When compounds containing conjugated double bonds undergo typical ionic alkene addition reactions (addition of HBr or Br₂, for example), the products which are obtained are not those which would be expected for addition to the individual double bonds in the molecule. For the addition of HBr to 1,3-butadiene, two products are obtained, 3-bromo-1-butene and 1-bromo-2-butene. These products can be seen to arise from a "standard" 1,2-addition to one of the terminal double bond (Markovnikov-style), and from a 1,4-addition of HBr to the two terminal carbons, with relocation of the double bond onto the central two carbons.

The formation of these products can be readily understood by examining the mechanism of the addition reaction. Protonation of the conjugated diene on either terminal carbon will generate a carbocation on the adjacent secondary carbon. This carbocation, however, can be stabilized by resonance with the adjacent double bond to give a delocalized carbocation (an allylic carbocation) in which there is positive character on both a secondary center and on the terminal, primary carbon. Since both of these centers share positive character in the resonance hybrid, both are subject to nucleophilic attack by bromide anion; attack on the secondary carbon gives the 1,2-addition product, and attack on the terminal carbon gives the 1,4-addition product.

Some further observations on this reaction reveal the following:

• The 1,2-addition product forms rapidly at low temperatures;
• the 1,4-addition product is predominant at higher temperatures;
• even at low temperatures, 1,4-addition products will predominate if given enough time;
• the addition of HBr to butadiene is reversible and isolated 1,2-addition product will convert to the 1,4-product at higher temperatures or at longer times.

These data can be explained using the reaction coordinates shown below. The pathway to form the 1,2-product must have a lower activation energy, because it forms more rapidly than the 1,4-product. The 1,4-product, however, must be more stable than the 1,2-product because it accumulates at equilibrium (note that the reaction appears freely reversible, since isolated 1,2-product reverts to 1,4-, given enough time).

The 1,2-addition product is referred to as the kinetic product since it is formed faster. The 1,4-product is the thermodynamic product since it is thermodynamically more stable. A similar product distribution is observed for Br₂ addition, through a similar mechanism.