Isomer Generator and Counter

In short: enter a molecular formula to count and draw every structural isomer, with IUPAC names, grouped by class. Add cis/trans and R/S stereoisomers with one tick. It is free, works in your web browser and needs no download.

Type a formula such as C₄H₁₀O. The tool builds every possible structure, removes duplicates and unstable forms, and lists the rest as alkanes, alkenes, alcohols, ethers, amines and so on.

Elements: C, H, N, O, S, F, Cl, Br, I with their usual valences (C 4, N 3, O 2, S 2, halogens 1). Up to 8 carbon atoms and 12 atoms other than hydrogen.

How to use the isomer generator

1. Type a molecular formula such as C4H10O, C5H10 or C3H9N, or press an example button.
2. Tick Include stereoisomers to count cis/trans and R/S forms. Keep Exclude unstable structures ticked for textbook counts. Tick Benzene ring only for aromatic questions like "isomers of C₇H₈O containing a benzene ring".
3. Read the total and the class chips. Click a chip to show one class, such as the alcohols or the secondary amines.
4. Press Open in MolDraw under any structure to edit it, check it in 3D or export an image.

Types of structural isomerism

Structural (constitutional) isomers have the same molecular formula but their atoms are joined in a different order. School courses split them into five kinds.

Tautomers, such as a ketone and its enol, are also structural isomers. They turn into each other quickly, so the unstable form (the enol) is left out of the count unless you untick the filter.

The degree of unsaturation comes first

The degree of unsaturation (DoU) tells you how many rings plus π bonds every isomer must have. For C_cH_hN_nO_oX_x:

Oxygen and sulfur do not change it.

C₄H₁₀O has DoU 0, so every isomer is open-chain and saturated: only alcohols and ethers. C₃H₆O has DoU 1, so each isomer has one C=C, one C=O or one ring. Use the degree of unsaturation calculator to check.

Stereoisomers: cis/trans, R/S and meso forms

Stereoisomers have the same bonds but a different arrangement in space. Tick Include stereoisomers to list them under each structure.

Geometrical (cis/trans, E/Z) isomers

A C=C double bond gives two isomers only when each carbon carries two different groups. But-2-ene has cis (Z) and trans (E) forms. But-1-ene and 2-methylpropene do not, because one carbon has two hydrogens. A ring with two substituted carbons also has cis and trans forms, for example 1,3-dimethylcyclobutane.

Optical isomers (R/S)

A carbon with four different groups is a stereocentre. It gives two mirror-image forms, R and S, called enantiomers. They are counted as two stereoisomers. Butan-2-ol and 2-chlorobutane each have one stereocentre, so each gives a pair. Use the R/S chirality finder to assign R or S to a drawn structure.

Meso forms and the 2ⁿ rule

A molecule with n stereocentres has at most 2ⁿ stereoisomers. It has fewer when the molecule is symmetric. A meso form has stereocentres but also an internal mirror plane, so it equals its mirror image and is counted once. Butane-2,3-diol has 2 stereocentres but 3 stereoisomers: (2R,3R), (2S,3S) and the meso (2R,3S) form. In the same way cis-1,2-dimethylcyclopropane is meso, while the trans form is a pair of enantiomers.

Nitrogen in an amine is not counted as a stereocentre. It flips (inverts) too fast for the two forms to be separated.

Worked examples (JEE and NEET style)

How the generator works

1. It reads the formula and works out the degree of unsaturation.
2. It builds every saturated skeleton one atom at a time, giving each atom its usual valence (C 4, N 3, O 2, S 2, halogens 1).
3. It then adds the unsaturation one unit at a time, either as an extra bond in a C=C, C=O or C≡C, or as a new bond that closes a ring.
4. After each step it compares the structures by a canonical code, so the same molecule drawn in two ways is kept once. Kekulé forms of a benzene ring count as one structure.
5. It removes unstable structures (rules below), sorts the rest by class and names them.
6. For stereoisomers it finds every stereocentre and stereogenic double bond, tries each arrangement and keeps the distinct ones. A meso form is kept once and two enantiomers count twice.

This is a real enumeration, not a stored list, so it also works for formulas that are not in textbooks. Names come from MolDraw's own naming rules: the main chain has the most principal groups, then the most multiple bonds, then the most carbons (as in most school textbooks), and a ring beats a chain when it carries as many principal groups. When the tool is not sure of a name it shows the SMILES instead of guessing.

Which structures count as unstable?

With Exclude unstable structures ticked (the default), these are left out:

• Bonds between two heteroatoms: no peroxides (O–O), hydroxylamines or oximes (N–O), hydrazines (N–N), O–S or N–S bonds, and no halogen on N, O or S. Disulfides (S–S) are allowed.
• Enols, ynols and enethiols: no O–H or S–H on a carbon that carries a C=C or C≡C bond. This also removes allenic alcohols. Phenols are kept.
• Enamines and ynamines with an N–H. Fully substituted enamines are kept, like enol ethers, and so are anilines.
• Imidic acids: no O–H on a carbon that is double-bonded to nitrogen.
• Gem-diols, hemiacetals and hemiaminals: a saturated carbon may not carry an O–H (or S–H) together with another O, N, S or halogen. Likewise no N–H and a halogen on the same carbon.
• Oxygen on an sp carbon of a C≡C or C=C=C unit: no ynol ethers and no ketenes (C=C=O). Isocyanates (N=C=O) are kept.
• A triple bond in a ring of fewer than 8 atoms, or a C=C=C unit in a ring of fewer than 9 atoms.
• A double bond shared by two rings when one of them has 3 or 4 atoms, and a double bond at a bridgehead when the ring it sits in has fewer than 8 atoms (Bredt's rule).
• Cyclobutadiene rings, and three-membered rings with a C=C and an O, N–H or S (oxirene, 1H-azirine, thiirene).

Every atom always gets its full valence, so carbenes, radicals and ions never appear. Strained but real molecules, such as cyclopropene and bicyclo[1.1.0]butane, are kept. For stereoisomers, ring fusions that cannot exist (a trans fusion of very small rings, or in/out bridgeheads) are not counted. Untick the filter to see every valence-correct structure; the left-out ones are listed under the results.

Common mistakes when counting isomers

• Counting one molecule twice. A chain drawn bent, or numbered from the other end, is the same molecule. 1-chloropropane and 3-chloropropane are one compound.
• Forgetting rings. C₃H₆ is propene and cyclopropane. Any formula with DoU 1 or more can have ring isomers.
• Forgetting functional isomers. Alcohols come with ethers, aldehydes with ketones, carboxylic acids with esters.
• Counting enols or gem-diols. They are not stable compounds, so textbook answers leave them out.
• Giving cis/trans to the wrong double bond. If one alkene carbon has two identical groups (but-1-ene, 2-methylpropene) there is no geometrical isomer.
• Mixing up meso forms. A meso form is one stereoisomer, not two. Enantiomers are two.
• Reading the question. "Structural isomers" means constitutional only. "Total isomers" or "stereoisomers included" means add cis/trans and R/S forms.

Related chemistry tools