Conjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in molecules with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the system. The compound may be cyclic, acyclic, linear or mixed.
Conjugation is the overlap of one p-orbital with another across an intervening sigma bond (in transition metals d-orbitals can be involved).
A conjugated system has a region of overlapping p-orbitals, bridging the adjacent single bonds. They allow a delocalization of pi electrons across all the adjacent aligned p-orbitals.
The pi electrons do not belong to a single bond or atom, but rather to a group of atoms.
The largest conjugated systems are found in graphene, graphite, conductive polymers, and carbon nanotubes.
Mechanism
Conjugation is possible by means of alternating single and double bonds. As long as each contiguous atom in a chain has an available p-orbital, the system can be considered conjugated. For example, furan (see picture) is a five-membered ring with two alternating double bonds and an oxygen in position 1. Oxygen has two lone pairs, one of which occupies a p-orbital on that position, thereby maintaining the conjugation of that five-membered ring. The presence of a nitrogen in the ring or groups α to the ring like a carbonyl group (C=O), an imine group (C=N), a vinyl group (C=C), or an anion will also suffice as a source of pi orbitals to maintain conjugation.
