How do you find the parent chain in IUPAC naming?

The parent chain is the longest continuous chain of carbon atoms in the molecule, even if it bends in the drawing. Its length sets the root name (meth, eth, prop, but, pent, and so on).

What is a locant in a chemical name?

A locant is the position number that tells you where a branch or functional group is attached, such as the '2' in 2-methylbutane. IUPAC rules require the lowest possible set of locants.

IUPAC Nomenclature

A naming system so precise that the name alone lets you rebuild the molecule. Learn its simple grammar.

High schoolIntro OrganicUni Year 1

⏱️ About 20 min

"2-methylbutane" looks like jargon, but it is really a set of assembly instructions: a four-carbon chain, with a one-carbon branch on carbon 2. Learn the grammar and every IUPAC name becomes a blueprint you can read and draw at a glance — no memorising required.

💡

The big idea: IUPAC names are built from three parts: a root for the longest carbon chain, a suffix for the main functional group, and prefixes with numbers (locants) for branches. Follow the rules — longest chain, lowest locants — and each molecule gets exactly one correct name.

🎯 By the end, you'll be able to

Identify the longest continuous carbon chain to set the root name
Use the meth/eth/prop/but… roots and the -ane/-ene/-yne/-ol suffixes
Number the chain to give substituents the lowest set of locants
Assemble a full name for a simple branched or substituted molecule

📎 Helpful to know first

A name in three parts

Every IUPAC name answers three questions: How long is the main chain? (root), What is the main functional group? (suffix), and What is attached, and where? (prefixes + numbers). Build those three and you have the name.

🔑 The chain-length roots

1 carbon = meth, 2 = eth, 3 = prop, 4 = but, then it turns regular: 5 = pent, 6 = hex, 7 = hept, 8 = oct. These roots run through the whole of organic naming.

The suffix names the main group

The ending tells you the key feature of the molecule:

-ane — only single bonds (alkane): propane.
-ene — a C=C double bond (alkene): propene.
-yne — a C≡C triple bond (alkyne): propyne.
-ol — a hydroxyl group, an alcohol: propan-1-ol.
-oic acid — a carboxyl group: propanoic acid.

✨ Two rules do most of the work

Rule 1 — longest chain: the root comes from the longest continuous chain of carbons, even if it bends around a corner in the drawing. Rule 2 — lowest locants: number the chain from whichever end gives the substituents (and the functional group) the smallest position numbers.

Naming a branch

A branch is named like a chain but ends in -yl: a one-carbon branch is methyl, two-carbon is ethyl. Put its position number in front. So a five-carbon chain with a methyl branch on carbon 2 is 2-methylpentane. If a branch repeats, use di-, tri-: two methyls give dimethyl.

\[ \ce{CH3-CH(CH3)-CH2-CH2-CH3} \]

2-methylpentane: a 5-carbon (pentane) main chain with a methyl branch on carbon 2.

⚠️ Number from the right end

Locants must be as low as possible. If a methyl branch sits one carbon from the left but two from the right, number from the left so it is "2-", not "4-". Choosing the wrong end is the most common naming slip — always check both directions before committing.

📝 Worked example: Name the molecule CH₃–CH₂–CH(CH₃)–CH₃.

Find the longest chain: the continuous carbon chain is 4 carbons long → root but.
Identify bonds/groups: all single bonds, no functional group → suffix -ane (butane).
Find the branch: a one-carbon methyl group hangs off the chain.
Number for the lowest locant: from the near end the methyl is on carbon 2 (from the far end it would be carbon 3), so use 2.
Assemble: 2 + methyl + butane.

✓ 2-methylbutane.

✏️ Practice: In the name 3-methylhexane, how many carbon atoms are in the main (parent) chain?

carbons

Solution

The parent chain is named by the root before the suffix: 'hex'.
The root 'hex' means 6 carbons.
So the main chain has 6 carbons (the methyl branch on carbon 3 is separate).

Check your understanding

1. What determines the root (parent) name of an organic molecule?

The root comes from the longest continuous carbon chain — even if it bends in the drawing. Everything else (branches, groups) is named relative to it.

2. Why do we number 2-methylbutane from that particular end of the chain?

IUPAC requires the lowest set of locants. Numbering from the end that puts the methyl on carbon 2 (not 3) gives the smaller number, so that is correct.

3. Which suffix signals that a molecule contains a carbon–carbon double bond?

The -ene ending means an alkene (a C=C double bond). -ane is all single bonds, -ol is an alcohol, and -oic acid is a carboxylic acid.

✅ Key takeaways

IUPAC names = root (longest chain) + suffix (main group) + prefixes/locants (branches).
Roots: meth, eth, prop, but, pent, hex, hept, oct; suffixes: -ane, -ene, -yne, -ol, -oic acid.
Rule 1: base the root on the longest continuous carbon chain.
Rule 2: number the chain to give the lowest set of locants.
Branches end in -yl (methyl, ethyl) with a position number in front.

➡️ Careful naming reveals a surprise: two molecules can share the exact same formula yet get different names, because their atoms are connected differently. Those are isomers — the subject of the next lesson.

Want to test yourself on this? Try the Chemistry practice test →