What it does
A carbon joined to four different groups is a chiral centre (stereocentre): the molecule and its mirror image cannot be turned to match, like your left and right hands. This page shows one such carbon in 3D. Drag to turn it; bold, larger groups point towards you and thin, dashed ones point away.
It ranks the four groups by the CIP priority rules and explains each step, then names the arrangement R or S. Press Put 4 at the back and the view turns so the lowest-priority group points away from you; a curved arrow runs 1 → 2 → 3. Clockwise is R, anticlockwise is S. Mirror image shows the other enantiomer, and Swap two groups shows that swapping any two flips the letter.
It follows the Class 12 Chemistry chapter on haloalkanes and haloarenes (optical isomerism, chirality, R and S, retention and inversion) and the Class 11 introduction to stereoisomerism. Find which isomers have a chiral carbon with the isomer explorer, see the tetrahedral shape behind it in the VSEPR shape viewer tool, and check the full symmetry of any molecule with the point group.
How to use it
- Choose a Molecule: CHFClBr, butan-2-ol, 2-chlorobutane, lactic acid, alanine, glyceraldehyde, 3-methylhexane, but-3-en-2-ol, or two that are not chiral. Or Build your own from 19 groups (H, D, halogens, OH, OCH₃, NH₂, CH₃, C₂H₅, propyl, isopropyl, CH₂OH, CH₂Cl, CHO, COOH, CN, CH=CH₂).
- Read the priorities (the blue numbers) and the steps that decide them.
- Press Put 4 at the back and follow the arrow.
- Try Mirror image and Swap two groups, and drag the picture to see that turning it never changes R or S.
Worked example: butan-2-ol
Carbon 2 holds H, OH, CH₃ and C₂H₅: four different groups, so it is chiral. First atoms: O (8) beats C (6), C (6) and H (1), so OH is 1 and H is 4. CH₃ and C₂H₅ tie on carbon; one atom further out, C₂H₅ has {C, H, H} and CH₃ has {H, H, H}, so C₂H₅ is 2 and CH₃ is 3. In the starting arrangement, putting H at the back gives OH → C₂H₅ → CH₃ going clockwise: (R)-butan-2-ol. Its mirror image is (S).
Tips
- Compare atoms, not whole groups. At the first difference, the higher atomic number wins; a big group of light atoms loses to a single heavy atom (Cl beats CH₂CH₂CH₃).
- Double bonds count twice: CHO is read as C attached to (O, O, H).
- If the lowest group points towards you, read the arrow and then take the opposite letter.
FAQ
Does R mean it turns light to the right? No. R and S describe the arrangement; the direction a compound rotates polarised light (+ or −) has to be measured.
Is this an NCERT tool? No. It is a free fibub page made for these chapters; it runs on your device.
Where the numbers come from
- Cahn–Ingold–Prelog priority rules (atomic number first, then the atoms one step further out, a double bond read as two single bonds to duplicate atoms) and the R/S convention are general organic chemistry, written fresh for fibub.
- Checked against the priority orders given in textbooks for lactic acid, alanine, glyceraldehyde, butan-2-ol and 3-methylhexane, and the standard comparisons CH=CH₂ > CH(CH₃)₂, CHO > CN and D > H; R/S checked by hand on a drawn CHFClBr, with mirror images and swaps flipping it.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.