1,2 and 1,4 Addition to Conjugated Diene

Instead of forming the triangular halonium ion typical of electrophilic addition of halogen to alkene, halogen adds to a conjugated diene to form a resonance stabilized allylic carbocation intermediate. The two resonance forms of the resonancestabilized allylic carbocation lead respectively to two different possible products, the 1,2 and 1,4 products of addition. A very interesting and significant discussion arises from the fact that while the pathway to the 1,4 product occurs with greatest free energy decrease, the pathway to the 1,2 product is achieved with less activation energy. This means that formation of 1,4 product is favored thermodynamically, but 1,2 product is favored kinetically. Higher temperatures, ‘thermodynamic conditions’, promote the formation of 1,4 product, because if a larger fraction of the molecules possess activation energy for either pathway, as would occur at higher temperature, a large portion of diene concentration continuously moves down the 1,4 pathway and forms the more stable 1,4 product. Lower temperatures, however, are ‘kinetic conditions’. At lower temperatures, fewer reagent diene particles have enough energy to get over the activated complex energy hump to form the 1,4 product, so 1,2 addition predominates.