About this dataset
Alkanes are saturated hydrocarbons with the acyclic formula CₙH₂ₙ₊₂. Alkane isomers have the same molecular formula but different carbon skeletons: one may be a straight chain while another is branched. These are constitutional, or structural, isomers because their atoms are connected differently.
Methane, ethane, and propane each have only one possible carbon skeleton. Chain isomerism begins with C₄H₁₀, which can form n-butane and 2-methylpropane. The number then rises quickly: pentane has 3 constitutional isomers, hexane 5, heptane 9, octane 18, nonane 35, and decane 75.
How branching changes an alkane
The three C₅H₁₂ isomers make the pattern easy to see. n-Pentane has one continuous five-carbon chain; 2-methylbutane has a four-carbon parent chain and one methyl branch; 2,2-dimethylpropane has a compact, highly symmetric skeleton centered on one carbon.
Branching usually lowers the boiling point among isomers because a compact molecule has less surface contact and weaker London dispersion attractions. Melting points do not follow the same simple trend: molecular symmetry and crystal packing can make a highly branched isomer melt at a higher temperature than less symmetrical alternatives.
How many isomers each alkane has
Methane, ethane, and propane have 1 constitutional isomer each. From there the count grows quickly: butane has 2, pentane 3, hexane 5, heptane 9, octane 18, nonane 35, and decane 75. This table lists every one of those 150 structures, generated by enumerating all possible carbon skeletons and matching each one to its PubChem record.
Beyond decane the numbers explode. Undecane (C₁₁H₂₄) has 159 constitutional isomers, dodecane 355, tridecane 802, pentadecane 4,347, and eicosane (C₂₀H₄₂) 366,319. For C₁₁ to C₂₀ the table therefore includes only the straight-chain alkane.
Stereoisomers are not counted separately. 3-Methylhexane, for example, has a stereocenter and exists as two mirror-image forms, but both share one carbon skeleton and appear as a single row. The notes column flags every isomer that has stereoisomers.
How alkane isomers are named
IUPAC naming starts with the longest continuous carbon chain. Number that chain from the end that gives substituents the lowest set of locants, identify each alkyl branch, combine repeated branches with prefixes such as di- or tri-, and alphabetize different substituent names.
The table uses PubChem IUPAC names, so 2-methylpropane appears under that name rather than isobutane; common names such as isobutane, neopentane, and isooctane are given in the notes. The SMILES value is useful when names become cumbersome because it records the molecular graph directly.
How this dataset was assembled
Carbon skeletons were enumerated exhaustively as trees in which each carbon has at most four carbon neighbors, then deduplicated by canonical SMILES with RDKit. That gives 1, 1, 1, 2, 3, 5, 9, 18, 35, and 75 isomers for C1 to C10. Each skeleton was matched to its PubChem record by structure and its InChIKey verified; names are PubChem IUPAC names. Straight-chain alkanes from C11 to C20 are included for reference only, because their branched isomers number from 159 (C11) to 366,319 (C20).
Each row retains source identifiers where available. Molecular weight is calculated from the recorded formula, and the original names, structures, classifications, and database links remain visible so individual entries can be checked before reuse.
License and citation
This compilation is free to download and reuse under the Creative Commons Attribution 4.0 International license. Credit ProteinIQ and the original sources when you share it or publish results that use it.
Suggested citation: ProteinIQ (2026). List of isomers of alkanes, version 2026.10.05. https://proteiniq.io/datasets/alkane-isomers
Sources and further reading
- PubChem: Canonical compound identifiers, names, formulas, structures, SMILES, and downloadable compound records. NCBI places no restrictions on PubChem data, though original submitters may retain rights in submitted records.
- NIST Chemistry WebBook: Reference physical chemistry data and diatomic constants for selected species. NIST SRD 69 should be cited and linked rather than mirrored wholesale.
- RDKit: Derived descriptors such as molecular weight, LogP, TPSA, hydrogen-bond counts, and canonicalized structures.