Q.Briefly explain geometrical isomerism in alkene by considering 2-butene as an example.

Geometrical isomerism is a form of stereoisomerism arising from RESTRICTED ROTATION -- about a C=C double bond, or about a single bond 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, in an alkene, is that the double-bonded carbons are sp2 hybridised: the double bond is a sigma bond (head-on sp2-sp2 overlap) plus a pi bond (sideways overlap of the remaining p orbitals), and that pi bond physically locks the two halves of the molecule in place -- free rotation about C=C simply is not possible without breaking the pi bond.

Cis-2-butene and trans-2-butene are the standard illustration: in the cis isomer, the two similar (methyl) groups sit on the SAME side of the double bond; in the trans isomer, they sit on OPPOSITE sides -- hence the whole phenomenon is often called cis-trans isomerism. Interconverting cis and trans genuinely requires energy -- heating to a high temperature, or exposing to light, supplies roughly 62 kcal/mole, enough to break the pi bond, allow free rotation about the remaining sigma bond, and on cooling the pi bond re-forms randomly, regenerating a MIXTURE of both isomers. Trans is generally the more stable of the two, because in the cis form the two bulky groups crowd together on the same side and suffer steric repulsion, which the trans arrangement avoids. …