Aromatic Hydrocarbons

The building block of aromatic hydrocarbons is the benzene ring.

The arrangement of atoms is shown on the left. The version in the center is often used to simplify diagrams of molecular structures.
The three double bonds are not restricted to the positions shown but are free to pass around the ring. This is sometimes indicated by drawing the benzene ring as it is on the far right.

Some examples of biological molecules that incorporate the benzene ring:

•       The amino acids tyrosine and phenylalanine
•       cholesterol and its various derivatives, such as the sex hormones
•       estrogens
•      testosterone
•        the herbicide, 2,4-D

The carotenoid, beta-carotene, is a hydrocarbon that has both aliphatic and aromatic portions.