# Isomer Generator - List All Structural Isomers of a Molecular Formula

> List every structural isomer of a formula like C5H12, C4H10O or C3H6Cl2, with drawings, IUPAC names, SMILES, class counts and stereoisomer totals.

- URL: https://qilingtools.com/en/math/isomers
- Category: Math Tools (https://qilingtools.com/en/math)
- Tags: [Math](https://qilingtools.com/en/tags/math), [Chemistry](https://qilingtools.com/en/tags/chemistry), [Education](https://qilingtools.com/en/tags/education)
- Last updated: 2026-10-26
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

Isomers share a molecular formula but differ in structure. Constitutional (structural) isomers differ in how the atoms are connected — chain, position and functional-group isomerism — so C₄H₁₀O has four alcohols (butan-1-ol, butan-2-ol, 2-methylpropan-1-ol, 2-methylpropan-2-ol) and three ethers, seven in total, and C₅H₁₂ has pentane, isopentane and neopentane. Type a formula and the generator first computes the degree of unsaturation, DBE = (2C + 2 + N − H − X)/2, then enumerates every constitutional isomer with a self-written graph algorithm: heavy atoms are joined into single-bond skeletons respecting valence and deduplicated by canonical labelling, then bonds are raised to double or triple bonds or rings are closed DBE times, deduplicating again. You get the total count, counts per class (alkanes, alkenes, alcohols, ethers, aldehydes, ketones, acids, esters, amines, …) that act as filters, and a card for each isomer with its skeletal drawing, SMILES and IUPAC name. Stereoisomers are counted too — cis/trans double bonds and enantiomers from stereocentres, with meso forms recognised. Results match known values: 5 isomers for C₆H₁₄, 9 for C₇H₁₆, 18 for C₈H₁₈, 75 for C₁₀H₂₂; C₄H₈ has 3 acyclic (4 with cis/trans) and 5 including rings; C₃H₆Cl₂ and C₄H₉Cl have 4 each; C₅H₁₀O₂ has 4 acids and 9 esters. Rings and unstable structures (enols, gem-diols, peroxides) can be switched on or off; everything runs in your browser.

## How to use

1. Enter a molecular formula such as C5H12, C4H10O or C3H6Cl2, or pick a common exercise
2. Choose whether to include rings, exclude unstable structures and count stereoisomers
3. Read the totals, click a class to filter, and browse the drawings, names and SMILES

## Features

- Lists every constitutional isomer of a formula with the total count
- Degree of unsaturation, with checks for impossible formulas
- Counts per class: alkanes, alkenes, alkynes, arenes, halides, alcohols, phenols, ethers, aldehydes, ketones, acids, esters, amines
- Skeletal drawing, IUPAC name and copyable SMILES for each isomer
- Stereoisomer totals with cis/trans, enantiomer pairs and meso forms
- Options to include or exclude rings and unstable structures
- Verified against textbook counts for alkanes up to C₁₀H₂₂, C₄H₁₀O and C₅H₁₀O₂
- Opens any isomer in the IUPAC name generator for step-by-step naming

## FAQ

### How do I work out how many isomers a formula has?

Start with the degree of unsaturation: DBE = 0 means only saturated structures (alkanes, alcohols, ethers), DBE = 1 means one double bond (alkene, aldehyde, ketone, acid, ester) or one ring. Then write chain isomers, then positional isomers of the functional group, then other functional-group classes. Useful shortcuts: there are 2 propyl, 4 butyl and 8 pentyl groups, so C₄H₉Cl has 4 isomers, C₅H₁₁Cl has 8 and the acids C₄H₉COOH (C₅H₁₀O₂) number 4.

### How many ester isomers does C₅H₁₀O₂ have?

Nine: four butyl formates (one per butyl group), two propyl acetates, one ethyl propanoate and two methyl butanoates (butanoic and 2-methylpropanoic acid), 4 + 2 + 1 + 2 = 9. Select C₅H₁₀O₂ and click the Esters filter to see them all.

### Are stereoisomers included in the count?

The main number counts constitutional isomers only. With “Count stereoisomers” on, a second total includes cis/trans isomers and enantiomers: C₄H₈ has 5 constitutional isomers including rings and 6 stereoisomers in total (cis- and trans-but-2-ene), and C₅H₁₀ has 10 and 13 (pent-2-ene is cis/trans; 1,2-dimethylcyclopropane has a meso cis form and a pair of trans enantiomers). Axial chirality of allenes is not counted.

### Why do I get more isomers than my textbook?

Textbook questions usually ask about one class (for example only alcohols and ethers), while the generator lists every valid structure for the formula, including rings, diols and multifunctional compounds; use the class filters to narrow it down. “Exclude unstable structures” removes enols, gem-diols, hemiacetals, peroxides, heteroatom–heteroatom bonds and triple or cumulated bonds in small rings, and turning off “Include rings” keeps only acyclic isomers.

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