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 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 names in the table identify distinct carbon skeletons, while the formula groups molecules that are isomers of one another. The SMILES value is useful when names become cumbersome because it records the molecular graph directly.
How this dataset was assembled
The starter list covers low-carbon alkanes and named structural isomers. PubChem identifiers anchor each molecule, while RDKit or Open Babel can canonicalize larger generated isomer sets.
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.
People also ask
What are the first ten straight-chain alkanes?
They are methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, and decane. Their formulas progress from CH₄ to C₁₀H₂₂ according to CₙH₂ₙ₊₂.
What type of isomerism do alkanes have?
Acyclic alkanes primarily demonstrate chain, or skeletal, constitutional isomerism. Larger substituted structures can also exhibit stereoisomerism, but those stereoisomers are not additional carbon skeletons.
How many structural isomers does each alkane have?
For methane through decane, the counts are 1, 1, 1, 2, 3, 5, 9, 18, 35, and 75. There is no simple classroom formula for the count; enumeration becomes a graph-counting problem as the number of carbons grows.
What are the four main types of isomerism?
A common introductory grouping is chain isomerism, position isomerism, functional-group isomerism, and stereoisomerism. The first three change connectivity and are constitutional isomerism; stereoisomers keep the same connectivity but differ in spatial arrangement.
Sources and further reading
- PubChem — Canonical compound identifiers, names, formulas, structures, SMILES, and downloadable compound records.
- NIST Chemistry WebBook — Reference physical chemistry data and diatomic constants for selected species.
- RDKit — Derived descriptors such as molecular weight, LogP, TPSA, hydrogen-bond counts, and canonicalized structures.