What Is Aromaticity?
At one time the term “aromatic” applied only to benzene. To some chemists it still simply means “like benzene.” An aromatic molecule is a planar ring with a circular cloud of delocalized π electrons. It is an unsaturated cyclic molecule stabilized by resonance. It is a very stable unsaturated ring that reacts by substitution instead of addition. Or, according to Hückel’s rule, it is a fully conjugated unsaturated ring that has 4n + 2 π electrons. How do you know whether or not a molecule is aromatic? If an unsaturated ring compound is aromatic, its heat of hydrogenation and its heat of combustion will both be considerably less than they would be if double bonds were present. Its bond distances (as measured by x-ray or electron diffraction or by microwave spectroscopy) will be uniform. It will have longer wavelength absorption bands in the ultraviolet region of the spectrum. An aromatic compound will have diamagnetic anisotropy (meaning that a crystal will have more magnetic susceptibil
