Assigning R and S Configuration: A Visual Guide
Practice CIP priorities, viewpoint and the clockwise/counterclockwise rule for stereocenters.
Understand the concept
The Cahn–Ingold–Prelog (CIP) rules assign priorities to four groups around a stereocenter. Higher atomic number wins at the first atom; when first atoms tie, compare atoms at the next layer in descending atomic number order.
With the lowest-priority group pointing away, clockwise movement from priority 1 → 2 → 3 indicates R, and counterclockwise indicates S. When priority 4 faces the viewer, reverse the apparent result.
A reliable four-step approach
- Identify a tetrahedral atom attached to four different substituents.
- Rank substituents using the first point of difference and atomic number.
- Reorient mentally so priority 4 points away; a dashed bond helps.
- Trace 1 → 2 → 3: clockwise gives R; counterclockwise gives S when 4 is away.
Visual explanation
Work through the examples
Rank OH, CH₂CH₃, CH₃ and H around the stereocenter of butan-2-ol.
Compare atoms directly attached: O beats C beats H. Both carbon groups begin with C; their next atom lists are (C,H,H) for ethyl and (H,H,H) for methyl.
Result: OH = 1; ethyl = 2; methyl = 3; H = 4.
The priority-4 group points away and 1 → 2 → 3 is counterclockwise.
The lowest-priority substituent is already oriented correctly, so apply the visible order directly.
Result: Configuration is S.
Common mistakes
- Ranking groups by total molecular size instead of first-point-of-difference atomic number.
- Forgetting to invert the apparent direction when priority 4 points toward you.
- Assuming R always corresponds to positive optical rotation.
Check your understanding
With priority 4 pointing away, a clockwise 1 → 2 → 3 sequence is:
Clockwise with the lowest priority away is R.
Complete the interactive practice below to check more questions and review explanations.
Interactive Practice
Answer curated questions, then review the scientific explanation.
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