When present, ring systems generally serve as the focal point of a chemical structure. Although there are exceptions, the "principal" ring system is normally positioned in a preferred orientation, and all other substituents are only then positioned accordingly.
In many cases -- and particularly when there is only one ring system in the entire structure -- selection of the principal ring system is obvious. In more complicated cases, there is substantial flexibility. Although it is always acceptable to select the principle ring system according to the hierarchy described in the Nomenclature of Fused and Bridged Fused Ring Systems guidelines, rigorous application of that hierarchy is unnecessary if the main intention is simply to produce an aesthetic diagram. For most practical purposes, it is sufficent to select the principal ring system as the one that:
Those four guidelines are a tremendous oversimplification of the full Fused Ring Systems guidelines, but they will serve most purposes. Phrased differently, the principal ring system may be considered to be "the ring system that seems most important." Those three guidelines will usually produce the ring system that most chemists would consider to be "most important" in a structure, but they need to be followed rigorously, either. In a discussion of several related substances, for example, it would be reasonable to select a single ring system to be considered as "most important" across the collection. So the single-ring benzoic acid might well be considered "more important" than a two-ring naphthyl group in a collection of structures that consisted of 2-fluorobenzoic acid, 2-chlorobenzoic acid, 2-bromobenzoic acid, 2-hydroxybenzoic acid, 2-cyclohexylbenzoic acid, 2-naphthylbenzoic acid, etc.
Nor should the simple presence of a ring be an absolute determining factor for the orientation of a structure! In a structure with a chain that is large compared to the size of the largest ring system, it is often reasonable to treat the chain as the primary system for orienting the structure, and allow the ring(s) to be drawn as simple substitutents once the main chain is positioned.
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If the structure features a ring system that is reasonable to treat as a primary ring, that ring should be presented in its preferred orientation. Single-ring systems should be drawn as regular polygons. Rings of 4, 6, or 8 members should be drawn with a vertical bond on the leftmost edge of the ring. Rings of 3, 5, or 7 members may be drawn with a vertical bond on their leftmost edge, or they may be drawn with a horizontal bond on their bottommost edge [Hm. Is this right? Or do we want to say that odd rings should always have flat edge on bottom?]:
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Single rings of more than 8 members should be draw with reentrant bond angles ("puckered"). Some examples are shown. For selection of preferred orientation, these should be treated analogously to the corresponding fused ring system. Note that there will often be more than one reasonable configuration as the rings get larger:
For ring systems containing at least two fused rings, the preferred orientation gets a little more complicated. For most practical purposes, it is sufficent to orient the ring system so that:
These guidelines intentionally differ from the Nomenclature of Fused and Bridged Fused Ring Systems. Those guidelines should be used to number the atoms within a fused ring system, but these should be used when depicting that ring system after its atoms have been numbered.
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It should also be noted that several classes of compounds, including steroids and many other natural products, have preferred orientations that are specific to those classes. In some cases (including most steroids), the preferred orientation of the class is very similar to the recommendations for fused rings in general. In other cases, the preferred orientation of the class may be very different. When working with one of these compounds, the class-based orientation is preferred. In rare cases, a single compound might contain elements of more than one class, for example a prostaglandin bound to a steroid. In such a compound, either class may be chosen as the principal one, depending which features the author wishes to emphasize.
As discussed in the section on selecting the principal ring system, these guidelines need not be followed slavishly. According to these guidelines, a quinoline system should be drawn horizontally, with the nitrogen toward the right and, most commonly, toward the bottom. Any further substituents would be positioned with respect to that ring system, as shown below, left. In some cases, however, it is useful to emphasize some other portion of the structure. In a context where the acid group is the most interesting feature, it might be reasonable to invert the ring system to position the acid group more prominently toward the top right. Those portions of these guidelines that make sense should still be followed, however -- the quinoline system should still be oriented horizontally, and not at an angle.
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In the absence of a different overriding concern, the largest ring system within a structure should be oriented toward the bottom-left of the structure and substituents should be arranged to spread horizontally rather than vertically
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This recommendation, combined with the previous one, implies that most ring systems are should be positioned so that their systematic numbering starts toward the top right and continues counterclockwise. However, that is simply a common consequence and not in any sense a requirement. Preferred orientations for polysubstituted six-membered rings are shown below. In most cases, the 1-position is indeed at the top right of the ring, but while the 1,2-disubstitued ring can be numbered clockwise, the 1,3-disubstitution is best numbered counterclockwise. It is more important to preserve the overall horizontal orientation than it is to enforce the absolute position of the individual atom numbers.
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| 1 | 1,2 | 1,3 | 1,4 | 1,2,3 | 1,2,4 | 1,3,5 |
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| 1,2,3,4 | 1,2,3,5 | 1,2,4,5 | 1,2,3,4,5 |
Similarly, because carbonyl groups should preferentially be drawn vertical, it is common to draw oxo-substituted rings with the carbonyl group pointing upward. It is still acceptable to draw them with the carbonyl group in the orientation preferred for non-carbonyl substituents, of course.
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