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Introduction

In the general context of structure-orientation issues, the positioning of double bonds in rings is a minor issue, far less important than the positioning of the rings themselves in relation to other portions of the structure. Still, there can be a question of positioning of double bonds in ring with highly symmetric substituents (or with no substituents at all). Also, there is always a question of the cyclic positioning of double bonds within mancude ring systems, including aromatic systems.

Positioning of isolated double bonds

Double bonds should be positioned toward the left (and secondarily toward the top) of a ring.

RECOMMENDED	RECOMMENDED	AVOID	AVOID	AVOID	AVOID	AVOID	AVOID

However, the positioning of double bonds is of less concern than the positioning of fused ring systems and of substituents.

RECOMMENDED	RECOMMENDED	AVOID	AVOID	AVOID

Alternating double bonds may be in either resonance form

In delocalized single-ring systems containing alternating single and double bonds, the bonds may often be drawn in either of two resonance forms. For structures where such a ring is treated as the primary ring and drawn in the standard orientation, it is somewhat more prevalent in common usage to draw the ring with a double bond on the leftmost side (rather than a single bond on the leftmost side). However, that is minor detail and either of these forms is acceptable in all cases.

RECOMMENDED	RECOMMENDED

In highly symmetrical structures, it's preferable for the double bonds within aromatic systems to be drawn in a symmetrical fashion as well.

RECOMMENDED	RECOMMENDED	AVOID	AVOID

Non-resonating aromatic rings may influence double bond positioning in fused rings

Some rings are traditionally considered to be aromatic, but can be represented by only a single representation of double bonds. This is most common in five-membered heterocycles such as furan, where the aromaticity is conferred by a lone pair from the heteroatom and that heteroatom, therefore, cannot be represented with an adjacent double bond. When such rings are present in larger fused ring systems, it is preferable to preserve the standard double-bond positioning in the odd-membered ring, and that preference may influence the double bond positioning in the larger system. As above, however, that is minor detail and either of these forms is acceptable in all cases.

RECOMMENDED	ACCEPTABLE

In some cases, of course, such a positioning may be structurally impossible.

RECOMMENDED

[After considerable discussion, we decided that any recommendation of this sort would be arbitrary, have frequent exceptions, and in general would be more trouble than it was worth. So, at the moment we propose sticking with the guideline two above: double bonds can go anywhere you like in alternating systems. Does anyone want to speak up for the rule below? Please provide supporting data if so.]

Alternating double bonds should prefer peripheral bonds

In fused ring systems containing delocalized alternating single and double bonds, the double bonds should be positioned on peripheral bonds to the greatest extent possible, and should avoid fusion bonds. As above, for structures where such a ring is treated as the primary ring and drawn in the standard orientation, it is somewhat more prevalent in common usage to draw the ring with a double bond on the left side of the leftmost bond. However, that is minor detail, and it should be clear that all of these configurations (including the ones labeled "AVOID") are acceptable, even if not preferred.

Morrison and Boyd has this to say: " I II III X-ray analysis shows that, in contrast to benzene, all carbon-carbon bonds in naphthalene are not the same; in particular, the C1-C2 bond is considerably shorter (1.365 Å) than the C2-C3 bond (1.404 Å). Examination of structures I, II, and III shows us that this difference in bond lengths is to be expected. The C1-C2 bond is double in two structures and single in only one; the C2-C3 bond is single in two structures and double in only one. We would therefore expect the C1-C2 bond to have more double-bond character than single, and the C2-C3 bond to have more single-bond character than double." That would suggest that the fusion bond has more single bond character than double, and would further argue for depicting the double bonds around the exterior of the fused ring system, with the internal fusion bonds preferentially single, not double. What do we think about this?

RECOMMENDED	RECOMMENDED	AVOID	AVOID	AVOID

RECOMMENDED	AVOID