The basic order of seniority of subunits is :
heterocyclic rings and rings systems > heteroatom chains > carbocyclic rings and ring systems > acyclic carbon chains .
Rule 2
Heterocyclic rings and ring systems
Among heterocyclic rings and ring systems, the descending order of seniority is :
a. a ring or ring system containing nitrogen;
b. a ring or ring system containing the teteroatom occurring earliest in the order given in Rule 4;
c. a ring or ring system containing the greatest number of ring;
d. a ring or ring system having the largest individual ringat the first point of difference when the rings are represented in decreasing order of size;
e. a ring or ring system having the greatest number of heteroatoms;f. a ring or ring system containing the greatest variety of heteroatoms;
g. a ring or ring system having the greatest number of heteroatoms earliest in the order given in Rule 4;
h. of the two rings or ring systems of the same size containing the same number and kind of heteroatoms the senior system is that one with the lower locant for the heteroatoms.
Examples :
 |
| > (2b) |
 |
| > (2d) |
 |
| > (2c) |
 |
| > (2h) |
 |
| > (2h) |
 |
| > (2d) |
 |
| > (2c) |
 |
| > (2e) |
 |
| > (2d) |
 |
| > (2e) |
 |
| > (2e) |
 |
| > (2a) |
 |
| > (2c) |
 |
| > (2b) |
@ appropriate rule in parentheses
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