Intuition The ONE core ideaA molecule is just a skeleton of carbon atoms wearing some special decorations (an –OH here, an extra bond there). IUPAC naming is nothing more than a way to count the skeleton and report the decorations in a fixed order — so that one structure always produces exactly one name, and one name always rebuilds exactly one structure.
Before you can name anything, you must be fluent in the alphabet of symbols the parent note throws at you: what a line-drawing means , what C H 3 is , what a double line between two carbons looks like , what a "locant" counts. This page earns every one of those from scratch.
Definition Atom, bond, and the letter symbolsAn atom is one ball of matter — one carbon (C ), one hydrogen (H ), one oxygen (O ), one nitrogen (N ). A bond is a shared connection holding two atoms together; we draw it as a line between the two letters. One line = one bond.
Look at the picture below. A single line is a handshake between two atoms — a pair of electrons they share. Every carbon wants exactly 4 handshakes (four bonds), no more, no less. That "always 4" rule is the hidden law behind every structure you will ever name.
Intuition Why carbon is the starBecause carbon insists on four bonds, it can hold hands with up to four neighbours — chaining into long backbones and branching sideways. That is exactly why organic chemistry (and therefore this topic ) is about carbon skeletons .
The parent note writes molecules like C H 3 − C H ( O H ) − C H 2 − C H 3 condensed notation — a shorthand that hides the boring hydrogens to save space.
Definition Condensed group symbols
==C H 3 3 hydrogens (it has 1 bond left to attach to something).
==C H 2 2 hydrogens (2 bonds left — sits in the middle of a chain).
==C H 1 hydrogen (3 bonds left — a branch point).
The little subscript number just counts how many hydrogens are stuck on that carbon.
Intuition The bookkeeping trickSince every carbon needs 4 bonds total: (bonds shown by dashes) + (hydrogens written) = 4. So C H 2 have to draw the H's; you can always recount them.
C H 3 − C H 2 − C H 3 C H 3 C H 2 C H 3 propane .
Before you can say how many carbons, you need the counting-words IUPAC uses. Each number of backbone carbons has a fixed root syllable that the parent's table relies on constantly.
A root word is the fixed syllable that stands for the number of carbons in the main chain. It is the middle of every name.
C
Root
C
Root
1
meth
6
hex
2
eth
7
hept
3
prop
8
oct
4
but
9
non
5
pent
10
dec
Intuition Where "propane" and "butane" come fromIn the example above, 3 carbons → root prop ; with all single bonds we add -ane → propane . Four carbons in a row → but + -ane → butane . The root is a carbon-counter , nothing more.
M onkeys E at P eeled B ananas P eacefully → meth, eth, prop, but, pent. Then Greek: H ex, H ept, O ct, N on, D ec.
Definition Bond multiplicity
A single bond (one line, −
A double bond (two lines, = same two atoms.
A triple bond (three lines, ≡
The picture shows all three between two carbons. Notice the atoms are pulled closer as bonds increase — more shared glue means a shorter, tighter grip.
Intuition Why the topic cares about thisThe middle suffix of every name (-ane / -ene / -yne ) is literally a report of bond multiplicity in the backbone : all singles → -ane, one double bond → -ene, one triple bond → -yne. So you cannot read that suffix without first seeing "= ≡ Degree of Unsaturation — each extra bond line is one "degree of unsaturation".
Definition Functional groupA functional group is a specific small cluster of atoms (like –OH, or C=O) that behaves as a recognizable unit and gives the molecule its personality. In naming, it is the thing we report as a suffix or prefix .
The picture below draws the four groups the parent note leans on most, so the symbol and the shape lock together in your mind.
Definition The key group symbols (full inventory)
==− O H
==C = O
==− C H O end of a chain, so it also carries one H: the aldehyde group.
==− C O O H and a hydroxyl on the same carbon: the acid group.
==− C O O R
==− C O N H 2 − N H 2
==− C ≡ N
==− N H 2
==− N O 2
==− O − between two carbons, always a prefix.
==− C l , − B r , − I , − F
Intuition Why we need a whole ranking of themA molecule can wear several decorations at once. IUPAC forces exactly one to be the "star" (the suffix) and the rest to be "supporting cast" (prefixes). That is the entire point of the seniority table in the parent — and of Priority and Seniority Rules (CIP vs IUPAC) . See the full menu in Functional Groups Overview .
A substituent is any group branching off the main chain that isn't the principal suffix group. It is reported as a prefix with its locant.
