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Conjugated Electrophilic Addition

Conjugated dienes do not behave as alkenes do. The conjugated C=C double bonds cooperate! Here is the example of 1,3-butadiene:

Addition of Br₂ to 1,3-butadiene: at −15 °C, 46% 1,2-product and 54% 1,4-product; at 60 °C, 10% and 90%

Isn't 1,4-dibromo-2-butene an unexpected product? In order to give it, the conjugated diene system has reacted as a special unit, not like common alkenes. The other product, 3,4-dibromo-1-butene, is the expected product of the normal bromine addition to one of the two double C=C bonds.

One of the two double C=C bonds attacks the bromine molecule, leading to an allylic carbocation, where the positive charge is delocalized between positions 1 and 3. That's why the reaction yields the normal (1,2) and conjugated (1,4) addition products.

Energy diagram of conjugate addition: the allylic carbocation gives the 1,2-product faster (kinetic control) and the more stable 1,4-product (thermodynamic control)

If the electrophilic addition to a conjugated diene is carried out under thermodynamic control conditions (high temperatures and long reaction times), the ratio of conjugated addition increases, the 1,4 product being the major one in most cases.