Some molecules have the same atomic connectivity but cannot freely interconvert between all spatial arrangements because rotation is restricted. This can produce geometric stereoisomers.

A carbon-carbon double bond is the most common example. The $\pi$ component of a C=C bond depends on side-by-side orbital overlap, so ordinary free rotation around the double bond would destroy that overlap and is therefore strongly restricted.

For a simple alkene in which each double-bond carbon bears two different substituents, two distinct configurations can result.

For 2-butene,

$$\mathrm{CH_3CH=CHCH_3},$$

the two methyl groups can lie on the same side of the double bond (cis) or on opposite sides (trans). These are different compounds, not rapidly interconverting conformations.

The more general E/Z notation works even when cis/trans language is ambiguous. Use the Cahn-Ingold-Prelog priority rules to identify the higher-priority substituent on each double-bond carbon.

• Z (zusammen, together): the two higher-priority substituents lie on the same side of the double bond.
• E (entgegen, opposite): they lie on opposite sides.

Example

For

$$\mathrm{CH_3CH=C(Cl)CH_3},$$

CIP ranking gives CH3 higher priority than H on the left alkene carbon and Cl higher priority than CH3 on the right alkene carbon. If CH3 on the left and Cl on the right lie on opposite sides, the alkene is E; if they lie on the same side, it is Z.

Restricted geometry in rings can also generate cis/trans relationships because substituents may be constrained to the same or opposite faces of a ring.

Geometric stereoisomers have the same molecular formula and connectivity but different spatial configuration. They are therefore stereoisomers, not constitutional isomers. Their different shapes and dipole arrangements can produce different physical properties and different interactions with other molecules.