R/S & E/Z Configuration Calculator
Find stereochemistry using CIP priorities. Choose R/S (one chiral center) or E/Z (one alkene), plug in where your priorities sit, and we’ll show the configuration + the exact rule used.
Background
R/S: Assign priorities (1 highest → 4 lowest). If #4 is away, then
1 → 2 → 3 clockwise = R, counterclockwise = S.
If #4 is toward you, you invert the result.
E/Z: Find the higher-priority group on the left alkene carbon and on the right alkene carbon.
Same side = Z, opposite sides = E.
How to use this calculator
- Pick a mode: R/S for one stereocenter, or E/Z for one alkene.
- R/S: Place priorities 1–4 at Top/Right/Bottom/Left. (Use each number once.)
- Don’t miss inversion: If #4 is toward you, the answer flips (R ↔ S).
- E/Z: Enter which substituent is higher priority on each carbon, then choose same side (Z) or opposite (E).
How this calculator works
- R/S step: We trace the path 1 → 2 → 3 on your drawing to see if it’s clockwise or counterclockwise.
- Invert rule: If #4 is toward you, we invert the result.
- Fischer shortcut: Top/bottom are away; left/right are toward you.
- E/Z step: We compare only the higher-priority group on each carbon: same side = Z, opposite = E.
Note: This calculator assumes you already assigned CIP priorities correctly. It focuses on the configuration step.
Formula & rules used
R/S rule: With #4 away, clockwise 1→2→3 = R, counterclockwise = S. If #4 is toward you, invert (R ↔ S).
E/Z rule: Determine the higher-priority substituent on each alkene carbon (CIP). Same side (Zusammen) = Z, opposite side (Entgegen) = E.
Example Problems & Step-by-Step Solutions
Example 1 — R/S (Fischer, no inversion)
In a Fischer projection, priority 4 is on top (vertical), so it is away. Trace 1→2→3. Clockwise = R, counterclockwise = S. (No flip.)
Example 2 — R/S (Fischer, inversion case)
If priority 4 is on left or right in a Fischer projection, it is toward you. Trace 1→2→3 to get a “base” R/S, then invert it.
Example 3 — E/Z (simple alkene)
Decide the higher-priority group on each carbon (CIP). If the two “high” groups are on the same side → Z. If they’re on opposite sides → E.
Example 4 — E/Z (different groups)
Suppose left carbon: Br beats CH3, and right carbon: Cl beats H. If Br and Cl are on the same side → Z. Opposite → E.
Frequently Asked Questions
Q: What does “invert” mean for R/S?
If the lowest priority group (#4) is pointing toward you, the clockwise/counterclockwise result flips: R becomes S and S becomes R.
Q: In a Fischer projection, which bonds point toward me?
In Fischer, horizontal bonds (left/right) point toward you, and vertical bonds (top/bottom) point away.
Q: Does this tool assign CIP priorities automatically?
Not in this version. It assumes you already know which substituents are #1–#4 (or hi/lo per alkene carbon).
Q: What if I accidentally use “2” twice?
The calculator blocks it. You must use 1, 2, 3, 4 exactly once.
Q: What’s the difference between E/Z and cis/trans?
Cis/trans works only when the same groups repeat on both alkene carbons. E/Z works in general using CIP priorities.
Q: Can I use this for multiple stereocenters?
This calculator handles one stereocenter (R/S) or one alkene (E/Z) at a time.
