Naming · Rule set

Nomenclature

The puzzle: you’re given a structure and asked for its IUPAC name. Work it through the same steps every time. Try the molecule yourself, then step through it: each step lights up on the drawing and points to the rule it uses.

Rules up to

Practice

    Exceptions & traps

    Where the obvious answer is wrong

    Each one loads a molecule that springs the trap.

    Words to know

    Plain-English glossary

    Parent
    The main chain (or ring) the name is built on. It gives the last word: hexane, cyclopentane.
    Substituent
    Anything attached to the parent other than H: carbon branches (methyl, ethyl…) and halogens (chloro, bromo…).
    Locant
    The number that says which parent carbon a substituent sits on. The 2 in 2-methylbutane.
    Prefix
    Everything in front of the parent name: the substituents with their locants.
    Alkyl group (-yl)
    A carbon branch, named by its length with -yl: methyl (1 C), ethyl (2), propyl (3), butyl (4).
    Wedge / dash
    Wedge = bond pointing toward you; dash (hashed) = pointing away. On a ring, they show which face a group is on.
    Stereocenter
    A carbon with four different groups. Also called a chiral center. Swapping any two groups gives a different molecule.
    Priority
    The 1–4 ranking of the groups on a stereocenter (the Cahn–Ingold–Prelog, or CIP, rules). Higher atomic number first.
    R / S
    The label for a stereocenter: R (rectus) if 1 → 2 → 3 runs clockwise with 4 pointing away; S (sinister) if counterclockwise.
    Chiral
    Not identical to its mirror image, like a hand. The mirror-image partner is its enantiomer.
    Fischer projection
    A flat way to draw stereocenters: each cross is a carbon, horizontal bonds come toward you, vertical bonds go away.
    Meso
    Has stereocenters but is identical to its mirror image (it has an internal mirror plane), so it isn’t chiral.

    Chain-length stems