Stereochemistry · Chemistry

R/S Configuration (CIP Rules) Simulation

Assign R or S step by step: rank by atomic number, break ties at the first point of difference, turn group 4 away and trace 1 → 2 → 3. Then decide whether two molecules are the same or mirror images, and watch one turned and reflected onto the other.

Interactive virtual lab · no download · works on phones and laptops

Chapter: Isomerism
For: Class 11 · Class 12 · JEE · NEET
Time: ~15 min

How to use it

1Assign R/S: pick a molecule and press Next step; the rank table explains each tie.
2Tick the shortcut to read it without turning, then see why you must reverse.
3Same or mirror image?: decide first, then tick “Turn B and lay it on A”, and “Reflect B” if needed.
4Fischer moves: turn, swap or rotate three groups, and watch whether R/S survives.

What to notice

Priority goes by atomic number of the first atom, never by the size of the group.
A tie is broken at the first point of difference; a double bond counts its partner twice.
With group 4 pointing at you, the direction you see is reversed.
Turning a molecule never changes R/S; reflecting it, or swapping two groups, always does.
In a Fischer projection a 180° turn keeps the molecule; 90° or one swap gives the enantiomer.

For teachers

Teach it in 23 minutes

A lesson plan, questions to put to the class, a quick quiz you can project or print, and the formulas behind R/S Configuration (CIP Rules).

1Hook
~2 min
- Hold up both hands and ask: “Same fingers, same order, so why can't you lay one exactly on the other?” Then open a chiral molecule.
2Demonstrate
~9 min
- Assign R/S: pick a molecule and press Next step; the rank table explains each tie.
- Tick the shortcut to read it without turning, then see why you must reverse.
- Same or mirror image?: decide first, then tick “Turn B and lay it on A”, and “Reflect B” if needed.
- Fischer moves: turn, swap or rotate three groups, and watch whether R/S survives.
3Pause and ask
~5 min
- Have a student say this in their own words: Priority goes by atomic number of the first atom, never by the size of the group.
- Have a student say this in their own words: A tie is broken at the first point of difference; a double bond counts its partner twice.
- Have a student say this in their own words: With group 4 pointing at you, the direction you see is reversed.
- Have a student say this in their own words: Turning a molecule never changes R/S; reflecting it, or swapping two groups, always does.
- Have a student say this in their own words: In a Fischer projection a 180° turn keeps the molecule; 90° or one swap gives the enantiomer.
4Check
~7 min
- Run the 7-question quiz below: project it and take answers by show of fingers (1–4), or print it as a worksheet.

About the R/S Configuration (CIP Rules) simulation

R/S (the Cahn–Ingold–Prelog system) names the arrangement of four different groups around a stereocentre. Rank the groups by the atomic number of the atom attached to the centre; break a tie at the first point of difference by comparing the sets of atoms attached next, highest first, with each double bond counting its partner twice. Then look with the lowest-priority group pointing away: if 1 → 2 → 3 runs clockwise the centre is R, anticlockwise S. If group 4 points towards you, the direction you see is reversed.

This R/S configuration simulation walks through those steps on a real 3D model: it shows the atomic numbers, explains each tie (vinyl against isopropyl, CHO against CH₂OH, CH₂Cl against isopropyl), turns the molecule slowly until group 4 is at the back, and draws the 1 → 2 → 3 arrow. The “Same or mirror image?” tab turns one molecule onto another so you can watch every group land on its twin, or two groups end up swapped, and reflects it when needed. The Fischer tab shows which moves of a Fischer projection keep the molecule (180° turn, two swaps, rotating three groups) and which make the enantiomer (90° turn, one swap).

Also covers: r s configuration, cip rules, cahn ingold prelog, priority rules, absolute configuration, chiral centre, stereocentre, enantiomers, identical or enantiomer, superimposable, mirror image, fischer projection r s, first point of difference.

Frequently asked questions

How do you break a tie in CIP priority rules?

List the three atoms attached to each tied atom, highest atomic number first, and compare them item by item. If they are still equal, move one sphere further out along the highest-ranked branch. The first difference decides.

Why does CHO outrank CH₂OH?

The C=O double bond is counted as two bonds to oxygen, so the CHO carbon carries (O, O, H) while CH₂OH carries (O, H, H).

How can I tell if two drawings are the same molecule or enantiomers?

Assign R/S to each. For a single stereocentre the same label means the same molecule and different labels mean enantiomers. Or turn one onto the other: if two groups always end up swapped, they are mirror images.