Question

Is `"benzene"` an aromatic species? What about `"cyclohexane"`, `"cyclohexene"`, and `"1.4-cyclohexadiene"`?

Answer 1

An aromatic compound requires `(4n+2)pi` electrons constrained in a ring. Benzene, fulfills these criteria; `C_6H_12`, `C_6H_10`, `C_6H_8` do not.

Explanation:

Benzene, `C_6H_6`, has `6` `pi` electrons, which are delocalized around a carbocycle. While a cyclohexatriene structure is often used to represent benzene, a hexagon enclosing a circle is probably a better representation of its aromaticity, and the non-locality of its `pi` electrons.

In neither the saturated nor the unsaturated carbocylic rings are the electrons delocalized, nor are any of these structures planar.

Chemists use the idea of aromaticity to rationalize the observed chemistry, and special stability of those compounds that they designate as `"aromatic"`. It turns out that this simple idea of `6pi` electrons is pretty powerful, and can be extended to rationalize the chemistry of heterocycles such as pyrrole, `C_4H_4NH` and furan `C_4H_4O`.