Aromatic hydrocarbons

The last class of hydrocarbons we will consider are known as aromatic compounds, so named originally because of the aromas they gave off. Aromatic compounds are generally cyclic conjugated systems. The simples example of such a system is 1,3,5-cyclohexatriene also known as benzene, whose formula is C$_6$H$_6$. Although the classical picture of this molecule is of a resonance structure:

Figure: Resonance in benzene.

However, quantum mechanics gives us a more accurate picture of the electronic structure of benzene. Each of the 6 carbons is $sp^2$ hybridized. Hence, the remaining unhybridized $2p_z$ orbitals can be combined in much the same way as was done for 1,3-butadiene. Here, we expect 6 new MOs, which are shown in the figure below:

Figure: Molecular orbitals that result in combining the $2p_z$ orbitals in benzene.

Note that, for each orbital, the plane of the 6 carbons is a nodal plane!! Unlike with 1,3-butadiene, however, each of the carbon-carbon bonds in benzene has the same length. This means that the bonds are really neither single nor double bonds but something intermediate between single and double. This is the origin of the third panel in the benzene resonance picture above. In fact, we can represent any organic molecule in this abstract fashion. We would eliminate the hydrogens and just show the bonding pattern between the carbons.