Definition The common substituent prefixes
==− C H 3 methyl · ==− C H 2 C H 3 ethyl · ==− C H 2 C H 2 C H 3 propyl .
iso- = a branch that forks into two C H 3 sec- ("secondary") = attached through a carbon that touches two other carbons.tert- ("tertiary") = attached through a carbon that touches three other carbons.Halogen branches: chloro (− C l bromo (− B r iodo (− I fluoro (− F
nitro (− N O 2 oxo (= O hydroxy (− O H
Intuition Why substituents are alphabetizedWhen several branches exist, IUPAC lists them in alphabetical order so two chemists always write them in the same sequence — again, uniqueness. The multiplying words di, tri, tetra are ignored when alphabetizing, but iso- and cyclo- DO count. Different branch arrangements are what Isomerism is all about.
Definition Parent / main chainThe main chain is the single continuous run of carbons we treat as the backbone; its length sets the root word.
The figure shows a case where the naïve "straight across" chain is 4 carbons but a corner-turning path is 5 — and only the correct pick contains the group.
Common mistake "The longest chain is just the most carbons drawn in a line."Why it feels right: structures are drawn flat. The fix: trace every path including ones that bend, and it must contain the senior group.
A locant is simply the house number given to a carbon along the chain. We number carbons 1 , 2 , 3 , … where a decoration sits.
Intuition Why numbering has a direction ruleThe chain has two ends, so there are two possible numberings — and they can disagree. Example: an − O H the senior group gets the smallest possible number. That single rule is why the parent's Example B lands on but-3-en-1-ol and not "but-1-en-4-ol".
Worked example Two numberings, one winnerC H 3 − C H ( O H ) − C H 2 − C H 3 − O H smaller number, so we number from the left → butan-2-ol .
Common mistake "I can number from whichever end I like."Why it feels right: both directions are physically the same molecule. The fix: the name must be unique, so IUPAC picks the direction that gives the principal group the lowest locant. Direction is decided, not free.
The finished name is not just letters and numbers thrown together; it obeys strict punctuation so it reads unambiguously.
Definition Name punctuation rules
A comma separates number from number : 2,3- means locants 2 and 3.
A hyphen separates a number from a letter : but-2-ene, 4-oxo.
Parentheses ( ) complex substituent so its internal numbers don't clash with the main chain, e.g. 4-(1-methylethyl).No spaces inside a single word-name (except before "acid": butanoic acid ).
Worked example Reading the punctuation of2,3-dimethylbutane
Comma between 2 and 3 (two locants) → methyls on C2 and C3; hyphen before "dimethyl" (number-to-letter); root but = 4 carbons; -ane = all single bonds.
You will soon meet italic letters in front of a name — these are stereodescriptors , symbols for the 3-D arrangement, not the connectivity.
E/Z label the two sides of a C=C double bond: Z (from German zusammen , "together") = the higher-priority groups on the same side; E (entgegen , "opposite") = on opposite sides.R/S label the 3-D handedness of a carbon with four different groups: R (Latin rectus , clockwise priority order) vs S (sinister , anticlockwise).
Two molecules can have identical connectivity yet be mirror images or geometric opposites — different shapes , sometimes wildly different smells or drug effects. The priority ranking behind E/Z and R/S is exactly the CIP system of Priority and Seniority Rules (CIP vs IUPAC) . You don't need to assign them yet — just recognise the italic E , Z , R , S as "shape tags".
Self-test: cover the right side and answer before revealing.
What does a single line between two letters mean? One bond — a shared pair of electrons.
How many bonds must every carbon have? Exactly 4.
In C H 2 2 hydrogens, 2 bonds left for chain neighbours.
What is the root word for 3 carbons? For 4? prop; but.
What does the symbol "=" between two carbons mean? A double bond (two shared electron pairs).
What does "≡" mean? A triple bond (three shared electron pairs).
What is a functional group? A recognizable cluster of atoms (e.g. –OH, C=O) that defines the molecule's behaviour and its name.
Write the symbol for the carboxyl (acid) group. –COOH
What group is –COOR? And –CONH₂? And –C≡N? ester; amide; nitrile.
What group is –NO₂, and can it ever be a suffix? nitro; no, always a prefix.
What are the prefixes for –Cl, –Br, –I, –F? chloro, bromo, iodo, fluoro.
Name the substituent –CH₃ and –CH₂CH₃. methyl; ethyl.
Two rules for choosing the main chain, in order? Must contain the principal group; then pick the longest (may turn a corner).
What is a locant? The number given to a carbon so we can say where a group sits.
What is the numbering rule when a senior group is present? Number so the senior (principal) group gets the lowest locant.
Comma vs hyphen in a name? Comma separates number from number; hyphen separates number from letter.
When are parentheses used? Around a complex substituent so its inner numbers don't clash.
What do E/Z label? What do R/S label? The two sides of a C=C; the 3-D handedness of a 4-different-group carbon.
The three ordered slots of an IUPAC name? prefix + root + suffix.