R/S Chirality Finder (CIP Rules, Step by Step)

In short: Draw a molecule or paste its SMILES. The tool marks every stereocentre, labels it R or S, and shows how each CIP priority was decided. Double bonds get E or Z.

Use it to check homework, to learn the Cahn–Ingold–Prelog rules, or to label a drug structure before you draw it for a report. Centres drawn without a wedge or dash are flagged as undefined.

How to use the R/S finder

1. Draw the molecule in the editor with wedge (toward you) and dash (away from you) bonds at each stereocentre, or paste a SMILES string with @/@@ and / \ marks.
2. The drawing on the right labels each centre R, S or (?) and each stereogenic double bond E or Z.
3. Each stereocentre gets a card with its four groups numbered 1–4 and the comparison that set every priority.
4. Click Open in MolDraw to keep the structure, add labels and export it.

CIP priority rules (Cahn–Ingold–Prelog)

1. Higher atomic number wins. Look at the four atoms joined to the stereocentre: I > Br > Cl > S > F > O > N > C > H.
2. Isotopes: if two atoms are the same element, the heavier isotope wins (D > H).
3. Ties: go one sphere further. List the three atoms attached to each tied atom, highest first, e.g. CH₂OH = {O, H, H} and CH(CH₃)₂ = {C, C, H}. Compare the lists at the first difference: {O, H, H} beats {C, C, H} because O beats C. If they are still equal, move out one more bond, always following the higher-ranked branch first.
4. Double and triple bonds count as two or three single bonds: in C=O the carbon gets a duplicate O, and the O gets a duplicate C. So –CHO = {O, O, H} and beats –CH₂OH = {O, H, H}.
5. Assign R or S. Point group 4 away from you and trace 1 → 2 → 3. Clockwise = R (rectus), anticlockwise = S (sinister).

E/Z for double bonds

Rank the two groups on each carbon of the C=C with the same rules. If the two higher-priority groups are on the same side, the bond is Z (zusammen, together). If they are on opposite sides, it is E (entgegen, opposite). E/Z can differ from cis/trans when the higher-priority group is not the larger alkyl chain.

How many stereoisomers?

A molecule with n stereocentres and stereogenic double bonds has at most 2ⁿ stereoisomers. A meso compound (such as meso-tartaric acid) has an internal mirror plane, so the real number is lower: tartaric acid has 3 stereoisomers, not 4.

Worked examples

Common mistakes

• Counting the size of a group instead of atomic number: –CH₂Br beats –C(CH₃)₃ because Br beats C at sphere 2.
• Adding up atomic numbers in a branch. Compare atom by atom at the first point of difference, not the total.
• Forgetting to duplicate atoms for C=O, C=C and C≡N.
• Reading the direction with the lowest group pointing toward you and not reversing the answer.
• Assuming L means S. It usually does for amino acids, but not for cysteine. D/L and R/S are different systems.

The tool applies the atomic-number and isotope rules (rules 1 and 2) with duplicate atoms. Rare cases that need the stereo-based rules 3–5 (for example pseudo-asymmetric centres) are reported as ties.

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