Aromatic compounds are often characterized by a particular smell and are defined as conjugated cycloalkenes. Their main property is increased stability due to the ability of electrons in the perpendicular p orbitals to delocalize and interact with each other.
The main requirements to define a compound as aromatic are met when:
The compound is cyclic.
The molecule is planar, hence each element in the ring has a perpendicular p orbital.
The molecule is conjugated.
Unless all three criteria are met, the molecule cannot be defined as aromatic. A common test to quantify the resonance energy in a compound is to compare the values of their heats of hydrogenation. Usually, these values increase with the higher number of double bonds, however, when taking into consideration benzene, its heat of hydrogenation is lower than 1,3-cyclohexadiene, a compound with only two double bonds. These experiments allow o quantify the effects of stabilization that are connected with the delocalization of p orbitals. To be fully considered aromatic, the compound also has to follow the Huckel’s rule, which connects the number of pi electrons to aromaticity.