Naming organic compounds

The three parts of a name

Every IUPAC name is built the same way:

$$\underbrace{\text{2-methyl}}_{\text{prefix}}\;\underbrace{\text{butan}}_{\text{stem}}\;\underbrace{\text{-1-ol}}_{\text{suffix}}$$

• The stem says how many carbons are in the longest chain — meth, eth, prop, but, pent, hex, hept, oct.

• The suffix names the highest-priority functional group, with a number saying where it is.

• Prefixes name everything else — branches, halogens and lower-priority groups — each with its own number.

• The standard restricts you to chains of no more than eight carbons.

The method

• Find the longest continuous carbon chain that contains the principal functional group. That gives the stem.

• Identify the highest-priority functional group present. That becomes the suffix.

• Number the chain from whichever end gives the suffix group the lowest possible number.

• Name everything else as a prefix, with its number, and put the prefixes in alphabetical order.

• Numbers are separated from words by hyphens and from other numbers by commas: 2,3-dimethylbutan-1-ol.

Which group wins

Highest priority first:

• carboxylic acid → -oic acid

• ester → alkyl ...-oate

• acyl chloride → -oyl chloride

• amide → -amide

• aldehyde → -al

• ketone → -one

• alcohol → -ol

• amine → -amine

• Only the highest one present becomes the suffix. Everything else becomes a prefix:

  • $-\text{OH}$ → hydroxy-
  • $-\text{NH}_2$ → amino-
  • $\text{C}=\text{O}$ (as a ketone) → oxo-
  • $-\text{Cl}$, $-\text{Br}$ → chloro-, bromo-
  • alkyl branches → methyl-, ethyl-

• So $\text{CH}_3\text{CH}_2\text{CH}(\text{OH})\text{COCl}$ is 2-hydroxybutanoyl chloride — the acyl chloride outranks the alcohol, so the –OH is demoted to a prefix.

Groups that need no number

• An aldehyde, a carboxylic acid, an acyl chloride and an amide are always at the end of a chain, so their carbon is always carbon 1 and no locant is written.
• Write propanal, not "propan-1-al"; butanoic acid, not "butan-1-oic acid".

Naming esters

• An ester name has two words, and they come from the two parts in a specific order:

  • the first word is the alkyl group from the alcohol
  • the second word comes from the acid, ending -oate

• $\text{CH}_3\text{CH}_2\text{COOCH}_3$ is methyl propanoate — the $\text{CH}_3$ on the oxygen came from methanol, and the three-carbon acid part is propanoate.

• The alkyl group named first is the one attached to the oxygen, not the one attached to the carbonyl. Getting these the wrong way round is the most common ester-naming error.

Naming amines

• The 2026 specification restricts systematic naming to primary amines.
• Name them as the alkane with -amine as the suffix and a locant: $\text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2$ is propan-1-amine.
• When an amine group is not the principal group it becomes the prefix amino-: $\text{H}_2\text{NCH}_2\text{COOH}$ is 2-aminoethanoic acid.