19.4: Nomenclature of Aryl and Heterocyclic Amines
The simplest aromatic amine is phenylamine, which contains an –NH2 functionality directly attached to an aromatic ring. The name aniline is designated for this skeleton. As shown in Figure 1, the common names of the functionalized anilines involve prefixes ortho-, meta-, and para- to indicate the substitution position. Different functionalized aniline derivatives also have notable trivial names.
Figure 1. Common names for functionalized anilines based on substitution.
For the systematic nomenclature, the ring is numbered from the nitrogen-bearing carbon in the direction that gives the lowest locant to the substituent. The order of numbering is to get the lowest number for the first point of difference in the case of amines with more than one substituent on the ring. As illustrated in Figure 2, the substituents are listed alphabetically. Alternatively, they are also referred to as derivatives of benzenamine.
Figure 2. Examples of substituents listed alphabetically in anilines.
Secondary and tertiary aromatic amines are named after the parent primary amines with N- substitutions prefixing the parent name of the amine, as presented in Figure 3.
Figure 3. Representative examples of secondary and tertiary aromatic amines.
Saturated and unsaturated heterocyclic amines, where N is a part of the heterocycle, have specific names based on the ring size and heteroatoms. Figure 4 presents some biological amines known by their common names—pyrrolidine, pyrrole, piperidine, pyridine, and imidazole.
Figure 4. Biological heterocyclic amines.
For the nomenclature of heterocyclic amines, the numbering starts at the heteroatom with higher atomic weight for amines containing other heteroatoms in addition to N. For example, as illustrated in Figure 5, the sulfur in thiazole is numbered one and continues in the direction to give the lowest numbering to the other heteroatom, nitrogen. A similar observation is noted in the presence of oxygen in morpholine.
Figure 5. Representative examples with the order of numbering in amines with other heteroatoms.
Alternatively, the prefixes aza- or diaza-, along with the locant number to indicate the number of carbons replaced with the nitrogen atom, are used before the parent alkane. This is more common in the nomenclature of a bicyclic system when the N atom is a part of a fused ring system, such as in DBU and DBN, as depicted in Figure 6.
Figure 6. Examples of nomenclature in bicyclic fused ring systems.