The five-step IUPAC naming procedure
- Find the parent chain. Find the longest continuous chain of carbon atoms. If the molecule has a functional group such as an alcohol, aldehyde, ketone, or carboxylic acid, the parent chain must be the longest chain that contains that group, even if a longer chain exists elsewhere in the molecule that skips it. Picking the longest chain visible instead of the longest chain that actually carries the group is the most common beginner error.
- Choose the principal functional group and suffix. Decide which functional group present is the most senior. Among the classes covered here, a carboxylic acid ranks above aldehyde and ketone, which rank above alcohol, which ranks above alkene or alkyne, which ranks above a plain alkane. That group becomes the suffix; everything else attached is named as a prefix.
- Number the chain for the lowest locants. Number the carbons from whichever end gives the lowest possible number, or locant, to the principal functional group first, then to any double or triple bonds, then to the substituent prefixes as a set. If a tie remains, compare the locant sets position by position — the first position where one set is lower decides it (the "first point of difference" rule).
- Name and number the substituents. Identify every branch or atom attached to the parent chain, such as methyl, ethyl, chloro, or bromo, and note which carbon number it sits on. If the same substituent appears more than once, use di-, tri-, tetra-, and so on, though these multiplying prefixes don't count toward alphabetical order.
- Assemble the full name. List substituent prefixes in alphabetical order, each with its locant number, hyphens between numbers and words, commas between numbers. Attach the parent-chain name, carbon-count root plus suffix, including the suffix's own locant where needed. Example finished format: 3-chloro-2-methylpentane.
Parent-chain prefixes by carbon count
| Carbons | Prefix | Example (alkane) |
|---|---|---|
| 1 | meth- | methane |
| 2 | eth- | ethane |
| 3 | prop- | propane |
| 4 | but- | butane |
| 5 | pent- | pentane |
| 6 | hex- | hexane |
| 7 | hept- | heptane |
| 8 | oct- | octane |
| 9 | non- | nonane |
| 10 | dec- | decane |
Meth-, eth-, prop-, and but- are retained historical names; from pent- onward, the prefixes follow standard Greek and Latin number words.
Functional-group suffixes at a glance
| Suffix | Functional group | Locant needed? |
|---|---|---|
| -oic acid | Carboxylic acid (-COOH) | No — always carbon 1 |
| -al | Aldehyde (-CHO) | No — always carbon 1 |
| -one | Ketone (C=O in the chain) | Usually (e.g. pentan-2-one) |
| -ol | Alcohol (-OH) | Yes (e.g. propan-1-ol) |
| -ene | Alkene (C=C) | Yes (e.g. but-1-ene) |
| -yne | Alkyne (C≡C) | Yes (e.g. but-1-yne) |
| -ane | Alkane (no functional group) | Not applicable |
This table covers the seven suffix classes most beginner courses start with. The full IUPAC seniority order also includes esters, amides, nitriles, amines, and other classes ranked above or below these — left out here as beyond first-pass beginner scope.
Worked example 1: a branched alkane
Structure: CH₃–CH(CH₃)–CH(CH₃)–CH₃
- Longest chain: four carbons → parent is butane.
- No functional group beyond the alkane itself → suffix stays -ane.
- Numbering from either end gives the two methyl branches positions 2 and 3, so direction doesn't matter here.
- Two identical methyl substituents at carbons 2 and 3 → "2,3-dimethyl."
Full name: 2,3-dimethylbutane.
Worked example 2: a simple alcohol
Structure: CH₃–CH(OH)–CH₂–CH₂–CH₃
- Longest chain: five carbons → parent is pentane.
- -OH is the principal functional group → suffix is -ol → "pentanol."
- Numbering from the left gives -OH position 2; from the right it's position 4. 2 is lower, so number from the left.
- No other substituents.
Full name: pentan-2-ol.
Worked example 3: a simple alkene
Structure: CH₃–CH=CH–CH₃
- Longest chain: four carbons → parent is butane; the C=C double bond changes the suffix to -ene → "butene."
- The double bond sits between carbons 2 and 3 regardless of which end you start from, so its locant is 2 either way.
- No substituents.
Full name: but-2-ene. Older-style naming, still common in many Nigerian textbooks, writes this as "2-butene," with the locant before the parent name instead of before the suffix. Both refer to the same molecule; "but-2-ene" is the locant placement IUPAC formally adopted.
Common beginner mistakes
- Picking the wrong parent chain — choosing the longest chain visible instead of the longest chain that actually contains the principal functional group produces a name for a different, wrong compound.
- Numbering from the wrong end out of habit — always numbering left-to-right instead of numbering for the lowest locant to the functional group first often produces a technically-formatted but incorrect name.
- Dropping a needed locant — writing "butanol" instead of "butan-2-ol" leaves the name ambiguous between two different molecules.
- Missing a substituent — overlooking one branch changes the molecular formula the name describes.
- Alphabetising incorrectly — listing substituents in the order they appear on the chain, or alphabetising by the multiplying prefix, is a common and easy-to-fix error.
People also ask
What is the difference between a compound's IUPAC name and its common name?
The IUPAC name is a systematic name that describes the exact structure and can only refer to one molecule. Common names, like acetone for propanone, are older traditional names that don't follow a predictable pattern.
Do you always number a chain starting from carbon 1 on the left?
No — you number from whichever end gives the lowest locant to the principal functional group first, then to double or triple bonds, then to substituents.
Why doesn't a carboxylic acid or aldehyde name include a locant number?
Because both groups can only exist at the very end of a carbon chain, so they're always at carbon 1 — there's no other possible position to number.