IUPAC Naming Conventions
High-Yield Summary
- An IUPAC name has four parts: parent chain (root), modifiers (functional groups), substituents, and numerical locants.
- Five-step process: (1) select the parent chain — longest chain containing the highest-priority functional group; (2) number the chain; (3) name the substituents; (4) assign locants; (5) assemble the name.
- Numbering priority: highest-priority functional group gets the lowest number first; otherwise minimize the full substituent locant set (compare carbon-by-carbon, first difference wins); rings start at the position of greatest substitution; double bonds outrank triple bonds in a tie.
- Functional group seniority (highest to lowest): carboxylic acids > anhydrides > esters > acid halides > amides > nitriles > aldehydes > ketones > alcohols > amines > alkenes/alkynes > alkanes. Ethers and halogens are never parent-defining — always substituent prefixes.
- Final name order: substituents alphabetically first (ignore di-/tri-/tetra- when alphabetizing) → locants → parent chain name last.
Parent Chain vs. Substituent Root Names
| Carbons | Parent name (-ane) / Substituent name (-yl) |
|---|---|
| 1 | Methane / Methyl |
| 2 | Ethane / Ethyl |
| 3 | Propane / Propyl |
| 4 | Butane / Butyl |
| 5 | Pentane / Pentyl |
| 6 | Hexane / Hexyl |
| 7 | Heptane / Heptyl |
| 8 | Octane / Octyl |
| 9 | Nonane / Nonyl |
| 10 | Decane / Decyl |
Key Terms
- Parent chain (root)
- Longest continuous carbon chain that includes the highest-priority functional group; sets the molecule's backbone. Ties go to the chain with more substituents.
- tert-Butyl / Isopropyl / Neopentyl / Isobutyl
- Named branched substituents: tert-butyl (4C, branch point with 3 carbons off one), isopropyl (3C, central carbon attaches to parent), neopentyl (5C, branch point near attachment), isobutyl (4C, single branch point).
- Multiplying prefixes
- di-, tri-, tetra- indicate how many times a substituent repeats (dimethyl, trimethyl); ignored when alphabetizing the name.
The Five Steps to Name Any Organic Molecule
- 1Select the parent chain: longest continuous chain containing the highest-priority functional group.
- 2Number the chain: lowest locant to the highest-priority functional group; otherwise lowest locant set overall for substituents; rings start at greatest substitution; double bonds beat triple bonds in a tie.
- 3Name the substituents using root prefixes ending in -yl (methyl through decyl), plus special names (tert-butyl, isopropyl, neopentyl, isobutyl).
- 4Assign each substituent its locant number from the fixed numbering direction; repeated substituents keep di-/tri-/tetra- alongside every position.
- 5Assemble the name: substituents alphabetically (ignoring multiplying prefixes) → locants with hyphens/commas → parent chain name.
Common MCAT Trap
- Comparing locant sets: {2,5,5} beats {3,3,6} because you stop at the first point of difference (2 < 3) — the rest of the set doesn't matter.
- Multiplying prefixes (di-, tri-, tetra-) are dropped when alphabetizing substituents — 'dimethyl' alphabetizes under 'm,' not 'd.'
- Ethers and halogens never become the parent suffix — they're always cited as substituent prefixes, regardless of what else is in the molecule.
Quick Recall
What are the four parts of an IUPAC name?
Between locant sets {2,2,4} and {3,5,5}, which wins and why?
What is the full functional-group seniority order (this chapter's list)?
What is the complete name for a hexane parent chain with two methyl groups on carbon 2 and one ethyl group on carbon 4?