Geometrical Isomerism

Geometrical isomerism is a kind of stereoisomerism. The isomerism due to the difference in spatial arrangements of groups about the doubly bonded carbon atoms is known as Geometrical Isomerism.

For e.g., consider the structure of 2-butene.

Space Models

 

fig.12.1 - Space Models of 2-Butenes

Note that the atoms are arranged in two different ways in I and II. The two CH₃ groups lie on the same side in I and on the opposite sides in II. When two similar atoms or groups lie on the same side, the isomerism is called cis isomerism and the isomers are called cis isomers (cis means "on same side" in Latin). When two similar atoms or groups lie on opposite sides, the isomerism is called trans isomerism and the isomers are called trans isomers (trans means "across" in Latin). Geometrical isomerism is also called Cis-trans isomerism.

The carbon-carbon double bond in 2-butene consists of one sigma bond and one pie bond. The pie bond is formed by the lateral overlapping of p orbitals that lie above and below the sigma orbital plane. To pass from the cis form to the trans form, the molecule must be twisted so that the p orbitals don't overlap any more and the pie bond is broken. To break this pie bond, around 167.2KJ of energy is required. Therefore the conversion from one geometric isomer to another is very rare in 2-butene.

 

 

fig.12.2 - Hindered rotation about the carbon carbon double bond

Similarly, maleic and fumaric acids exhibit geometric isomerism or they are geometric isomers.

Just as the compounds containing carbon-carbon double bond can't rotate freely and hence exhibit geometric isomerism, those containing carbon nitrogen double bonds also exhibit geometrical isomerism. For e.g., alpha benzaldoxime and beta benzaldoxime are geometrical isomers.