We shall meet many of the derivatives of these aromatic compounds
in later chapters, but a few of the more common ones are shown right.
Toluene, phenol, aniline, and benzoic acid are derivatives in which
different chemical groups are substituted onto the benzene ring.
Pyridine is a variant in which a nitrogen atom replaces a C-H in
the ring without destroying delocalization.
As we saw in Chapter 10, pyridine is a base because it can use
its nitrogen lone pair to bind a proton. Ring compounds with more
extensive nitrogen substitution are important components of DNA
and other nucleic acids.
The large porphyrin ring at the far right is the parent compound
of chlorophyll and heme. The entire 24-atom ring is one large delocalized
system, which makes the molecule a good absorber of light energy.
Porphyrin has the same number of delocalized double bonds as P-carotene
does, eleven, but is a cyclic rather than a linear molecule (click
button for more information).