When determining R/S of stereoisomers, the group with the smallest atomic number needs to be in the back (dashed line). But if no group attached to a chiral center is drawn in dashed line, do we assume that the undrawn hydogren is dashed? I have attached an example below, the chiral center is circled in red. Thanksenter image description here

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    $\begingroup$ We don't assume anything. The configuration of this center is unknown. $\endgroup$ Jul 22, 2018 at 21:03
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    $\begingroup$ .. btw, it's not true that "the group with the smallest atomic number needs to be in the back (dashed line)". $\endgroup$
    – mykhal
    Jul 22, 2018 at 22:13
  • $\begingroup$ so how would the R/S be determined in this molecule? $\endgroup$
    – Jon
    Jul 22, 2018 at 23:05
  • $\begingroup$ Jon: oh, there might be some confusion, I meant only for the structure representation. When determining the stereochemistry.. feel free to use your 3D spatial imagination and rotate your molecule in your head, you don't have to redraw the formula. But you cannot determine stereochemistry for chiral centers with unknown stereochemistry. $\endgroup$
    – mykhal
    Jul 23, 2018 at 6:17

2 Answers 2


When no bond from the chiral center has up or down graphical indication, it can mean any or unknown stereochemical configuration. Which might be confusing, because it can also mean, that the stereochemical configuration on this atom was mistakenly omitted.

To explicitly indicate that any or unknown configuration, wavy line bond can be used:

structure formula

(Hydrogen atom with the dashed bond on the (other) chiral center can be can be omitted at all, the stereochemical meaning is unambiguous, much more if the bond wedge was filled with black.)


Better practice is to indicate a specific stereochemistry explicitly, if you know or specifically presume it. In the molecule presented you could, of course, do this by drawing the methyl group with a wedge pointing into the chiral center or, for the opposite configuration, a hashed line.

In some situations where specific stereochemistry is unimportant, you can get away with generic lines. You could render the reaction of 2-butanol with acetic acid, to form an ester, quite well without fussing over the stereochremistry of the chiral center in the alcohol. But in a case where you have multiple chiral centers and their relative stereochemistries are important (like d-l versus meso tartaric acid), or if you are tracking the configuration around a single chiral center (like $S_N2$ substitution on 2-butanol), putting the stereochemistry explicitly into the structure is best whenever it it possible.


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