Standard chemical structure diagrams imply a valence-bond interpretation, with two electrons localized in the general vicinity of a single bond, four electrons near a double bond, and six electrons near a triple bond. It is well known that the physical reality isn't that simple, and that many substances exhibit significant electron mobility.
Possibly the most common class is the collection of compounds characterized as having the characteristic of aromaticity. Aromatic compounds have a cyclic π system generally containing 4n + 2 electrons, for n = 0, 1, 2, etc. Benzene is the archetypical example. Conventionally, benzene is commonly drawn with three single bonds and three double bond, but physically, the six bonds of benzene are indistinguishable from each other. One way to depict this equivalence explicitly is to draw benzene not as a single structure but as a pair of resonance forms:
However, benzene is a very simple case. It is rarely convenient to provide diagrams of all equivalent resonance forms. Instead, a shorthand has been developed that indicates all resonance forms by a curve or circle:
These recommendations discuss preferred usages for these sorts of circles, and by extension, similar curves that represent other types of electron delocalization
Above all else, curves represent delocalization. Accordingly, they may be used to depict systems that have delocalized electrons. They should not be used to depict systems whose electrons are primarily localized.
| RECOMMENDED | RECOMMENDED | RECOMMENDED | RECOMMENDED | AVOID | AVOID |
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Curves only represent the delocalized electrons associated with their adjacent bonds. Most systems will have, at the very least, a σ framework that is not delocalized; it is this framework that is represented by the "single" bonds adjacent to the curve. Structures with additional unsaturation will need further representation. For example, a depiction of the delocalized form of benzyne would have a "double" bond replacing one of the "single" bonds.
Curves represent delocalization, but lots of structures have some delocalized elements. Although all such elements could be depicted with curves, there is little gained by doing so. On the contrary, the arbitrary use of curves can draw the viewers attention to insignificant portions of a structural diagram, and away from areas that are chemically more important such as an active site or a reactive group. Accordingly, curves should only be used when the delocalization is specifically being highlighted as a important feature of the structure.
When curves are not used, any alternating configuration of double bonds is acceptible. There is no preference for a specific form.
| RECOMMENDED | RECOMMENDED | AVOID (without specific reason) |
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By the same token, curves should be used for any system that cannot be adequately represented by alternating single and double bonds.
| RECOMMENDED | AVOID | AVOID | AVOID |
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Curves should be drawn to approach all associates bonds at a consistent distance. That means, basically, that a circle should be well-centered within its framework of bonds. At its closest approach, the circle should remain separated from each bond by a distance at least equivalent to the width of the bond, and no greater than twice the distance by which a double bond would be specified.
| RECOMMENDED (with specific reason) | AVOID | AVOID | AVOID |
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Additionally, there is no restriction that curves be circular. It is extremely common for curves to be drawn as ellipses when they are associated with ring systems that are viewed in perspective. Curves do not have to be convex either (although non-convex delocalized systems are uncommon). Curves don't even have to be closed: delocalization can also occur in non-cyclic systems such as the allyl radical.
| RECOMMENDED | RECOMMENDED | RECOMMENDED |
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Curves should be drawn as solid lines the same width as a single bond.
| RECOMMENDED | AVOID | AVOID |
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