# Do you assume undrawn hydrogens are pointing away from you when determining R/S?

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. Thanks

• We don't assume anything. The configuration of this center is unknown. – Ivan Neretin Jul 22 '18 at 21:03
• .. btw, it's not true that "the group with the smallest atomic number needs to be in the back (dashed line)". – mykhal Jul 22 '18 at 22:13
• so how would the R/S be determined in this molecule? – Jon Jul 22 '18 at 23:05
• 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. – mykhal Jul 23 '18 at 6:17

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.