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. Aromaticity is a special type of conjugation describing a conjugated system in a planar ring where the number of π delocalized electrons that is even, but not a multiple of 4. That is, 4n + 2 number of π electrons, where n=0, 1, 2, 3, and so on. This is known as Hückel's Rule. Aromaticity allows for the electrons in the molecule's pi system to be completely delocalized around the ring. Aromaticitiy increases the molecule's stability to a greater degree than normal conjugation.
From the role of stable conjugated intermediates in enzyme reactions, to the in behavior of electron transporters such as NADH or ubiquinone, to the absorption qualities of pigments, concepts from conjugation and aromaticity come up again and again within biochemistry. For this reason, this is an area of particular focus within organic chemistry for the new MCAT.
Conjugated Species Mechanisms
Conceptual Vocabulary Self-Test
Basic Terms Crossword Puzzle
Basic Puzzle Solution
Conceptual Vocabulary for Conjugation and Aromaticity