Geometrical Isomerism

Geometrical isomerism is a form of stereoisomerism arising specifically from RESTRICTED ROTATION -- either about a carbon-carbon double bond, or about a single bond that happens to be locked inside a ring -- so that two different, non-interconvertible (without breaking a bond) arrangements of the same substituents are possible around that rigid framework.

The root cause, for an alkene, is the nature of the C=C bond itself: the double-bonded carbons are sp2 hybridised, and the double bond is built from a sigma bond (head-on overlap of sp2 orbitals) PLUS a pi bond (sideways overlap of the remaining unhybridised p orbitals). That pi bond physically locks the two halves of the molecule in a fixed orientation relative to each other -- rotating one carbon relative to the other would have to break the pi overlap, so free rotation about a C=C bond simply is not possible at ordinary temperatures. Cis-2-butene and trans-2-butene are the standard illustration: in the cis isomer, the two similar (here, methyl) groups sit on the SAME side of the double bond; in the trans isomer, they sit on OPPOSITE sides -- hence this whole phenomenon is often called cis-trans isomerism.

Because interconverting cis and trans requires breaking the pi bond, it takes real energy to do it: heating to a high temperature, or exposing the compound to light, supplies roughly 62 kcal/mole -- enough to break the pi bond and allow free rotation about the remaining sigma bond -- after which, on cooling, the pi bond re-forms randomly, regenerating a MIXTURE of both cis and trans isomer rather than restoring the original pure form. Of the two isomers, trans is generally the more stable, because in the cis form the two bulky substituent groups sit crowded together on the same side of the double bond and suffer steric (space-clash) repulsion, which the trans arrangement avoids by keeping them on opposite sides. Cis and trans isomers, having genuinely different physical properties as a consequence, can be physically separated by techniques such as fractional distillation or gas chromatography.

Not every alkene can show geometrical isomerism -- it requires each of the two double-bonded carbons to carry two DIFFERENT groups. Propene is the clearest counter-example: one of its double-bonded carbons carries two identical hydrogen atoms, so swapping the other substituent's position produces nothing new, and propene simply has no cis/trans forms.

The same restricted-rotation logic extends to single bonds inside a conjugated diene system: 1,3-butadiene (CH2=CH-CH=CH2) can in principle adopt infinitely many rotational conformations about its central single bond, though in practice only two extreme (s-cis and s-trans) conformations matter chemically. …