Members of the phenothiazine class of classic antipsychotics share the same basic phenothiazine ring but differ in substitutions at both their R1 and R2 positions (Figure 27–1). Based on the side chain attached to the nitrogen atom in the middle ring (R1), the phenothiazines are further subdivided into three subtypes: aliphatic, piperidine, and piperazine phenothiazines. There is much more variety in the substitutions at the second carbon atom; generally more negatively charged (electrophilic) groups tend to result in stronger binding to dopamine receptors. These electrophilic groups weaken the aromatic bonds within the phenothiazine ring by partially drawing bonding electrons into their orbit.