E and Z configuration
/ E and Z /
A carbon-carbon double bond is stiff — unlike a single bond, you cannot freely rotate around it without breaking the pi bond. That stiffness locks the two ends in place, so the groups attached can be frozen on the same side or on opposite sides of the bond. This gives two different molecules, and chemists need clear names to tell them apart.
The old labels cis (same side) and trans (opposite sides) work fine when each double-bond carbon carries one hydrogen and one obvious group, but they break down when all four positions hold different groups — same side as what? The E/Z system fixes this with the Cahn-Ingold-Prelog rules. On each carbon of the double bond, rank its two attached groups by CIP priority. If the two higher-priority groups (one from each carbon) sit on opposite sides, the alkene is E (from German entgegen, 'opposite'). If they sit on the same side, it is Z (from zusammen, 'together'). A simple memory aid: 'Z = ze zame zide'.
E/Z is the rigorous, universal way to specify double-bond geometry, and it appears throughout IUPAC names like (2Z,4E)-hexadienal. The geometry is not cosmetic: it changes a molecule's shape, polarity, and biology. The cis and trans forms of fatty acids behave very differently in the body, and the light-driven flip of an alkene from cis to trans in retinal is the first chemical event of vision.
In 2-butene, CH3-CH=CH-CH3, the cis form has both methyls on the same side (this is Z, since CH3 outranks H on each carbon) and the trans form has them on opposite sides (E). Here cis matches Z and trans matches E, but that lucky coincidence fails as soon as the four groups get more complicated.
E/Z is decided by CIP priorities, so it works even when cis/trans becomes ambiguous.
Do not assume Z always equals cis and E always equals trans. They agree only in simple cases; with four different substituents the labels can flip, because E/Z compares CIP priorities while cis/trans loosely compares 'like' groups. When in doubt, assign priorities and use E/Z.