IUPAC nomenclature

Two chemists on opposite sides of the world, looking at the same jar, must arrive at the same name — or organic chemistry collapses into guesswork. That is the job IUPAC (International Union of Pure and Applied Chemistry) nomenclature does: a small, mechanical rulebook that turns any structure into one unambiguous name, and any correctly formed name back into one unambiguous structure. It replaces the older world of trivial names (acetic acid, formaldehyde) that had to be memorised one compound at a time. This topic sits inside the ISC Class 11 unit "Organic Chemistry — Some Basic Principles and Techniques," and every later organic topic — isomerism, mechanisms, reactions — depends on being able to name and decode structures correctly first.

The core insight

Every IUPAC name is built as prefix(es)–parent chain(root+ending)–suffix, chosen by finding the longest chain through the highest-priority functional group and numbering it to give that group the lowest possible locant.

The core idea

An IUPAC name is assembled from four decisions, always taken in the same order.

1. Pick the principal characteristic group. Scan the molecule for functional groups and rank them by a fixed priority order (highest to lowest, ISC-level list): carboxylic acids $>$ ... $>$ aldehydes $>$ ketones $>$ alcohols $>$ amines $>$ hydrocarbons. Only the single highest-ranked group present becomes the suffix; every other group present is demoted to a substituent prefix (for example $-\text{OH}$ becomes "hydroxy" if a $-\text{COOH}$ is also present).

2. Select the parent chain. This is the longest continuous carbon chain that passes through the carbon(s) bearing the principal characteristic group. If two chains tie in length, the one carrying more substituents wins. Choosing a long chain that misses the principal group is the single most common structural error.

3. Number the chain. Numbering runs so that the principal characteristic group receives the lowest possible locant. If no principal group is present, number instead to give the lowest locant set to substituents and multiple bonds together.

4. Name and assemble. The root names the chain length — meth(1), eth(2), prop(3), but(4), pent(5), hex(6), hept(7), oct(8), non(9), dec(10) carbons. The ending marks saturation: $-\text{ane}$ (all single bonds), $-\text{ene}$ (a C=C present), $-\text{yne}$ (a C≡C present), with the bond's locant inserted just before the ending, e.g. but-2-ene. The suffix names the principal group with its locant, e.g. $-\text{ol}$ (alcohol), $-\text{al}$ (aldehyde), $-\text{oic acid}$ (carboxylic acid), $-\text{one}$ (ketone), $-\text{amine}$ (amine). Any remaining group is written as a locant-and-prefix (methyl, ethyl, chloro, bromo, nitro, hydroxy, oxo...), all prefixes ordered alphabetically, with di-, tri-, tetra- used for repeats but ignored while alphabetising.

\text{[locants + prefixes (alphabetical)]} - \text{[root]} - \text{[unsaturation ending]} - \text{[locant + suffix]}

where: the prefixes carry the locants of every substituent not chosen as the principal group; the root fixes the carbon count of the parent chain; the unsaturation ending records any C=C or C≡C in that chain; the suffix names the one principal characteristic group with its own locant.

Figure

A four carbon chain labelled carbon one through carbon four with a hydroxyl group on carbon one and a methyl branch on carbon three, illustrating locant numbering for 3 methylbutan 1 ol

Why it works

Two conventions carry the whole system, and both exist to remove ambiguity rather than to add rules for their own sake.

Why lowest locant to the principal group, always first. If numbering were chosen freely, the same molecule could be numbered two different ways and get two different-looking names for one structure — defeating the entire purpose of a systematic name. Fixing "lowest locant to the suffix group first" gives every chemist, working independently, the same numbering every time. Only once that is settled do substituent locants get a say, and even then the rule is the same idea one level down: choose the numbering direction that gives the lowest locant set overall.

Why only one suffix, even with several functional groups present. A name can carry only one suffix position, so when a molecule has two or more functional groups, the priority order acts as a tie-breaker: the single highest-ranked group is promoted to suffix and fixes both the parent chain's endpoint and its numbering, while every other group is written as an ordinary prefix. This is why $-\text{COOH}$ always outranks a ketone's $\text{C=O}$ : naming the acid group as the suffix, and the ketone as "oxo-", still lets both groups be read unambiguously from the name, whereas trying to give two groups suffix status at once would not.

