Organic Nomenclature Prefixes — IUPAC Carbon Chain Names
| Carbon Atoms | Prefix | Alkane (-ane) | Alkane Formula | Alkene (-ene) | Alkyne (-yne) |
|---|---|---|---|---|---|
| 1 | meth- | Methane | CH₄ | — | — |
| 2 | eth- | Ethane | C₂H₆ | Ethene | Ethyne |
| 3 | prop- | Propane | C₃H₈ | Propene | Propyne |
| 4 | but- | Butane | C₄H₁₀ | Butene | Butyne |
| 5 | pent- | Pentane | C₅H₁₂ | Pentene | Pentyne |
| 6 | hex- | Hexane | C₆H₁₄ | Hexene | Hexyne |
| 7 | hept- | Heptane | C₇H₁₆ | Heptene | Heptyne |
| 8 | oct- | Octane | C₈H₁₈ | Octene | Octyne |
| 9 | non- | Nonane | C₉H₂₀ | Nonene | Nonyne |
| 10 | dec- | Decane | C₁₀H₂₂ | Decene | Decyne |
| 11 | undec- | Undecane | C₁₁H₂₄ | Undecene | Undecyne |
| 12 | dodec- | Dodecane | C₁₂H₂₆ | Dodecene | Dodecyne |
| 13 | tridec- | Tridecane | C₁₃H₂₈ | Tridecene | Tridecyne |
| 14 | tetradec- | Tetradecane | C₁₄H₃₀ | Tetradecene | Tetradecyne |
| 15 | pentadec- | Pentadecane | C₁₅H₃₂ | Pentadecene | Pentadecyne |
| 16 | hexadec- | Hexadecane | C₁₆H₃₄ | Hexadecene | Hexadecyne |
| 17 | heptadec- | Heptadecane | C₁₇H₃₆ | Heptadecene | Heptadecyne |
| 18 | octadec- | Octadecane | C₁₈H₃₈ | Octadecene | Octadecyne |
| 19 | nonadec- | Nonadecane | C₁₉H₄₀ | Nonadecene | Nonadecyne |
| 20 | icos- | Icosane | C₂₀H₄₂ | Icosene | Icosyne |
Memorize meth-/eth-/prop-/but- — they're historical, not Greek. From C5 upward, prefixes follow Greek numerals predictably (pent, hex, hept, oct, non, dec, undec, dodec...). The IUPAC 2013 recommendations renamed the C20 prefix from 'eicos-' to 'icos-' (so eicosane is now icosane, and the lipid name 'eicosanoid' is now technically deprecated, though still in widespread use). Methane has no alkene or alkyne form because C=C and C≡C require two carbons. The same prefixes apply to cycloalkanes (cyclopentane, cyclohexane) and to substituent names by changing -ane to -yl (methyl, ethyl, propyl). Source: IUPAC Recommendations on Organic Nomenclature (2013).
Frequently Asked Questions
Why are the first four prefixes irregular?
Meth-, eth-, prop-, and but- predate systematic nomenclature and were retained when IUPAC formalized organic naming in 1892. Methane comes from methanol (Greek methy + hyle, 'wine wood', referring to wood-spirit alcohol). Ethane traces to ether/ethyl. Propane comes from propionic acid (Greek protos pion, 'first fat'). Butane comes from butyric acid (Latin butyrum, 'butter'). From pentane onward, prefixes are pure Greek numerals (penta, hexa, hepta...), so the system becomes regular and learnable after C4.
How do you name branched alkanes?
Four steps: (1) find the longest continuous carbon chain — that's the parent; (2) number from whichever end gives the substituents the lowest set of locants; (3) name each branch as an alkyl substituent (methyl, ethyl, propyl, isopropyl) with its locant; (4) list substituents alphabetically as prefixes to the parent name, with multiplying prefixes (di-, tri-) ignored when alphabetizing. Example: CH₃CH(CH₃)CH₂CH₂CH₃ → longest chain 5 carbons, methyl on C2 → 2-methylpentane. Use commas between locants and hyphens between locants and names.
What is the difference between an alkyl group and an alkane?
An alkane is a complete molecule with formula CₙH₂ₙ₊₂ (methane CH₄, ethane C₂H₆, propane C₃H₈). An alkyl group is the same skeleton minus one hydrogen, used as a substituent: methyl –CH₃, ethyl –C₂H₅, propyl –C₃H₇, with general formula CₙH₂ₙ₊₁. Convert by swapping -ane to -yl. Branched alkyl groups also have common names (isopropyl for (CH₃)₂CH–, tert-butyl for (CH₃)₃C–) that IUPAC 2013 still permits, though systematic 1-methylethyl and 1,1-dimethylethyl are unambiguous alternatives.