Stereoisomer Enumerator (2ⁿ Rule, Meso Forms, Enantiomers)

In short: Draw a molecule or paste its SMILES. The tool finds every stereocentre and stereogenic double bond, builds all 2ⁿ arrangements, removes the duplicates so meso forms count once, labels each isomer R/S and E/Z, and sorts them into enantiomer pairs and diastereomers.

Use it to answer "how many stereoisomers?" questions, to see why tartaric acid has 3 and not 4, or to draw all 16 aldohexoses. Stereo marks you give in the SMILES are kept as a filter: the matching isomers are marked.

How to use the stereoisomer enumerator

1. Draw the molecule in the editor, paste a SMILES and press Enter, or press an example such as Tartaric acid. Stereo marks are optional.
2. Read the count: the number of stereocentres and stereogenic double bonds, the 2ⁿ upper limit, and how many real stereoisomers remain.
3. Each stereoisomer is drawn with its R/S and E/Z labels and tagged chiral, meso or achiral. Enantiomers are shown in mirror pairs.
4. The table at the end says whether any two isomers are enantiomers or diastereomers. Press Open in MolDraw under a structure to edit or export it.

Counting stereoisomers: the 2ⁿ rule

A stereocentre is usually a carbon with four different groups. A stereogenic double bond has two different groups on each end, so it can be E or Z. Each one has two choices, so n of them give at most 2ⁿ stereoisomers.

The real number is lower when the molecule is symmetric:

• Meso forms. If the two halves of the molecule are the same, an arrangement such as (2R,3S) can have an internal mirror plane. It is then identical to its mirror image, so it is counted once and is optically inactive.
• Symmetric dienes. In hexa-2,4-diene, (2E,4Z) and (2Z,4E) are the same molecule, so there are 3 isomers, not 4.
• Rings. cis-1,2-Dimethylcyclohexane is meso; the trans form is a pair of enantiomers, so 3 in total. 1,4-Disubstituted rings have only cis and trans, with no R/S.

Enantiomers, diastereomers and meso compounds

• Enantiomers are non-superimposable mirror images. Every stereocentre is inverted: (2R,3R) and (2S,3S). They have the same physical properties except the direction they rotate plane-polarised light.
• Diastereomers are stereoisomers that are not mirror images. Some centres differ and some do not: (2R,3R) and (2R,3S). Cis/trans (E/Z) isomers are diastereomers too. They have different melting points, boiling points and solubilities.
• Meso compounds have stereocentres but are achiral because of an internal mirror plane (or centre of inversion).
• Epimers are diastereomers that differ at only one centre, such as glucose and galactose (C4).

How the tool finds them

It tries every R/S and E/Z combination, then writes each one as a stereo-aware canonical code. Two arrangements with the same code are the same molecule, so a meso form is kept once and two enantiomers stay as two. Centres that only become stereogenic once their neighbours are fixed (pseudo-asymmetric centres, labelled r or s) are found by repeating the search.

Worked examples

Common mistakes

• Using 2ⁿ as the answer for a symmetric molecule. Check for meso forms first.
• Missing E/Z double bonds. They count in n just like stereocentres.
• Counting a ring C=C in a small ring as E/Z. In rings of fewer than eight atoms only the cis (Z) form can exist.
• Calling (2R,3S) and (2S,3R) two isomers when the molecule is symmetric. They are one meso compound.
• Counting nitrogen in an amine as a stereocentre. It inverts too fast to keep R and S apart.

Elements: C, H, N, O, S, F, Cl, Br, I. Up to 40 atoms other than hydrogen and 256 stereoisomers.

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