IUPAC naming rules: A step-by-step summary

A systematic name is built from a few parts: a parent (the main chain or ring), an ending that says whether it contains double or triple bonds, a suffix for the most important functional group, and prefixes for everything else. This page sums up the rules in the order you apply them when you name a structure. It follows the current IUPAC recommendations (2013), and so do the exercise names, with the deliberate exceptions described below under "Preferred IUPAC names"; the topic guides explain each family of compounds in more detail.

The parts of a name

Each number (locant) stands directly before the part it describes. The final "e" of a parent such as "pentene" is dropped before a suffix that starts with a vowel or "y": pent-3-en-2-one, propan-2-ol, hex-1-en-5-yne, but kept before a consonant: butane-1,4-diol.

Step 1: find the principal characteristic group

Only one kind of functional group can be named as the suffix: the most senior one present. All other groups become prefixes. The order of seniority, from the highest, for the groups on this site:

Some groups are only ever prefixes: halogens (fluoro-, chloro-, bromo-, iodo-), nitro-, alkoxy groups such as methoxy- in ethers, and alkyl groups. The "-carboxylic acid", "-carbonitrile" and "-carbaldehyde" forms are used when the carbon of the group is not part of the parent, as in cyclohexanecarboxylic acid. (Acid anhydrides, which rank between acids and esters, are not covered on this site.)

Step 2: choose the parent

• The parent must contain the principal characteristic group, and as many of them as possible.
• Then it is the longest chain, wherever that chain runs in the drawing; it is often not the one drawn in a straight line.
• If two chains are equally long, the one with more multiple bonds is the parent. Between equally long chains with no suffix and no multiple bonds, as in alkanes, the one with more substituents is chosen. (The full recommendations have further tie-breakers, which are rarely needed.)
• Rings: when the principal group is on a ring, or there is no principal group, the ring is the parent and chains on it are prefixes (ethylcyclohexane, 2-methylcyclohexan-1-one). When the principal group is on the chain, the chain is the parent and the ring is a prefix (phenylmethanol).

Older recommendations (1979 and 1993), still found in many textbooks, chose the chain with the most double and triple bonds first, even when a longer chain existed. Both rules choose the same parent whenever the longest chain contains every multiple bond.

Step 3: number the parent

Number from the end that gives the lowest locants, applying these criteria in order until one decides:

1. The principal characteristic group (the suffix).
2. Double and triple bonds together; if that is still a tie, the double bond gets the lower locant.
3. All prefixes together, compared term by term at the first point of difference: {2,2,4} is lower than {2,4,4}.
4. The prefix cited first in alphabetical order.

In a ring with a suffix, the carbon that carries it is C1. In a cycloalkene without a suffix, the double bond is between C1 and C2. In acids, aldehydes, amides and nitriles named with -oic acid, -al, -amide or -nitrile (not -carboxylic acid, -carbaldehyde or -carbonitrile), the carbon of the group is C1 of the chain.

Step 4: write the name

• Prefixes go in alphabetical order, ignoring multiplying prefixes such as di-, tri- and tetra-: 3-ethyl-2,2-dimethylpentane (e before m). The prefixes sec- and tert- are ignored too, but iso- counts as part of the name.
• One locant for every group, even when two groups share a carbon: 2,2-dimethyl, not 2-dimethyl.
• Commas between numbers, hyphens between numbers and letters.
• Esters are two words: the alkyl group on the oxygen first, then the acid part ending in -oate: methyl propanoate. Acids are two words as well: propanoic acid.

Preferred IUPAC names

The 2013 recommendations choose one preferred IUPAC name (PIN) for each compound. Usually it is the systematic name built with the steps above. For a few common compounds, however, the preferred name is a retained name: toluene rather than methylbenzene, acetic acid rather than ethanoic acid. The acetic acid names carry over to its esters, its amide and its acyl chloride, to acetaldehyde and to acetonitrile.

The exercises on this site use the systematic name that many school courses teach, and the worked answer shows the preferred IUPAC name next to it. Both names describe the same structure.

Phenol, aniline, benzoic acid and benzaldehyde are preferred IUPAC names as well, and the exercises use them. Toluene is preferred only for the unsubstituted compound: its derivatives are named as substituted benzenes, such as 1-ethyl-4-methylbenzene. Several other familiar names are not preferred IUPAC names: acetone (propan-2-one), styrene (ethenylbenzene), glycerol (propane-1,2,3-triol), chloroform (trichloromethane) and the substituent prefix isopropyl (propan-2-yl).

In a preferred IUPAC name each stereodescriptor also carries its locant: (2E)-but-2-ene and (2R)-butan-2-ol. The stereochemistry exercises use the shorter form, such as (E)-but-2-ene, and the worked answer lists the form with locants among the other names.

Mistakes to avoid

• Numbering from the wrong end, for example 3-methylbutane instead of 2-methylbutane.
• Taking the chain that is drawn in a straight line instead of the longest one: 2-ethylbutane is really 3-methylpentane.
• Sorting prefixes by their multiplier: dimethyl is filed under m, not d.
• Giving the lowest locant to a prefix instead of the suffix: 4-methylpentan-2-ol, not 2-methylpentan-4-ol.
• Forgetting that the carbon of –COOH, –CHO, –CONH₂ and –C≡N counts as C1 of a chain named with -oic acid, -al, -amide or -nitrile.
• Swapping the two words of an ester name: ethyl methanoate and methyl ethanoate are different compounds.

Practice

Read a name and draw the structure, or look at a structure and choose its name. Both give feedback on every answer.