E/Z vs cis/trans isomers explained

When can an alkene have isomers?

Rotation about a C=C is restricted, so a double bond can have two arrangements if each carbon of the C=C carries two different groups. If either carbon has two identical groups (like =CH₂ or =C(CH₃)₂), there's only one arrangement.

Cis and trans

Cis means two matching groups on the same side; trans means opposite sides. This is clear when each carbon has one H and one other group, as in but-2-ene.

How to assign E and Z

1. Look at the left carbon of the C=C. Rank its two groups by the atomic number of the atom directly attached. Higher atomic number = higher priority.
2. If those atoms are the same, move outwards: compare the sets of atoms attached to them, highest first, until there's a difference.
3. Do the same for the right carbon.
4. If the two higher-priority groups are on the same side, it's Z (from German zusammen, together). If they're on opposite sides, it's E (entgegen, opposite).

Double and triple bonds count as duplicated atoms: a C=O carbon is treated as having two oxygens attached.

Where cis is not Z

In 2-bromobut-2-ene, the left carbon has Br and CH₃. Bromine (atomic number 35) beats carbon (6). The right carbon has CH₃ and H, so CH₃ wins. If the two methyl groups are on the same side, the molecule looks "cis", but the higher-priority groups (Br and CH₃) are on opposite sides, so it's E.

Another example

In 3-methylpent-2-ene, C3 carries a methyl and an ethyl group. Both start with carbon, so look one atom further: the ethyl's CH₂ has (C,H,H) attached, which beats the methyl's (H,H,H). So ethyl has priority.

Writing the name

Put the descriptor in brackets at the front: (E)-but-2-ene. With more than one double bond, add the locant: (2E,4Z)-hexa-2,4-diene.

The same priority rules give R and S at stereocentres: see how to assign R and S.