An IUPAC name is not a label someone chose — it is a set of instructions for rebuilding the molecule. Once you can read it in both directions, naming stops being memory work and becomes a procedure you can follow every time.
📚 What you need to know
A name has three parts: a prefix (branches), a stem (longest chain), and a suffix (functional group).
Stems for 1–6 carbons: meth, eth, prop, but, pent, hex.
Number the chain from the end that gives the lowest possible numbers.
Side chains are named as alkyl groups: methyl, ethyl, propyl.
Repeats get di–, tri–, tetra–; different branches are listed alphabetically.
Numbers are separated from words by hyphens and from each other by commas.
The anatomy of a name
An IUPAC name is a set of build instructions. Read it left to right and the molecule assembles itself.
Read left to right and the name builds the molecule for you: put a methyl group on carbon 2, make the main chain four carbons long, make all the bonds single, and hang an OH off carbon 1.
Carbons
Stem
Alkane
Alkyl branch
1
meth–
methane
methyl
2
eth–
ethane
ethyl
3
prop–
propane
propyl
4
but–
butane
butyl
5
pent–
pentane
pentyl
6
hex–
hexane
hexyl
Good news: IB only asks you to name molecules with up to six carbon atoms, so these six stems are all you need.
The method
🧩 Naming any structure
Find the longest continuous carbon chain. That gives the stem. It does not have to be drawn in a straight line.
Identify the functional group. That gives the suffix.
Number the chain from the end that gives the functional group the lowest number. If there is no functional group, use the branches instead.
Name each branch as an alkyl group and note its number.
Assemble: branches (alphabetical) – stem – suffix, with hyphens between numbers and words.
Which end do you number from?
This is the step that decides the mark, and the rule is short: number from whichever end gives the smaller number.
Both numberings describe the same molecule, but only the one giving the smaller locant is the accepted name.
Both numberings describe the same molecule, so both are “correct” as descriptions — but only one is the accepted name. When there is a functional group, it takes priority over branches when deciding which end to count from.
Naming by family
Alkanes, alkenes and alkynes
Alkanes end in –ane. Alkenes end in –ene and alkynes in –yne, and once the chain has four or more carbons you must say where the multiple bond starts, using the lower of the two carbon numbers it joins.
Position matters from four carbons up
CH2=CHCH2CH3 is but-1-ene · CH3CH=CHCH3 is but-2-ene
Halogenoalkanes
The halogen becomes a prefix: fluoro–, chloro–, bromo–, iodo–, with a number if the chain has three or more carbons. Multiple halogens get di–, tri– and so on, and are listed alphabetically.
CH3CHBrCH3 is 2-bromopropane · CHCl2CH2Cl is 1,1,2-trichloroethane
Alcohols
Replace the final –e of the alkane with –ol, and give the position of the –OH from propan-1-ol onwards. Two –OH groups make a diol.
Aldehydes and ketones
Aldehydes take –al and need no number, because the group can only ever be on carbon 1. Ketones take –one; they need no number until you reach five carbons, where pentan-2-one and pentan-3-one are both possible.
Carboxylic acids, esters and amines
Carboxylic acids take –oic acid, and like aldehydes need no number — the carboxyl carbon is always carbon 1. Esters take –oate, named as “alkyl alkanoate”, with the alkyl part coming from the alcohol. Amines take –amine.
WORKED EXAMPLE
Name the compound CH3CH(CH3)CH2CH2OH.
Longest chain4 carbons in a row (the bracketed CH₃ is a branch) → but–Functional group–OH, so the suffix is –ol.Number from the end nearest the OHOH on C1, methyl branch on C33–methylbutan–1–olCounting from the other end would put the OH on C4 — higher, so rejected.
WORKED EXAMPLE
Name the compound with two methyl groups on carbon 2 and one on carbon 3 of a pentane chain.
Stem: 5 carbons → pentaneThree identical branches → use tri–Positions are 2, 2 and 3. Adjacent numbers take commas; numbers meet words with a hyphen.2,2,3–trimethylpentane
WORKED EXAMPLE
Draw the skeletal structure of 4-ethyl-2-methylhexane.
Work backwards through the name“hexane” → draw a 6-carbon zig-zag and number it.Add the branches at their numbersA CH₃ on carbon 2, and a CH₂CH₃ on carbon 4.C₉H₂₀ — check every carbon has 4 bondsEthyl is listed before methyl because e comes before m, not because of its position.
💡 Exam tip
The longest chain is often not the horizontal one. Trace round the corners before deciding the stem.
Alphabetical order ignores the multiplying prefixes: dimethyl files under m, not d.
Punctuation is marked. 2,2-dimethylpropane — comma between numbers, hyphen between number and word.
Naming and drawing are the same skill in reverse. Practise both directions.
⚠️ Common mix-up
Numbering from the wrong end. 4-methylpentane doesn’t exist — it is 2-methylpentane.
Missing the longest chain because the molecule is drawn bent.
Aldehydes and carboxylic acids never need a number — adding “1-” is wrong, not just unnecessary.
Alkenes with 4+ carbons always need a position number. “Butene” on its own is incomplete.
A branch on carbon 1 usually means you chose the wrong chain — it should have been part of the main chain.
Up next: Structural Isomerism — where one molecular formula turns out to describe several genuinely different compounds, and naming becomes the way to tell them apart.
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