IUPAC Nomenclature

Naming is a fixed algorithm: pick the parent chain, number it for lowest locants, choose the suffix by seniority, and cite the rest as alphabetical prefixes. Do the steps in order and the name follows.

1. Select the parent (principal) chain

• Choose the longest continuous chain that contains the principal characteristic group (and the maximum number of skeletal double/triple bonds).
• Key point: the parent chain must include the principal group even if that isn't the longest carbon chain overall.
• If two chains are equal length, pick the one with more substituents / lower locants.

2. Number for lowest locants

• Number from the end that gives the lowest locant to the principal characteristic group first.
• Then, if still tied, lowest locants to multiple bonds, then to substituents (lowest set / "first point of difference"), then to the substituent cited first alphabetically.

3. Seniority of the principal characteristic group

Only one group becomes the suffix — the most senior. Everything less senior is expressed as a prefix.

Example: a molecule with both —OH and —COOH is named as an acid; the —OH is cited as the hydroxy- prefix (e.g. 2-hydroxypropanoic acid).

4. Cite substituent prefixes alphabetically

• List substituent prefixes in alphabetical order (ethyl before methyl).
• When alphabetising, ignore the multiplying prefixes di-, tri-, tetra- (alphabetise "dimethyl" under m).
• Complex substituents in enclosing marks (e.g. 1,2-dimethylpropyl) are alphabetised by their first letter of the complete name.

5. Indicating unsaturation

• Double bond → suffix —ene; triple bond → —yne, each with a locant (e.g. but-2-ene, pent-1-yne).
• Both present: —en—yne, with lowest locants (e.g. pent-1-en-4-yne); the double bond gets priority for the lower number if there's a choice.

6. Cyclic and polyfunctional naming

• Ring parent takes the prefix cyclo- (cyclohexane, cyclohex-1-ene). Number to give lowest locants to the principal group / substituents.
• Polyfunctional: use a multiplying prefix + locants for repeated groups (e.g. butane-1,4-diol, propane-1,2,3-triol).
• The senior group is still the suffix; all others are prefixes.

7. Stereodescriptors in the name

• E/Z for C=C geometry (CIP priorities), cis/trans for simple cases; placed in italics in front: (Z)-, (E)-.
• R/S for chiral centres, with the locant: (2R,3S)-.
• Combine as needed, e.g. (2E,4R)-... — listed at the very front in parentheses.

Summary table