Conjugated Dienes:

• A conjugated system considers to a system of pi electrons that contains three or more 2p_z AO's on directly connected atoms. The resultant pi MO's are then delocalized over three or more atoms.
• A conjugated diene is a molecule that consists of two alkene linkages which are in conjugation, providing a four atom (also 4 AO) delocalized system.
• Simple instance of a conjugated system is 1, 3-butadiene, where the two pi bonds are directly connected so as to permit continuous overlap over the complete system of four carbon atoms.

• Dienes that are not conjugated dienes are exemplified through 1, 2-propadiene (allene) and 1, 5-pentadiene. In the later, the 2 alkene linkages are divided by a saturated carbon atom, thus that the pi overlap is not continuous. In the previous, the two pi bonds are perpendicular and do not interact or delocalize. However 1, 3-pentadiene is a conjugate diene.