Note: Each of the six orbitals overlaps equally with its two neighbors. As a result, the delocalized electrons form two π electron clouds (one above and one below the plane of the ring), as shown in Figure 6.6. This delocalization is characteristic of all aromatic molecules, and because of this, aromatic molecules are fairly unreactive.

Mnemonic: To remember the types and structures of these two classes of nitrogenous bases, remember to CUT the PYe (as C, U, and T are pyrimidines). You can also note that pie has one ring of crust, and pyrimidines have only one ring in their structure. You can also remember PURe As Gold (as A and G are purines); think of gold wedding rings. It takes two gold rings at a wedding, just like purines have two rings in their structure.