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

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Organic Chemistry

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

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

The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s rule.


With conjugated systems like 1,3-butadiene, the addition of one equivalent of HBr yields a mixture of products: 1,2 and 1,4-addition products. As shown below, the mechanism involves the addition of H+ across one of the double bonds of the conjugated diene to form a resonance stabilized allyl cation. This is followed by the nucleophilic attack of Br at either carbon of the allyl cation bearing a positive charge to form the 1,2 or the 1,4-addition product.


The ratio of the products formed depends on the reaction temperature. Low temperature favors the 1,2-adduct, whereas the 1,4-adduct is preferentially formed at higher temperatures.


Electrophilic Addition HX 1,2-addition 1,4-addition HBr Alkenes Nonconjugated Dienes Markovnikov's Rule Conjugated Systems 1,3-butadiene Mixture Of Products Mechanism Resonance Stabilized Allyl Cation Nucleophilic Attack Reaction Temperature

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