Worked example

Question: Give the IUPAC name of $\text{CH}_3-\text{CO}-\text{CH}_2-\text{COOH}$ , a molecule carrying two functional groups.

Step 1. Identify every functional group present. There is a ketone ( $\text{C=O}$ flanked by carbons on both sides, in the middle of the chain) and a carboxylic acid ( $-\text{COOH}$ , at the end).

Step 2. Apply the priority order. Carboxylic acid outranks ketone, so $-\text{COOH}$ is the principal group and takes the suffix $-\text{oic acid}$ ; the ketone must be expressed as a prefix, "oxo-".

Step 3. Select and number the parent chain. The chain is $\text{C}_1\text{OOH}-\text{C}_2\text{H}_2-\text{C}_3(=\text{O})-\text{C}_4\text{H}_3$ , four carbons, so the root is but-. The carboxylic acid carbon is, by convention, always $C_1$ since it must be a terminal carbon of the parent chain.

Step 4. Locate the ketone. Its $\text{C=O}$ sits on $C_3$ , so the prefix is "3-oxo".

Step 5. Assemble the name: root but-, suffix -oic acid (no locant needed, since $-\text{COOH}$ is always terminal), prefix 3-oxo.

Answer: 3-oxobutanoic acid (also known by its trivial name, acetoacetic acid).

Competitive stretch

Now add a methyl branch on $C_2$ of the same skeleton: $\text{CH}_3-\text{CO}-\text{CH}(\text{CH}_3)-\text{COOH}$ . The priority order and numbering are unchanged — $C_1$ is still the acid carbon, $C_3$ still carries the oxo group — but a second prefix, "2-methyl", must now be alphabetised alongside "3-oxo". Since m precedes o, the prefixes are listed methyl before oxo: 2-methyl-3-oxobutanoic acid. The lesson: adding a substituent never disturbs which group is the suffix or how the chain is numbered; it only adds one more alphabetically placed prefix.

Common traps

Choosing chain locants for substituents before locking in the lowest locant for the principal (suffix) group — the suffix group's locant always takes priority first.
Giving every functional group present its own suffix; only the single highest-priority group gets the suffix, the rest become prefixes (hydroxy, oxo, and so on).
Alphabetising a multiplying prefix like "di-" or "tri-" as part of the substituent name — "dimethyl" is alphabetised under "m," not "d."
Picking the longest chain in the whole structure without checking that it actually runs through the carbon bearing the principal characteristic group.

Ask the tutor: Want a few structures to name (or names to decode back into structures) with instant feedback on your locants and priority choices?

Why can't a molecule just get two suffixes if it has two functional groups?

A name is built as one prefix block, one root, one ending, and one suffix slot — there is only ever a single suffix position. If a molecule carries two functional groups, the priority order picks the higher-ranked one for that single suffix slot, and the lower-ranked group is written instead as a prefix with its own locant, such as "oxo-" or "hydroxy-". This keeps every name in the same fixed shape, so it can always be parsed back into a unique structure.

Why is the carboxylic acid carbon always numbered C1 without writing a locant for it?

Because $-\text{COOH}$ can only ever sit on a terminal carbon of the parent chain — it uses up that carbon's bonds in a way that leaves no room for the chain to continue past it. Since there is no other place it could be, giving it a locant would be redundant, so IUPAC convention omits the "1" and simply writes "-oic acid". Any other suffix group that could, in principle, sit at more than one position (like -ol) always keeps its explicit locant.

If a name looks correct but I numbered the chain the wrong way, is it still wrong?

Yes — numbering direction is not a free choice, it is fixed by the lowest-locant rule for the principal group, so a differently-numbered version of the same structure is a different, incorrect name, not just a stylistic variant. Always number the parent chain in the direction that gives the smallest locant to the suffix group first, and only use the other direction's locants for substituents if the suffix group is genuinely symmetric or absent.