Conjugated system

The stability of a conjugated diene arises because of an interaction between the p orbitals of the two double bonds. To review briefly, when two p atomic orbitals combine to form a π bond, two π molecular orbitals (MOs) result. One is lower in energy than the starting p orbitals and is therefore bonding; the other is higher in energy, has a node between nuclei, and is antibonding. The two p electrons occupy the low-energy, bonding orbital, resulting in formation of a stable bond between atoms.

— Conjugated system

Dienes are named according to the rules formulated for ordinary alkenes. Conjugated dienes are more stable than dienes containing two isolated double bonds, as measured by their heats of hydrogenation. (...) The molecular-orbital picture of the π system in 1,3-butadiene shows two bonding and two antibonding orbitals. The four electrons are placed in the first two bonding levels.

— Conjugated system

Conjugated dienes are electron rich and are attacked by electrophiles to give intermediate allylic cations on the way to 1,2- and 1,4-addition products. These reactions may be subject to kinetic control at relatively low temperatures. At relatively higher temperatures, the kinetic product ratios may change to thermodynamic product ratios, when such product formation is reversible.

— Conjugated system

Cyclic conjugated polyenes are aromatic if their p electron count is 4n + 2. This number corresponds to a completely filled set of bonding molecular orbitals. Conversely, 4n p systems have open-shell, antiaromatic structures that are unstable, are reactive, and lack aromatic ring-current effects in H NMR. Finally, when steric constraints impose nonplanarity, cyclic polyenes behave as nonaromatic alkenes.

— Conjugated system