ELECTROPHILIC CONJUGATED ADDITION TO DIENES
Conjugated dienes do not behave as alkenes do.
The conjugated double C=C bonds cooperate!!!
Here you are the example of 1,3-butadiene.
Isn't 1,4-dibromo-2-butene an unexpected product?
In order to give it, the conjugated diene system has reacted as an 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 (1,3) addition products.
If the electrophilic addition to a conjugated diene is carried out in 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.