide ion, giving the 1,2 -adduct.

Alternatively, an attack at C4 moves the double bond to the adjacent position, displacing the bromonium ion at C2 and forming the 1,4 -adduct.

Lastly, the 1,4-dihalide is dominant at higher temperatures, whereas the 1,2-dihalide is favored at lower temperatures.

16.7: Electrophilic 1,2- and 1,4-Addition of X₂ to 1,3-Butadiene

Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide.

Conjugated dienes react with halogens in a similar manner. However, in addition to the 1,2-dihalide, they also form a 1,4-dihalide. The mechanism involves two steps.

First, a nucleophilic attack by one of the diene π bonds on the electrophilic center of the polarized halogen molecule forms a halonium ion intermediate. This is followed by a nucleophilic attack of the displaced halide ion at C2 of the intermediate to give the 1,2-product or at C4 to form the 1,4-addition product. The ratio of the products formed depends on the reaction temperature. Low temperature favors the 1,2-product, whereas high temperature favors the 1,4-product.

Tags

Electrophilic Addition 1,2-Addition 1,4-Addition X2 1,3-Butadiene Halogens Cyclic Halonium Ion 1,2-dihalide Vicinal Dihalide Conjugated Dienes Nucleophilic Attack Electrophilic Center Polarized Halogen Molecule Halonium Ion Intermediate Halide Ion Reaction Temperature

X

Simple Hit Counter