Conjugated system
Different types of molecular structures can result in conjugation. For conjugation, all the neighboring atoms in a chain must have an available p-orbital. In many cases, conjugated systems are a series of a single bonds each followed by a double bond. Other systems exist. For example, furan (see picture) is a five-membered ring with two alternating double bonds and an oxygen in position 1. The Oxygen in furan has two lone pairs, one of which fills a p-orbital on that position. The lone pair electron makes conjugation possible on a five-membered ring. The presence of a nitrogen in the ring can be a source of pi orbitals needed for conjugation. Also, pi orbitals needed for conjugation can come from substituent groups α to the ring like: a carbonyl group (C=O), an imine group (C=N), a vinyl group (C=C), or an anion.