Locants

The double or triple bond position is given the lowest possible locant. The locant refers to the lower-numbered carbon of the multiple bond.

Example: but-1-ene (double bond between C1 and C2), but-2-ene (double bond between C2 and C3).

E/Z Geometry

When a double bond has two different groups on each carbon, it can exist as E or Z isomers.

Z (zusammen) — higher-priority groups on the same side of the double bond.

E (entgegen) — higher-priority groups on opposite sides.

Priority is assigned by atomic number (Cahn–Ingold–Prelog rules): higher atomic number = higher priority. When the attached atoms are the same, move outward along the chain until a difference is found.

Cis/trans is an older convention still used for simple cases: cis = same side, trans = opposite side. E/Z is unambiguous and applies to all cases.

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