Lesson 4 of 13 · 7 min
IUPAC nomenclature
NCERT § "Nomenclature of Organic Compounds"
'Acetic acid' comes from acetum, Latin for vinegar. A lovely name, but it tells you nothing about the structure. The systematic name, ethanoic acid, tells you everything: two carbons, all single bonds, a –COOH group.
The lesson in notes
In short
An IUPAC name is built from a root word (length of the parent chain), a primary suffix (ane, ene, yne), a secondary suffix for the principal functional group and prefixes for substituents.
Choose the longest carbon chain as the parent; if two chains are equally long, choose the one with more substituents.
Number the chain so that the substituents (or the principal group, or multiple bonds) get the lowest set of locants.
Substituent prefixes are written in alphabetical order; multiplying prefixes such as di-, tri- and tetra- are ignored when alphabetising.
When several functional groups are present, the principal group takes the suffix; the order of preference is –COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂ > >C=C< > –C≡C–.
Halogens (–F, –Cl, –Br, –I), –NO₂ and –OR are always written as prefixes (fluoro, chloro, bromo, iodo, nitro, alkoxy), never as suffixes.
Example: CH₂=CH–CH₂–CHBr–CH₃ is numbered from the end nearer the double bond, giving 4-bromopent-1-ene rather than 2-bromopent-4-ene.
Ring compounds take the prefix cyclo-, as in cyclohexane.
In disubstituted benzenes, 1,2-, 1,3- and 1,4- positions are also called ortho (o-), meta (m-) and para (p-).