Fused Ring and Bridged Fused Ring Nomenclature
(IUPAC Recommendations 1998)

# FR-5.Numbering

Continued from FR-4.5 Choice of Locants

Contents of Section

Continue with FR-5.3 Peripheral Numbering.

FR-5 Numbering

The older rules for numbering fused ring systems (ref 2) were based on the concept of converting all systems into an approximate two-dimensional hexagonal grid framework. As the complexity of fused ring systems increased these arbitrary rules have been extended as needed. It is recognised that there are problems which are not covered by this document with some systems containing three-membered rings or with rings greater than seven.

Note

All ring systems in this document have been drawn so that the peripheral numbering (not always shown) is clockwise and starts approximately in the top right hand ring.

Before the fused ring system can be orientated it must be drawn according to the following specifications. If more than one drawing meets these specifications the one chosen is that which gives the more preferred orientation (FR-5.2) and if there is still a choice the more preferred numbering (FR-5.3).

In the preferred orientation of a ring system, as many rings as possible should be in the same horizontal row (FR-5.2.a). Permitted shapes for three- to six-membered rings which form the horizontal row are:

The five-membered ring may be considered as a six-membered ring with one atom eliminated and replaced by an elongated bond (shown thickened above). Internal ring angles of 120o are marked with an 'x'.

Rings with an even number of members greater than six are considered as modified six-membered rings with additional pairs of horizontal bonds inserted between the two halves. This arrangement ensures that rings fused on both sides in the horizontal row are fused to bonds as far apart as possible. For eight- and ten-membered rings permitted shapes are:

A ring with an odd number of members greater than six is considered as a modification of the corresponding even membered ring with one more or less member above or below so that rings fused on both sides in the horizontal row are fused to bonds as far apart as possible. For the seven- and nine-membered rings permitted shapes are:

The three left-hand forms correspond to insertion into a smaller ring with one less member. The two right-hand forms correspond to deletion from larger rings with one more member. In each case the rings may also be orientated with the modified region at the top instead of the bottom.

Example:

(axis of horizontal row shown by dotted line)

Rings ortho- or ortho- and peri-fused to the horizontal row may also be drawn as shown in FR-5.1.1 but other shapes or orientations are permitted. With a five-membered ring the shape may need to be orientated as follows:

Additional modifications of rings with more than seven members are permitted. For eight-, nine- and ten-membered rings permitted shapes are:

These modifications may be required for fusion to the horizontal row (see FR-5.3 example 3), or, if additional rings are fused above or below the nine- or ten-membered rings, they may need to be used for rings in the main horizontal row provided the rings on either side in the horizontal row are fused to bonds as far apart as possible (i.e. to the bond with a dot at each end for a ten-membered ring).

In drawing an ortho- and peri-fusion the individual rings should be orientated so that each internal ring angle involved is 120o. Such angles are marked with an 'x' in the above diagrams. If this is not possible see FR- 5.1.5. Elongated bonds, shown thickened in the diagrams above, should not be used for fusion. If this is not possible see FR-5.1.6.

Example:

not
Only preferred shapes used not Distorted ring fused to elongated bond

If the fused ring system can be drawn in more than one way using the preferred ring shapes (FR-5.1.1 and FR-5.1.2) preference is given to those forms which do not involve overlap of bonds. If, when using these preferred ring shapes two bonds overlap, it is necessary to draw one (or both) of the rings involving those bonds with a modified shape to show that the two rings are not ortho-fused together. This modification is not considered to represent distortion in the sense of FR-5.1.5.

Example:

not
Only preferred shapes used, slightly modified not Distorted ring fused to elongated bond

If the fused ring system can be drawn in more than one way using the preferred ring shapes (FR-5.1.1 and FR-5.1.2) preference is given to those forms which do not involve overlap of rings. If, when using these preferred ring shapes two rings overlap, it may be necessary to draw one or more rings with a slight modification to show the two overlapping rings. This modification does not constitute distortion in the sense of FR-5.1.5.

Examples:

not
No overlap of rings not Overlap of rings

not
Only preferred shapes used not Distorted ring shape

If the fused ring system cannot be drawn using only the above shapes then the minimum number of distorted ring shapes should be used. Where there is a choice distortion of a smaller ring is preferred to distortion of a larger ring. If possible the rings in the horizontal row should not be distorted. The following shapes are quite often required:

The 120o angles marked with an 'x' are suitable for peri-fusion. The thickened (elongated bonds) should not be used for fusion.

Example:

not
Distorted four-membered ring preferred to distorted six-membered ring

If the fused ring system cannot be drawn following FR-5.1.1, FR-5.1.2 or FR-5.1.5 then the minimum number of elongated bonds may be used for fusion. If there is a choice then elongated bonds of a smaller ring are preferred to those of a larger ring.

Example:

Before the drawing (or drawings if there is more than one of equal preference) is numbered it needs to be orientated. In considering possible orientations it should be remembered that the system can be rotated in the plane of the ring system and can also be turned over. The preferred orientation is established by consideration of the following points in order. If a choice remains, the final decision is made by consideration of the preferred numbering (see FR-5.4).

a. Maximum number of rings in a horizontal row

The preferred drawing is that with the greatest number of ortho-fused rings, with vertical common bonds, in a horizontal row. The relevant vertical bonds are always those furthest apart. If the correct orientation is not immediately apparent bisect the horizontal row by a horizontal axis and a vertical axis to form four quadrants. Rings which are bisected by the horizontal axis but are not directly ortho-fused to the main row are not considered when counting how many rings are in the row (see example 2 in FR- 5.2.b).

Example:

is preferred to
3 rings in row is preferred to 2 rings in a row

b. Maximum number of rings in upper right quadrant

In the preferred orientation, the maximum number of rings should appear above the horizontal row on the right hand side (upper right quadrant). For this purpose the centre of the row is defined as the central common bond if there is an even number of rings in the row, and the centre of the central ring if there is an odd number of rings. When counting rings in a quadrant those rings which are divided by an axis are considered as two halves and the ring at the centre of the row, if bisected by both axes, counts as four quarters.

Examples:

is preferred to
2 rings top right is preferred to 1 ring top right

is preferred to
3.5 rings top right is preferred to 3 rings top right

Notes

1. The right hand structure only has two rings ortho-fused in a row. The third ring in the row is not directly ortho-fused to the other two.

2. Rings are counted in the quadrant in which they are drawn.

c. Minimum number of rings in lower left quadrant

In the preferred orientation the minimum number of rings should appear below the row on the left hand side (lower left quadrant).

Example:

is preferred to
0.75 ring lower left is preferred to 1.75 rings lower left

d. Maximum number of rings above the horizontal row

In the preferred orientation, the maximum number of rings should appear above the horizontal row identified in FR-5.2.a.

Example:

is preferred to or
3.5 rings above row is preferred to 2.5 rings above row

is preferred to
3.5 rings above row is preferred to 2.5 rings above row

e. Preferred peripheral numbering
The preferred orientation is that which gives preferred peripheral numbering (see FR-5.3).

Reference for this Section

2. A.M. Patterson, Proposed international rules for numbering organic ring systems, J. Am. Chem. Soc. 47, 543-561 (1925); Rec. trav. chim. Pays-Bas 45, 1-18 (1926); A.M. Patterson, Nomenclature of parent ring systems, J. Am. Chem. Soc. 50, 3074-3087 (1928); A.M. Patterson and L.T. Capell, The Ring Index, Reinhold, New York, 1940, p. 599-611; The Chemical Society (London); The French Chemical Society.

Continue with FR-5.3 Peripheral Numbering.