A stereocenter has four groups ranked 1–4 by CIP rules. Group 4 is on a wedge (pointing toward the viewer), and tracing 1→2→3 appears clockwise. What is the correct designation?
AR, because the 1→2→3 arc is clockwise
BS, because group 4 is toward you so the observed clockwise arc must be reversed
CR, because wedge bonds always indicate R configuration
DCannot be determined without knowing the atomic numbers
R/S requires group 4 pointing AWAY from you. When it points toward you (wedge), you are viewing from the wrong side — the configuration appears mirrored. Whatever direction you observe for 1→2→3, reverse it: clockwise becomes S, counterclockwise becomes R. Ignoring the viewing correction (option A) is the most common error when reading wedge-dash structures.
Question 2 Multiple Choice
A compound is determined to have (S) configuration at its only stereocenter. What can you conclude about its optical rotation?
AIt rotates plane-polarized light to the left (−)
BIt rotates plane-polarized light to the right (+)
CNothing — R/S designation has no predictable relationship to the direction of optical rotation
DIts optical rotation is opposite to that of the corresponding R enantiomer
R/S is a purely geometric label based on spatial arrangement of substituents. Optical rotation direction depends on the actual interaction of the molecule with polarized light and must be measured experimentally. The S designation does NOT predict levorotatory (−) behavior — that confusion conflates the Latin 'sinister' (left) with the optical property. Option D is also wrong: enantiomers do have opposite optical rotations, but this doesn't tell you the sign of either one.
Question 3 True / False
The CIP priority rules rank substituents on a stereocenter by atomic number, expanding outward to break ties — double bonds are treated as if each bonded atom appears twice.
TTrue
FFalse
Answer: True
This is the 'phantom atom' rule for handling unsaturation. A C=O carbon is treated as bonded to two oxygens (one real, one phantom), and the oxygen is treated as bonded to two carbons. This lets the standard atomic-number ranking handle alkenes, carbonyls, and other unsaturated groups without any special-case rules.
Question 4 True / False
A molecule designated (S) is expected to be levorotatory — it rotates plane-polarized light to the left.
TTrue
FFalse
Answer: False
R/S describes absolute configuration (spatial geometry); (+)/(−) describes optical rotation (an experimentally measured physical property). There is no systematic relationship between the two. Some S-configured compounds are dextrorotatory (+), others are levorotatory (−). The Latin origins of S (sinister = left) are a historical naming coincidence, not a rule about optical behavior.
Question 5 Short Answer
When determining R/S configuration, why must group 4 point away from the viewer? What adjustment do you make when it is on a wedge bond instead?
Think about your answer, then reveal below.
Model answer: Group 4 must point away because the R/S rule is defined for a specific viewing direction — looking at the stereocenter from the side opposite to group 4, so groups 1–3 face you. If group 4 is on a wedge (pointing toward you), you are looking from the wrong direction and see a mirror image of the correct view. In this case, determine the apparent 1→2→3 direction and then reverse it: if it looks clockwise (R), the true designation is S, and vice versa.
The R/S assignment is fundamentally a statement about spatial geometry as seen from a defined viewpoint. Misidentifying the viewpoint (forgetting to correct for group 4 pointing toward you) is one of the most common errors in stereochemistry problems. A systematic habit — always check where group 4 is before reading the arc direction — prevents this mistake.