IUPAC nomenclature

The IUPAC system gives each compound one systematic name. For alkanes and substituents:

• Find the longest carbon chain (the parent chain). If two chains are equally long, choose the one with more side chains.
• Number the chain from the end that gives the substituents the lowest locants. Compare the sets term by term and pick the one that is lower at the first difference: 2,5,6 beats 3,4,7, because 2 is lower than 3.
• Write the substituents in alphabetical order. 'Iso' and 'neo' are counted in the alphabet; 'sec', 'tert' and the multiplying prefixes di, tri, tetra are not.
• If two substituents would get the same numbers from either end, the one first in the alphabet gets the lower number: 3-ethyl-5-methylheptane.

With functional groups, one group is chosen as the principal group, named as the suffix, and given the lowest possible number. The order of priority is:

–COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂ > C=C > C≡C.

The other groups become prefixes: –OH becomes 'hydroxy', >C=O becomes 'oxo'. For example, 5-oxohexanoic acid has an acid as its principal group and a ketone as a prefix.

Some groups are always prefixes, never suffixes: the halogens (chloro, bromo…), –NO₂ (nitro) and –OR (alkoxy). When a group appears more than once, use di, tri… with all its locants: hexane-2,4-dione.

For benzene compounds, the substituents are written as prefixes to 'benzene', numbered to give the lowest locants and listed alphabetically: 4-ethyl-1-fluoro-2-nitrobenzene. When there are just two substituents, the positions can also be named ortho (1,2), meta (1,3) and para (1,4). Some trivial names are still used: methylbenzene is usually called toluene.