Naming organic compounds (IUPAC)

How an IUPAC name is built

An IUPAC name is a code. Read it properly and you know exactly which molecule is meant — every part of the name carries information.

• Prefix — the branches, each with the number of the carbon it sits on.
• Stem — how many carbons are in the longest chain.
• Locant — the number saying where the functional group is.
• Suffix — which family the compound belongs to.

Step 1 — Find the longest chain

• Find the longest continuous chain of carbon atoms that contains the functional group.
• The chain does not have to be drawn in a straight line — follow it round corners.
• The number of carbons gives the stem:

Carbons	1	2	3	4	5	6	7	8
Stem	meth-	eth-	prop-	but-	pent-	hex-	hept-	oct-

• Any carbon not in that chain becomes a branch.

Step 2 — Number the chain

• Number from the end that gives the functional group the lowest number.
• If there is no functional group (a plain alkane), number from the end that gives the first branch the lowest number.

• Always try both ends and compare. The molecule has not changed — only your counting.

Step 3 — Add the suffix for the family

Family	Ending	Example
alkane	-ane	propane
alkene	-ene	prop-1-ene
alkyne	-yne	prop-1-yne
alcohol	-ol	propan-1-ol
primary amine	-amine	propan-1-amine
carboxylic acid	-oic acid	propanoic acid
haloalkane	prefix: fluoro-, chloro-, bromo-, iodo-	2-bromopropane

• The haloalkane is the odd one out: the halogen goes in the prefix, not the suffix.
• Carboxylic acids need no locant — the –COOH carbon is always carbon 1, because the chain must end there.

Step 4 — Name the branches

• A branch is named after the number of carbons in it, ending in -yl:
  • $CH_3$– = methyl, $CH_3CH_2$– = ethyl, $CH_3CH_2CH_2$– = propyl.
• Put the number of the carbon it sits on in front: 2-methyl, 3-ethyl.
• If the same branch appears more than once, use di- (2), tri- (3), tetra- (4), and list every position.
  • $2,3$-dimethylbutane — two methyl branches, one on carbon 2 and one on carbon 3.
• If there are different branches, list them in alphabetical order.
  • 4-ethyl-2-methylhexane — ethyl before methyl.

Step 5 — Punctuation

• Numbers are separated from each other by commas: 2,3-dimethyl.
• Numbers are separated from letters by hyphens: 2-bromo, propan-1-ol.
• No spaces, except before "acid".

Putting it together

• $CH_3CH_2CH_2OH$ → 3 carbons, –OH on carbon 1 → propan-1-ol.
• $CH_3CHClCH_3$ → 3 carbons, Cl on carbon 2 → 2-chloropropane.
• $CH_3CH{=}CHCH_3$ → 4 carbons, C=C starting at carbon 2 → but-2-ene.
• $CH_3CH_2COOH$ → 3 carbons, –COOH → propanoic acid.
• $CH_3CH_2CH_2NH_2$ → 3 carbons, –NH₂ on carbon 1 → propan-1-amine.