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

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Organic Chemistry

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

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.



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

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